Evolving Together: Cassandra Retrotransposons Gradually Mirror Promoter Mutations of the 5S rRNA Genes

The 5S rRNA genes are among the most conserved nucleotide sequences across all species. Similar to the 5S preservation we observe the occurrence of 5S-related nonautonomous retrotransposons, so-called Cassandras. Cassandras harbor highly conserved 5S rDNA-related sequences within their long terminal repeats, advantageously providing them with the 5S internal promoter. However, the dynamics of Cassandra retrotransposon evolution in the context of 5S rRNA gene sequence information and structural arrangement are still unclear, especially: (1) do we observe repeated or gradual domestication of the highly conserved 5S promoter by Cassandras and (2) do changes in 5S organization such as in the linked 35S-5S rDNA arrangements impact Cassandra evolution? Here, we show evidence for gradual co-evolution of Cassandra sequences with their corresponding 5S rDNAs. To follow the impact of 5S rDNA variability on Cassandra TEs, we investigate the Asteraceae family where highly variable 5S rDNAs, including 5S promoter shifts and both linked and separated 35S-5S rDNA arrangements have been reported. Cassandras within the Asteraceae mirror 5S rDNA promoter mutations of their host genome, likely as an adaptation to the host's specific 5S transcription factors and hence compensating for evolutionary changes in the 5S rDNA sequence. Changes in the 5S rDNA sequence and in Cassandras seem uncorrelated with linked/separated rDNA arrangements. We place all these observations into the context of angiosperm 5S rDNA-Cassandra evolution, discuss Cassandra's origin hypotheses (single or multiple) and Cassandra's possible impact on rDNA and plant genome organization, giving new insights into the interplay of ribosomal genes and transposable elements.

Mechanism of 5S RNP recruitment and helicase-surveilled rRNA maturation during pre-60S biogenesis

Ribosome biogenesis proceeds along a multifaceted pathway from the nucleolus to the cytoplasm that is extensively coupled to several quality control mechanisms. However, the mode by which 5S ribosomal RNA is incorporated into the developing pre-60S ribosome, which in humans links ribosome biogenesis to cell proliferation by surveillance by factors such as p53-MDM2, is poorly understood. Here, we report nine nucleolar pre-60S cryo-EM structures from Chaetomium thermophilum, one of which clarifies the mechanism of 5S RNP incorporation into the early pre-60S. Successive assembly states then represent how helicases Dbp10 and Spb4, and the Pumilio domain factor Puf6 act in series to surveil the gradual folding of the nearby 25S rRNA domain IV. Finally, the methyltransferase Spb1 methylates a universally conserved guanine nucleotide in the A-loop of the peptidyl transferase center, thereby licensing further maturation. Our findings provide insight into the hierarchical action of helicases in safeguarding rRNA tertiary structure folding and coupling to surveillance mechanisms that culminate in local RNA modification.

5S rRNA pseudogene transcripts are associated with interferon production and inflammatory responses in alcohol-associated hepatitis

BACKGROUND AND AIMS

Interferon (IFN) signaling is critical to the pathogenesis of alcohol-associated hepatitis (AH), yet the mechanisms for activation of this system are elusive. We hypothesize that host-derived 5S rRNA pseudogene (RNA5SP) transcripts regulate IFN production and modify immunity in AH.

APPROACH AND RESULTS

Mining of transcriptomic datasets revealed that in patients with severe alcohol-associated hepatitis (sAH), hepatic expression of genes regulated by IFNs was perturbed and gene sets involved in IFN production were enriched. RNA5SP transcripts were also increased and correlated with expression of type I IFNs. Interestingly, inflammatory mediators upregulated in sAH, but not in other liver diseases, were positively correlated with certain RNA5SP transcripts. Real-time quantitative PCR demonstrated that RNA5SP transcripts were upregulated in peripheral blood mononuclear cells (PBMCs) from patients with sAH. In sAH livers, increased 5S rRNA and reduced nuclear MAF1 (MAF1 homolog, negative regulator of RNA polymerase III) protein suggested a higher activity of RNA polymerase III (Pol III); inhibition of Pol III reduced RNA5SP expression in monocytic THP-1 cells. Expression of several RNA5SP transcript-interacting proteins was downregulated in sAH, potentially unmasking transcripts to immunosensors. Indeed, siRNA knockdown of interacting proteins potentiated the immunostimulatory activity of RNA5SP transcripts. Molecular interaction and cell viability assays demonstrated that RNA5SP transcripts adopted Z-conformation and contributed to ZBP1-mediated caspase-independent cell death.

CONCLUSIONS

Increased expression and binding availability of RNA5SP transcripts was associated with hepatic IFN production and inflammation in sAH. These data identify RNA5SP transcripts as a potential target to mitigate inflammation and hepatocellular injury in AH.

High production of transfer RNAs identifies the presence of developing oocytes in ovaries and intersex testes of teleost fish

5S rRNA is highly transcribed in fish oocytes and this transcription levels can be used to identify the presence of oocytes in the intersex testes of fish exposed to xenoestrogens. Similar to 5S rRNA, tRNAs are transcribed by RNA polymerase III (Pol-III) in eukaryotes, so this study focuses in the analysis of the levels of expression of tRNAs in the gonads (ovaries and testes) of eight teleost species as a possible new oocyte molecular marker. Total RNA extracted from gonads of six commercial teleost species in the Biscay Bay, from the pollution sentinel species thicklip grey mullet (Chelon labrosus) known present intersex testes in response to xenoestrogens in Gernika estuary and from the laboratory model species Danio rerio were analysed through capillary electrophoresis. Bioanalyzer electropherograms were used to quantify the concentrations of tRNAs, 5S and 5.8S rRNA. All studied ovaries expressed significantly higher levels of tRNAs and 5S rRNA than testes. A tRNA to 5.8S rRNA index was calculated which differentiates ovaries from testes, and identifies some intersex testes in between testes and ovaries in mullets. The tRNA/5.8S ratio was highest in ovaries in previtellogenic stage, decreasing towards maturity. Thus, strong oocyte expression of tRNAs is an additional proof of high activity levels of Pol-III during early stages of oocyte development in teleost ovaries. Incidentally, we observed that miRNA concentrations were always higher in testes than ovaries. The indexing approach developed in the present study could have multiple applications in teleost reproduction research and in the development of early molecular markers of intersex condition.

Five Species of Taxus Karyotype Based on Oligo-FISH for 5S rDNA and (AG3T3)3

As a relict plant, Taxus is used in a variety of medicinal ingredients, for instance to treat a variety of cancers. Taxus plants are difficult to distinguish from one another due to their similar morphology; indeed, some species of Taxus cytogenetic data still are unclear. Oligo-FISH can rapidly and efficiently provide insight into the genetic composition and karyotype. This is important for understanding the organization and evolution of chromosomes in Taxus species. We analysed five Taxus species using two oligonucleotide probes. (AG₃T₃)₃ signals were distributed at the chromosome ends and the centromere of five species of Taxus. The 5S rDNA signal was displayed on two chromosomes of five species of Taxus. In addition to Taxus wallichiana var. mairei, 5S rDNA signals were found proximal in the remaining four species, which signals a difference in its location. The karyotype formula of Taxus wallichiana was 2n = 2x = 24m, its karyotype asymmetry index was 55.56%, and its arm ratio was 3.0087. Taxus × media's karyotype formula was 2n = 2x = 24m, its karyotype asymmetry index was 55.09%, and its arm ratio was 3.4198. The karyotype formula of Taxus yunnanensis was 2n = 2x = 24m, its karyotype asymmetry index was 55.56%, and its arm ratio was 2.6402. The karyotype formula of Taxus cuspidate was 2n = 2x = 24m, its karyotype asymmetry index was 54.67%, its arm ratio was 3.0135, and two chromosomes exhibited the 5S rDNA signal. The karyotype formula of T. wallichiana var. mairei was 2n= 2x = 22m + 2sm, its karyotype asymmetry index was 54.33%, and its arm ratio was 2.8716. Our results provide the karyotype analysis and physical genetic map of five species of Taxus, which contributes to providing molecular cytogenetics data for Taxus.

Genomic architecture of 5S rDNA cluster and its variations within and between species

BACKGROUND

Ribosomal DNAs (rDNAs) are arranged in purely tandem repeats, preventing them from being reliably assembled onto chromosomes during generation of genome assembly. The uncertainty of rDNA genomic structure presents a significant barrier for studying their function and evolution.

RESULTS

Here we generate ultra-long Oxford Nanopore Technologies (ONT) and short NGS reads to delineate the architecture and variation of the 5S rDNA cluster in the different strains of C. elegans and C. briggsae. We classify the individual rDNA's repeating units into 25 types based on the unique sequence variations in each unit of C. elegans (N2). We next perform assembly of the cluster by taking advantage of the long reads that carry these units, which led to an assembly of 5S rDNA cluster consisting of up to 167 consecutive 5S rDNA units in the N2 strain. The ordering and copy number of various rDNA units are consistent with the separation time between strains. Surprisingly, we observed a drastically reduced level of variation in the unit composition in the 5S rDNA cluster in the C. elegans CB4856 and C. briggsae AF16 strains than in the C. elegans N2 strain, suggesting that N2, a widely used reference strain, is likely to be defective in maintaining the 5S rDNA cluster stability compared with other wild isolates of C. elegans or C. briggsae.

CONCLUSIONS

The results demonstrate that Nanopore DNA sequencing reads are capable of generating assembly of highly repetitive sequences, and rDNA units are highly dynamic both within and between population(s) of the same species in terms of sequence and copy number. The detailed structure and variation of the 5S rDNA units within the rDNA cluster pave the way for functional and evolutionary studies.

Karyological and nuclear DNA content variation of the genus Asparagus

Asparagus wild relatives could be a promising possibility to extent the genetic variability of garden asparagus and for new cultivars with favorable traits such as high yield stability, disease resistance and stress tolerance. In order to achieve an efficient use in breeding, a detailed cytogenetic characterization of the accessions is necessary. This study worked on 35 Asparagus accessions, including A. officinalis cultivars (‘Darlise’, ‘Ravel’ and ‘Steiners Violetta’) and Asparagus wild relatives, for which the number of chromosomes, their size, the nuclear DNA content, and the genomic distribution of 5S and 45S rDNA were analyzed. Different ploidy levels (diploid, triploid, tetraploid, pentaploid and hexaploid) were found. Furthermore, the size of the chromosomes of all diploid Asparagus accessions was determined which led to differences in the karyotypic formula. A. plocamoides harbors the smallest chromosome with 1.21 μm, whereas the largest chromosome with 5.43 μm was found in A. officinalis. In all accessions one 5S rDNA locus per genome was observed, while the number of 45S rDNA loci varied between one (A. albus, A. plumosus, A. stipularis) to four (A. setaceus). In most Asparagus accessions, the 5S and 45S rDNA signals were located on different chromosomes. In contrast, the genomes of A. africanus, A. plocamoides, A. sp. (a taxonomically unclassified Asparagus species from Asia) and A. verticillatus (diploid accessions) have one 5S and one 45S rDNA signal on the same chromosome. The measured 2C DNA content ranges from 1.43 pg (A. plocamoides, diploid) to 8.24 pg (A. amarus, hexaploid). Intraspecific variations for chromosome number, karyotypic formula, signal pattern with 5S and 45s rDNA probes and DNA content were observed. Interspecific variations were also recognized in the genus Asparagus.

5S-IGS rDNA in wind-pollinated trees (Fagus L.) encapsulates 55 million years of reticulate evolution and hybrid origins of modern species

Standard models of plant speciation assume strictly dichotomous genealogies in which a species, the ancestor, is replaced by two offspring species. The reality in wind-pollinated trees with long evolutionary histories is more complex: species evolve from other species through isolation when genetic drift exceeds gene flow; lineage mixing can give rise to new species (hybrid taxa such as nothospecies and allopolyploids). The multi-copy, potentially multi-locus 5S rDNA is one of few gene regions conserving signal from dichotomous and reticulate evolutionary processes down to the level of intra-genomic recombination. Therefore, it can provide unique insights into the dynamic speciation processes of lineages that diversified tens of millions of years ago. Here, we provide the first high-throughput sequencing (HTS) of the 5S intergenic spacers (5S-IGS) for a lineage of wind-pollinated subtropical to temperate trees, the Fagus crenata - F. sylvatica s.l. lineage, and its distant relative F. japonica. The observed 4963 unique 5S-IGS variants reflect a complex history of hybrid origins, lineage sorting, mixing via secondary gene flow, and intra-genomic competition between two or more paralogous-homoeologous 5S rDNA lineages. We show that modern species are genetic mosaics and represent a striking case of ongoing reticulate evolution during the past 55 million years.

Oligo-FISH Can Identify Chromosomes and Distinguish Hippophaë rhamnoides L. Taxa

Oligo-fluorescence in situ hybridization (FISH) facilitates precise chromosome identification and comparative cytogenetic analysis. Detection of autosomal chromosomes of Hippophaë rhamnoides has not been achieved using oligonucleotide sequences. Here, the chromosomes of five H. rhamnoides taxa in the mitotic metaphase and mitotic metaphase to anaphase were detected using the oligo-FISH probes (AG3T3)3, 5S rDNA, and (TTG)6. In total, 24 small chromosomes were clearly observed in the mitotic metaphase (0.89–3.03 μm), whereas 24–48 small chromosomes were observed in the mitotic metaphase to anaphase (0.94–3.10 μm). The signal number and intensity of (AG3T3)3, 5S rDNA, and (TTG)6 in the mitotic metaphase to anaphase chromosomes were nearly consistent with those in the mitotic metaphase chromosomes when the two split chromosomes were integrated as one unit. Of note, 14 chromosomes (there is a high chance that sex chromosomes are included) were exclusively identified by (AG3T3)3, 5S rDNA, and (TTG)6. The other 10 also showed a terminal signal with (AG3T3)3. Moreover, these oligo-probes were able to distinguish one wild H. rhamnoides taxon from four H. rhamnoides taxa. These chromosome identification and taxa differentiation data will help in elucidating visual and elaborate physical mapping and guide breeders’ utilization of wild resources of H. rhamnoides.

Physical localization of 45S rDNA in Cymbopogon and the analysis of differential distribution of rDNA in homologous chromosomes of Cymbopogon winterianus

Cymbopogon, commonly known as lemon grass, is one of the most important aromatic grasses having therapeutic and medicinal values. FISH signals on somatic chromosome spreads off Cymbopogon species indicated the localization of 45S rDNA on the terminal region of short arms of a chromosome pair. A considerable interspecific variation in the intensity of 45S rDNA hybridization signals was observed in the cultivars of Cymbopogon winterianus and Cymbopogon flexuosus. Furthermore, in all the varieties of C. winterianus namely Bio-13, Manjari and Medini, a differential distribution of 45S rDNA was observed in a heterologous pair of chromosomes 1. The development of C. winterianus var. Manjari through gamma radiation may be responsible for breakage of fragile rDNA site from one of the chromosomes of this heterologous chromosome pair. While, in other two varieties of C. winterianus (Bio-13 and Medini), this variability may be because of evolutionary speciation due to natural cross among two species of Cymbopogon which was fixed through clonal propagation. However, in both the situations these changes were fixed by vegetative method of propagation which is general mode of reproduction in the case of C. winterianus.

Comparative cytogenetics of Serrasalmidae (Teleostei: Characiformes): The relationship between chromosomal evolution and molecular phylogenies

Serrasalmidae has high morphological and chromosomal diversity. Based on molecular hypotheses, the family is currently divided into two subfamilies, Colossomatinae and Serrasalminae, with Serrasalminae composed of two tribes: Myleini (comprising most of pacus species) and Serrasalmini (represented by Metynnis, Catoprion, and remaining piranha’s genera). This study aimed to analyze species of the tribes Myleini (Myloplus asterias, M. lobatus, M. rubripinnis, M. schomburgki, and Tometes camunani) and Serrasalmini (Metynnis cuiaba, M. hypsauchen, and M. longipinnis) using classical and molecular cytogenetic techniques in order to understand the chromosomal evolution of the family. The four species of the genus Myloplus and T. camunani presented 2n = 58 chromosomes, while the species of Metynnis presented 2n = 62 chromosomes. The distribution of heterochromatin occurred predominantly in pericentromeric regions in all species. Tometes camunani and Myloplus spp. presented only one site with 5S rDNA. Multiple markers of 18S rDNA were observed in T. camunani, M. asterias, M. lobatus, M. rubripinnis, and M. schomburgkii. For Metynnis, however, synteny of the 18S and 5S rDNA was observed in the three species, in addition to an additional 5S marker in M. longipinnis. These data, when superimposed on the phylogeny of the family, suggest a tendency to increase the diploid chromosome number from 54 to 62 chromosomes, which occurred in a nonlinear manner and is the result of several chromosomal rearrangements. In addition, the different karyotype formulas and locations of ribosomal sequences can be used as cytotaxonomic markers and assist in the identification of species.

Large-scale comparative analysis of cytogenetic markers across Lepidoptera

Fluorescence in situ hybridization (FISH) allows identification of particular chromosomes and their rearrangements. Using FISH with signal enhancement via antibody amplification and enzymatically catalysed reporter deposition, we evaluated applicability of universal cytogenetic markers, namely 18S and 5S rDNA genes, U1 and U2 snRNA genes, and histone H3 genes, in the study of the karyotype evolution in moths and butterflies. Major rDNA underwent rather erratic evolution, which does not always reflect chromosomal changes. In contrast, the hybridization pattern of histone H3 genes was well conserved, reflecting the stable organisation of lepidopteran genomes. Unlike 5S rDNA and U1 and U2 snRNA genes which we failed to detect, except for 5S rDNA in a few representatives of early diverging lepidopteran lineages. To explain the negative FISH results, we used quantitative PCR and Southern hybridization to estimate the copy number and organization of the studied genes in selected species. The results suggested that their detection was hampered by long spacers between the genes and/or their scattered distribution. Our results question homology of 5S rDNA and U1 and U2 snRNA loci in comparative studies. We recommend the use of histone H3 in studies of karyotype evolution.

Structural-profiling of low molecular weight RNAs by nanopore trapping/translocation using Mycobacterium smegmatis porin A

Folding of RNA can produce elaborate tertiary structures, corresponding to their diverse roles in the regulation of biological activities. Direct observation of RNA structures at high resolution in their native form however remains a challenge. The large vestibule and the narrow constriction of a Mycobacterium smegmatis porin A (MspA) suggests a sensing mode called nanopore trapping/translocation, which clearly distinguishes between microRNA, small interfering RNA (siRNA), transfer RNA (tRNA) and 5 S ribosomal RNA (rRNA). To further profit from the acquired event characteristics, a custom machine learning algorithm is developed. Events from measurements with a mixture of RNA analytes can be automatically classified, reporting a general accuracy of ~93.4%. tRNAs, which possess a unique tertiary structure, report a highly distinguishable sensing feature, different from all other RNA types tested in this study. With this strategy, tRNAs from different sources are measured and a high structural conservation across different species is observed in single molecule.

Connecting structure to function with the recovery of over 1000 high-quality metagenome-assembled genomes from activated sludge using long-read sequencing

Microorganisms play crucial roles in water recycling, pollution removal and resource recovery in the wastewater industry. The structure of these microbial communities is increasingly understood based on 16S rRNA amplicon sequencing data. However, such data cannot be linked to functional potential in the absence of high-quality metagenome-assembled genomes (MAGs) for nearly all species. Here, we use long-read and short-read sequencing to recover 1083 high-quality MAGs, including 57 closed circular genomes, from 23 Danish full-scale wastewater treatment plants. The MAGs account for ~30% of the community based on relative abundance, and meet the stringent MIMAG high-quality draft requirements including full-length rRNA genes. We use the information provided by these MAGs in combination with >13 years of 16S rRNA amplicon sequencing data, as well as Raman microspectroscopy and fluorescence in situ hybridisation, to uncover abundant undescribed lineages belonging to important functional groups.

Expansion segments in bacterial and archaeal 5S ribosomal RNAs

The large ribosomal RNAs of eukaryotes frequently contain expansion sequences that add to the size of the rRNAs but do not affect their overall structural layout and are compatible with major ribosomal function as an mRNA translation machine. The expansion of prokaryotic ribosomal RNAs is much less explored. In order to obtain more insight into the structural variability of these conserved molecules, we herein report the results of a comprehensive search for the expansion sequences in prokaryotic 5S rRNAs. Overall, 89 expanded 5S rRNAs of 15 structural types were identified in 15 archaeal and 36 bacterial genomes. Expansion segments ranging in length from 13 to 109 residues were found to be distributed among 17 insertion sites. The strains harboring the expanded 5S rRNAs belong to the bacterial orders Clostridiales, Halanaerobiales, Thermoanaerobacterales, and Alteromonadales as well as the archael order Halobacterales When several copies of a 5S rRNA gene are present in a genome, the expanded versions may coexist with normal 5S rRNA genes. The insertion sequences are typically capable of forming extended helices, which do not seemingly interfere with folding of the conserved core. The expanded 5S rRNAs have largely been overlooked in 5S rRNA databases.

CRISPR/Cas9-mediated genome editing in Penicillium oxalicum and Trichoderma reesei using 5S rRNA promoter-driven guide RNAs

OBJECTIVE

To construct convenient CRISPR/Cas9-mediated genome editing systems in industrial enzyme-producing fungi Penicillium oxalicum and Trichoderma reesei.

RESULTS

Employing the 5S rRNA promoter from Aspergillus niger for guide RNA expression, the β-glucosidase gene bgl2 in P. oxalicum was deleted using a donor DNA carrying 40-bp homology arms or a donor containing no selectable marker gene. Using a markerless donor DNA as editing template, precise replacement of a small region was achieved in the creA gene. In T. reesei, the A. niger 5S rRNA promoter was less efficient than that in P. oxalicum when used for gene editing. Using a native 5S rRNA promoter, stop codons were introduced into the lae1 coding region using a markerless donor DNA with an editing efficiency of 36.67%.

CONCLUSIONS

Efficient genome editing systems were developed in filamentous fungi P. oxalicum and T. reesei by using heterologous or native 5S rRNA promoters for guide RNA expression.

Quorum sensing regulates rRNA synthesis in Saccharomyces cerevisiae

Ribosome biogenesis requires the concerted activities of three nuclear RNA polymerases, (Pols) I, II, and III, to produce 25S, 18S and 5.8S ribosomal RNA (rRNA), messenger RNA (mRNA) encoding ribosomal proteins, and the 5S rRNA, respectively. The rRNA is processed and modified before being assembled with ribosomal proteins to produce a ribosome. Ribosome biogenesis requires extensive energetic investment by the cell, so it is critical that this process is tightly regulated in accord with cellular growth potential. Previous work revealed that rRNA synthesis in Saccharomyces cerevisiae is repressed prior to the cells shift from active growth (log phase) to limited/static growth (stationary phase). The mechanism(s) by which cells anticipate imminent stationary phase are unknown. In this study, we demonstrate that growing cells produce one or more compounds that accumulate in the growth medium, and that this compound induces repression of rRNA synthesis. When cells encounter this compound, rRNA synthesis is rapidly repressed. We further show that subunits of Pols I and II are degraded during the transition from log to stationary phase growth, but this degradation does not account for the observed repression of rRNA synthesis. Interestingly, repression of rRNA synthesis by spent media requires the nuclear exosome, implying that spent media stimulates rapid rRNA degradation. Together, these data suggest that yeast use quorum sensing to regulate rRNA synthesis in anticipation of high cell density in stationary phase.

Chromosome Mapping of 5S Ribosomal Genes in Indo-Pacific and Atlantic Muraenidae: Comparative Analysis by Dual Colour Fluorescence In Situ Hybridisation

The Muraenidae is one of the largest and most complex anguilliform families. Despite their abundance and important ecological roles, morays are little studied, especially cytogenetically, and both their phylogenetic relationships and the taxonomy of their genera are controversial. With the aim of extending the karyology of this fish group, the chromosomal mapping of the 5S ribosomal gene family was performed on seven species belonging to the genera Muraena and Gymnothorax from both the Atlantic and Pacific oceans. Fluorescence in situ hybridisation (FISH) experiments were realized using species-specific 5S rDNA probes; in addition, two-colour FISH was performed to investigate the possible association with the 45S ribosomal gene family. Multiple 5S rDNA clusters, located either in species-specific or in possibly homoeologous chromosomes, were found. Either a syntenic or different chromosomal location of the two ribosomal genes was detected. Our results revealed variability in the number and location of 5S rDNA clusters and confirmed a substantial conservation of the number and location of the 45S rDNA.

Cryo-electron microscopy visualization of a large insertion in the 5S ribosomal RNA of the extremely halophilic archaeon Halococcus morrhuae

The extreme halophile Halococcus morrhuae (ATCC® 17082) contains a 108-nucleotide insertion in its 5S rRNA. Large rRNA expansions in Archaea are rare. This one almost doubles the length of the 5S rRNA. In order to understand how such an insertion is accommodated in the ribosome, we obtained a cryo-electron microscopy reconstruction of the native large subunit at subnanometer resolution. The insertion site forms a four-way junction that fully preserves the canonical 5S rRNA structure. Moving away from the junction site, the inserted region is conformationally flexible and does not pack tightly against the large subunit. The high-salt requirement of the H. morrhuae ribosomes for their stability conflicted with the low-salt threshold for cryo-electron microscopy procedures. Despite this obstacle, this is the first cryo-electron microscopy map of Halococcus ribosomes.

The 5S rRNA genes in Alexandrium: their use as a FISH chromosomal marker in studies of the diversity, cell cycle and sexuality of dinoflagellates

Chromosomal markers of the diversity and evolution of dinoflagellates are scarce because the genomes of these organisms are unique among eukaryotes in terms of their base composition and chromosomal structure. Similarly, a lack of appropriate tools has hindered studies of the chromosomal localization of 5S ribosomal DNA (rDNA) in the nucleosome-less chromosomes of dinoflagellates. In this study, we isolated and cloned 5S rDNA sequences from various toxin-producing species of the genus Alexandrium and developed a fluorescence in situ hybridization (FISH) probe that allows their chromosomal localization. Our results can be summarized as follows: 1) The 5S rDNA unit is composed of a highly conserved 122-bp coding region and an intergenic spacer (IGS), the length and sequence of which are variable even within strains. 2) Three different IGS types, one containing the U6 small nuclear RNA (snRNA) gene, were found among four of the studied species (A. minutum, A. tamarense, A. catenella and A. pacificum). 3) In all strains investigated by FISH (A. minutum, A. tamarense, A. pacificum, A. catenella, A. andersonii and A. ostenfeldii), 5S rDNA gene arrays were separate from the nucleolar organizer region, which contains the genes for the large 45S pre-ribosomal RNA. 4) One to three 5S rDNA sites per haploid genome were detected, depending on the strains/species. Intraspecific variability in the number of 5S rDNA sites was determined among strains of A. minutum and A. pacificum. 5) 5S rDNA is a useful chromosomal marker of mitosis progression and can be employed to differentiate vegetative (haploid) vs. planozygotes (diploid) cells. Thus, the FISH probe (oligo-Dino5Smix5) developed in this study facilitates analyses of the diversity, cell cycle and life stages of the genus Alexandrium.

Evolutionary insights in Amazonian turtles (Testudines, Podocnemididae): co-location of 5S rDNA and U2 snRNA and wide distribution of Tc1/Mariner

Eukaryotic genomes exhibit substantial accumulation of repetitive DNA sequences. These sequences can participate in chromosomal reorganization events and undergo molecular cooption to interfere with the function and evolution of genomes. In turtles, repetitive DNA sequences appear to be accumulated at probable break points and may participate in events such as non-homologous recombination and chromosomal rearrangements. In this study, repeated sequences of 5S rDNA, U2 snRNA and Tc1/Mariner transposons were amplified from the genomes of the turtles, Podocnemis expansa and Podocnemis unifilis, and mapped by fluorescence in situ hybridization. Our data confirm the 2n=28 chromosomes for these species (the second lowest 2n in the order Testudines). We observe high conservation of the co-located 5S rDNA and U2 snRNA genes on a small chromosome pair (pair 13), and surmise that this represents the ancestral condition. Our analysis reveals a wide distribution of the Tc1/Mariner transposons and we discuss how the mobility of these transposons can act on karyotypic reorganization events (contributing to the 2n decrease of those species). Our data add new information for the order Testudines and provide important insights into the dynamics and organization of these sequences in the chelonian genomes.

Long Tandem Arrays of Cassandra Retroelements and Their Role in Genome Dynamics in Plants

Retrotransposable elements are widely distributed and diverse in eukaryotes. Their copy number increases through reverse-transcription-mediated propagation, while they can be lost through recombinational processes, generating genomic rearrangements. We previously identified extensive structurally uniform retrotransposon groups in which no member contains the gag, pol, or env internal domains. Because of the lack of protein-coding capacity, these groups are non-autonomous in replication, even if transcriptionally active. The Cassandra element belongs to the non-autonomous group called terminal-repeat retrotransposons in miniature (TRIM). It carries 5S RNA sequences with conserved RNA polymerase (pol) III promoters and terminators in its long terminal repeats (LTRs). Here, we identified multiple extended tandem arrays of Cassandra retrotransposons within different plant species, including ferns. At least 12 copies of repeated LTRs (as the tandem unit) and internal domain (as a spacer), giving a pattern that resembles the cellular 5S rRNA genes, were identified. A cytogenetic analysis revealed the specific chromosomal pattern of the Cassandra retrotransposon with prominent clustering at and around 5S rDNA loci. The secondary structure of the Cassandra retroelement RNA is predicted to form super-loops, in which the two LTRs are complementary to each other and can initiate local recombination, leading to the tandem arrays of Cassandra elements. The array structures are conserved for Cassandra retroelements of different species. We speculate that recombination events similar to those of 5S rRNA genes may explain the wide variation in Cassandra copy number. Likewise, the organization of 5S rRNA gene sequences is very variable in flowering plants; part of what is taken for 5S gene copy variation may be variation in Cassandra number. The role of the Cassandra 5S sequences remains to be established.

Chromosomal inheritance of parental rDNAs distribution pattern detected by FISH in diploid F1 hybrid progeny of Cobitis (Teleostei, Cobitidae) species has non-Mendelian character

This study was conducted to describe the major and the minor rDNA chromosome distribution in the spined loach Cobitis taenia (2n = 48) and the Danubian loach Cobitis elongatoides (2n = 50), and their laboratory-produced diploid reciprocal F₁ hybrid progeny. It was tested by fluorescence in situ hybridisation (FISH) whether the number of 28s and 5s rDNA sites in the karyotypes of diploid hybrids corresponds to the expectations resulting from Mendelian ratio and if nucleolar organiser regions (NOR)were inherited from both parents or nucleolar dominance can be observed in the induced F₁ hybrid progeny. Ten (females) or twelve (males) 28s rDNA loci were located in nine uniarm chromosomes of C. taenia. Two of such loci terminally bounded on one acrocentric chromosome were unique and indicated as specific for this species. Large 5s rDNA clusters were located on two acrocentric chromosomes. In C. elongatoides of both sexes, six NOR sites in terminal regions on six meta-submetacentric chromosomes and two 5s rDNA sites on large submetacentrics were detected. The F₁ hybrid progeny (2n = 49) was characterised by the intermediate karyotype with the sites of ribosome synthesis on chromosomes inherited from both parents without showing nucleolar dominance. 5s rDNA sites were detected on large submetacentric and two acrocentric chromosomes. The observed number of both 28s and 5s rDNAs signals in F₁ diploid Cobitis hybrids was disproportionally inherited from the two parental species, showing inconsistency with the Mendelian ratios. The presented rDNA patterns indicate some marker chromosomes that allow the species of the parental male and female to be recognised in hybrid progeny. The 5s rDNA was found to be a particularly effective diagnostic marker of C. elongatoides to partially discern genomic composition of diploid Cobitis hybrids and presumably allopolyploids resulting from their backcrossing with one of the parental species. Thus, the current study provides insight into the extent of rDNA heredity in Cobitis chromosomes and their cytotaxonomic character.

Effects of environmental pollution on the rDNAomics of Amazonian fish

Pollution is a growing environmental problem throughout the world, and the impact of human activities on biodiversity and the genetic variability of natural populations is increasingly preoccupying, given that adaptive processes depend on this variability, in particular that found in the repetitive DNA. In the present study, the mitochondrial DNA (COI) and the distribution of repetitive DNA sequences (18S and 5S rDNA) in the fish genome were analysed in fish populations inhabiting both polluted and unpolluted waters in the northern Amazon basin. The results indicate highly complex ribosomal sequences in the fish genome from the polluted environment because these sequences are involved primarily in the maintenance of genome integrity, mediated by a systematic increase in the number of copies of the ribosomal DNA in response to changes in environmental conditions.

5S rRNA Promoter for Guide RNA Expression Enabled Highly Efficient CRISPR/Cas9 Genome Editing in Aspergillus niger

The CRISPR/Cas9 system is a revolutionary genome editing tool. However, in eukaryotes, search and optimization of a suitable promoter for guide RNA expression is a significant technical challenge. Here we used the industrially important fungus, Aspergillus niger, to demonstrate that the 5S rRNA gene, which is both highly conserved and efficiently expressed in eukaryotes, can be used as a guide RNA promoter. The gene editing system was established with 100% rates of precision gene modifications among dozens of transformants using short (40-bp) homologous donor DNA. This system was also applicable for generation of designer chromosomes, as evidenced by deletion of a 48 kb gene cluster required for biosynthesis of the mycotoxin fumonisin B1. Moreover, this system also facilitated simultaneous mutagenesis of multiple genes in A. niger. We anticipate that the use of the 5S rRNA gene as guide RNA promoter can broadly be applied for engineering highly efficient eukaryotic CRISPR/Cas9 toolkits. Additionally, the system reported here will enable development of designer chromosomes in model and industrially important fungi.

[Whole Genome Sequence Determination and Analysis of Bacillus Subtilis Strain CGMCC 12426]

Objective To describe the microbiological characteristics of Bacillus subtilis(B. subtilis)CGMCC 12426 and determine and analyze its complete genome sequences.Methods B. subtilis strain CGMCC 12426 genomic DNA sequencing was performed on a single molecule real-time sequencing(SMRT)platform and the annotation was completed in the NCBI Prokaryotic Genomic Annotation Pipeline(pGAP).Results The complete genomic sequences of the released B. subtilis CGMCC 12426 consisted of a 4 138 265-bp circular chromosome and a 74 165-bp plasmid,which resulted in the prediction of 4581 genes including 4222 coding sequences,87 tRNAs,and 30 rRNAs(which included 5S rRNA,16S rRNA,and 23S rRNA).Conclusion The genome sequencing provided a basis for further investigations on the genetic background of B. subtilis and on the metabolic and regulatory mechanisms.

Roles of RAD51 and RTEL1 in telomere and rDNA stability in Physcomitrella patens

Telomeres and ribosomal RNA genes (rDNA) are essential for cell survival and particularly sensitive to factors affecting genome stability. Here, we examine the role of RAD51 and its antagonist, RTEL1, in the moss Physcomitrella patens. In corresponding mutants, we analyse their sensitivity to DNA damage, the maintenance of telomeres and rDNA, and repair of double-stranded breaks (DSBs) induced by genotoxins with various modes of action. While the loss of RTEL1 results in rapid telomere shortening, concurrent loss of both RAD51 genes has no effect on telomere lengths. We further demonstrate here the linked arrangement of 5S and 45S rRNA genes in P. patens. The spacer between 5S and 18S rRNA genes, especially the region downstream from the transcription start site, shows conspicuous clustering of sites with a high propensity to form quadruplex (G4) structures. Copy numbers of 5S and 18S rDNA are reduced moderately in the pprtel1 mutant, and significantly in the double pprad51-1-2 mutant, with no progression during subsequent cultivation. While reductions in 45S rDNA copy numbers observed in pprtel1 and pprad51-1-2 plants apply also to 5S rDNA, changes in transcript levels are different for 45S and 5S rRNA, indicating their independent transcription by RNA polymerase I and III, respectively. The loss of SOL (Sog One-Like), a transcription factor regulating numerous genes involved in DSB repair, increases the rate of DSB repair in dividing as well as differentiated tissue, and through deactivation of G2/M cell-cycle checkpoint allows the cell-cycle progression manifested as a phenotype resistant to bleomycin.

Fluorescence In Situ Hybridization (FISH) Analysis of the Locations of the Oligonucleotides 5S rDNA, (AGGGTTT)3, and (TTG)6 in Three Genera of Oleaceae and Their Phylogenetic Framework

We report the cytogenetic map for a collection of species in the Oleaceae, and test similarities among the karyotypes relative to their known species phylogeny. The oligonucleotides 5S ribosomal DNA (rDNA), (AGGGTTT)₃, and (TTG)₆ were used as fluorescence in situ hybridization (FISH) probes to locate the corresponding chromosomes in three Oleaceae genera: Fraxinus pennsylvanica, Syringa oblata, Ligustrum lucidum, and Ligustrum × vicaryi. Forty-six small chromosomes were identified in four species. (AGGGTTT)₃ signals were observed on almost all chromosome ends of four species, but (AGGGTTT)₃ played no role in distinguishing the chromosomes but displayed intact chromosomes and could thus be used as a guide for finding chromosome counts. (TTG)₆ and 5S rDNA signals discerned several chromosomes located at subterminal or central regions. Based on the similarity of the signal pattern (mainly in number and location and less in intensity) of the four species, the variations in the 5S rDNA and (TTG)₆ distribution can be ordered as L. lucidum < L. × vicaryi < F. pennsylvanica < S. oblata. Variations have observed in the three genera. The molecular cytogenetic data presented here might serve as a starting point for further larger-scale elucidation of the structure of the Oleaceae genome, and comparison with the known phylogeny of Oleaceae family.

Chromosomal Evolution in the Amolops mantzorum Species Group (Ranidae; Anura) Narrated by Repetitive DNAs

In an attempt to analyze the organization of repetitive DNAs in the amphibian genome, 7 microsatellite motifs and a 5S rDNA sequence were synthesized and mapped in the karyotypes of 5 Amolops species. The results revealed nonrandom distribution of the microsatellite repeats, usually in the heterochromatic regions, as found in other organisms. These microsatellite repeats showed rapid changes among Amolops species, documenting the recent evolutionary history within this lineage. In contrast, 5S rDNA was localized in chromosomes 5 of all species, suggesting that these chromosomes are homologous within the monophyletic clade. Furthermore, the heteromorphic X and Y sex chromosomes (chromosomes 5) of A.mantzorum, had identical patterns of 5S rDNA, indicating that the subtelocentric Y resulted from a pericentric inversion. Several microsatellite repeats were found in the heteromorphic sex chromosomes, verifying the association of repetitive DNAs with sex chromosome differentiation in A. mantzorum.

Low coverage sequencing for repetitive DNA analysis in Passiflora edulis Sims: citogenomic characterization of transposable elements and satellite DNA

BACKGROUND

The cytogenomic study of repetitive regions is fundamental for the understanding of morphofunctional mechanisms and genome evolution. Passiflora edulis a species of relevant agronomic value, this work had its genome sequenced by next generation sequencing and bioinformatics analysis performed by RepeatExplorer pipeline. The clusters allowed the identification and characterization of repetitive elements (predominant contributors to most plant genomes). The aim of this study was to identify, characterize and map the repetitive DNA of P. edulis, providing important cytogenomic markers, especially sequences associated with the centromere.

RESULTS

Three clusters of satellite DNAs (69, 118 and 207) and seven clusters of Long Terminal Repeat (LTR) retrotransposons of the superfamilies Ty1/Copy and Ty3/Gypsy and families Angela, Athila, Chromovirus and Maximus-Sire (6, 11, 36, 43, 86, 94 and 135) were characterized and analyzed. The chromosome mapping of satellite DNAs showed two hybridization sites co-located in the 5S rDNA region (PeSat_1), subterminal hybridizations (PeSat_3) and hybridization in four sites, co-located in the 45S rDNA region (PeSat_2). Most of the retroelements hybridizations showed signals scattered in the chromosomes, diverging in abundance, and only the cluster 6 presented pericentromeric regions marking. No satellite DNAs and retroelement associated with centromere was observed.

CONCLUSION

P. edulis has a highly repetitive genome, with the predominance of Ty3/Gypsy LTR retrotransposon. The satellite DNAs and LTR retrotransposon characterized are promising markers for investigation of the evolutionary patterns and genetic distinction of species and hybrids of Passiflora.

Metabolic programming a lean phenotype by deregulation of RNA polymerase III

As a master negative regulator of RNA polymerase (Pol) III, Maf1 modulates transcription in response to nutrients and stress to balance the production of highly abundant tRNAs, 5S rRNA, and other small noncoding RNAs with cell growth and maintenance. This regulation of Pol III transcription is important for energetic economy as mice lacking Maf1 are lean and resist weight gain on normal and high fat diets. The lean phenotype of Maf1 knockout (KO) mice is attributed in part to metabolic inefficiencies which increase the demand for cellular energy and elevate catabolic processes, including autophagy/lipophagy and lipolysis. A futile RNA cycle involving increased synthesis and turnover of Pol III transcripts has been proposed as an important driver of these changes. Here, using targeted metabolomics, we find changes in the liver of fed and fasted Maf1 KO mice consistent with the function of mammalian Maf1 as a chronic Pol III repressor. Differences in long-chain acylcarnitine levels suggest that energy demand is higher in the fed state of Maf1 KO mice versus the fasted state. Quantitative metabolite profiling supports increased activity in the TCA cycle, the pentose phosphate pathway, and the urea cycle and reveals changes in nucleotide levels and the creatine system. Metabolite profiling also confirms key predictions of the futile RNA cycle hypothesis by identifying changes in many metabolites involved in nucleotide synthesis and turnover. Thus, constitutively high levels of Pol III transcription in Maf1 KO mice reprogram central metabolic pathways and waste metabolic energy through a futile RNA cycle.

Differential kinetics of transcription complex assembly distinguish oocyte and somatic 5S RNA genes of Xenopus

Differential transcription of the Xenopus gene families encoding the oocyte and somatic 5S ribosomal RNAs can be reproduced in vitro with cell-free extracts prepared from Xenopus oocytes and unfertilized eggs. The transcriptional activities of these genes as assayed in these in vitro systems are a consequence of large differences in the rates of assembly of active transcription complexes. The somatic 5S genes sequester limiting transcription factors much more rapidly than the corresponding oocyte 5S genes and, as a consequence, are far more active. However, once transcription complexes are formed, these complexes are stable on both of these genes. Previous studies have established that transcription factors IIIA and IIIC are sufficient to form a stable protein-DNA complex on the somatic 5S gene. The rate of formation of the stable TFIIIA+C complex for the oocyte gene is far slower than that for the somatic 5S gene. Insertion of the DNA binding site for TFIIIC2 (the B-block promoter element from tRNA genes) into the 3' flanking region of a synthetic oocyte 5S gene increases the transcription efficiency and rate of transcription complex assembly of this gene relative to the parent gene lacking the B-block element. Our results support a model in which competition for limiting transcription factors plays a pivotal role in establishing differential transcription of the two classes of 5S genes during early embryogenesis.

Small noncoding RNAs in FSHD2 muscle cells reveal both DUX4- and SMCHD1-specific signatures

Facioscapulohumeral muscular dystrophy (FSHD) is caused by insufficient epigenetic repression of D4Z4 macrosatellite repeat where DUX4, an FSHD causing gene is embedded. There are two forms of FSHD, FSHD1 with contraction of D4Z4 repeat and FSHD2 with chromatin compaction defects mostly due to SMCHD1 mutation. Previous reports showed DUX4-induced gene expression changes as well as changes in microRNA expression in FSHD muscle cells. However, a genome wide analysis of small noncoding RNAs that might be regulated by DUX4 or by mutations in SMCHD1 has not been reported yet. Here, we identified several types of small noncoding RNAs including known microRNAs that are differentially expressed in FSHD2 muscle cells compared to control. Although fewer small RNAs were differentially expressed during muscle differentiation in FSHD2 cells compared to controls, most of the known myogenic microRNAs, such as miR1, miR133a and miR206 were induced in both FSHD2 and control muscle cells during differentiation. Our small RNA sequencing data analysis also revealed both DUX4- and SMCHD1-specific changes in FSHD2 muscle cells. Six FSHD2 microRNAs were affected by DUX4 overexpression in control myoblasts, whereas increased expression of tRNAs and 5S rRNAs in FSHD2 muscle cells was largely recapitulated in SMCHD1-depleted control myoblasts. Altogether, our studies suggest that the small noncoding RNA transcriptome changes in FSHD2 might be different from those in FSHD1 and that these differences may provide new diagnostic and therapeutic tools specific to FSHD2.

RNase III Processing of rRNA in the Lyme Disease Spirochete Borrelia burgdorferi

The rRNA genes of Borrelia (Borreliella) burgdorferi are unusually organized; the spirochete has a single 16S rRNA gene that is more than 3 kb from a tandem pair of 23S-5S rRNA operons. We generated an rnc null mutant in B. burgdorferi that exhibits a pleiotropic phenotype, including decreased growth rate and increased cell length. Here, we demonstrate that endoribonuclease III (RNase III) is, as expected, involved in processing the 23S rRNA in B. burgdorferi The 5' and 3' ends of the three rRNAs were determined in the wild type and rncBb mutants; the results suggest that RNase III in B. burgdorferi is required for the full maturation of the 23S rRNA but not for the 5S rRNA nor, curiously, for the 16S rRNA.IMPORTANCE Lyme disease, the most common tick-borne zoonosis in the Northern Hemisphere, is caused by the bacterium Borrelia (Borreliella) burgdorferi, a member of the deeply branching spirochete phylum. B. burgdorferi carries a limited suite of ribonucleases, enzymes that cleave RNA during processing and degradation. Several ribonucleases, including RNase III, are involved in the production of ribosomes, which catalyze translation and are a major target of antibiotics. This is the first study to dissect the role of an RNase in any spirochete. We demonstrate that an RNase III mutant is viable but has altered processing of rRNA.

Anti-replicative recombinant 5S rRNA molecules can modulate the mtDNA heteroplasmy in a glucose-dependent manner

Mutations in mitochondrial DNA are an important source of severe and incurable human diseases. The vast majority of these mutations are heteroplasmic, meaning that mutant and wild-type genomes are present simultaneously in the same cell. Only a very high proportion of mutant mitochondrial DNA (heteroplasmy level) leads to pathological consequences. We previously demonstrated that mitochondrial targeting of small RNAs designed to anneal with mutant mtDNA can decrease the heteroplasmy level by specific inhibition of mutant mtDNA replication, thus representing a potential therapy. We have also shown that 5S ribosomal RNA, partially imported into human mitochondria, can be used as a vector to deliver anti-replicative oligoribonucleotides into human mitochondria. So far, the efficiency of cellular expression of recombinant 5S rRNA molecules bearing therapeutic insertions remained very low. In the present study, we designed new versions of anti-replicative recombinant 5S rRNA targeting a large deletion in mitochondrial DNA which causes the KSS syndrome, analyzed their specific annealing to KSS mitochondrial DNA and demonstrated their import into mitochondria of cultured human cells. To obtain an increased level of the recombinant 5S rRNA stable expression, we created transmitochondrial cybrid cell line bearing a site for Flp-recombinase and used this system for the recombinase-mediated integration of genes coding for the anti-replicative recombinant 5S rRNAs into nuclear genome. We demonstrated that stable expression of anti-replicative 5S rRNA versions in human transmitochondrial cybrid cells can induce a shift in heteroplasmy level of KSS mutation in mtDNA. This shift was directly dependent on the level of the recombinant 5S rRNA expression and the sequence of the anti-replicative insertion. Quantification of mtDNA copy number in transfected cells revealed the absence of a non-specific effect on wild type mtDNA replication, indicating that the decreased proportion between mutant and wild type mtDNA molecules is not a consequence of a random repopulation of depleted pool of mtDNA genomes. The heteroplasmy change could be also modulated by cell growth conditions, namely increased by cells culturing in a carbohydrate-free medium, thus forcing them to use oxidative phosphorylation and providing a selective advantage for cells with improved respiration capacities. We discuss the advantages and limitations of this approach and propose further development of the anti-replicative strategy based on the RNA import into human mitochondria.

Amplification and adaptation of centromeric repeats in polyploid switchgrass species

Centromeres in most higher eukaryotes are composed of long arrays of satellite repeats from a single satellite repeat family. Why centromeres are dominated by a single satellite repeat and how the satellite repeats originate and evolve are among the most intriguing and long-standing questions in centromere biology. We identified eight satellite repeats in the centromeres of tetraploid switchgrass (Panicum virgatum). Seven repeats showed characteristics associated with classical centromeric repeats with monomeric lengths ranging from 166 to 187 bp. Interestingly, these repeats share an 80-bp DNA motif. We demonstrate that this 80-bp motif may dictate translational and rotational phasing of the centromeric repeats with the cenH3 nucleosomes. The sequence of the last centromeric repeat, Pv156, is identical to the 5S ribosomal RNA genes. We demonstrate that a 5S ribosomal RNA gene array was recruited to be the functional centromere for one of the switchgrass chromosomes. Our findings reveal that certain types of satellite repeats, which are associated with unique sequence features and are composed of monomers in mono-nucleosomal length, are favorable for centromeres. Centromeric repeats may undergo dynamic amplification and adaptation before the centromeres in the same species become dominated by the best adapted satellite repeat.

Viral unmasking of cellular 5S rRNA pseudogene transcripts induces RIG-I-mediated immunity

The sensor RIG-I detects double-stranded RNA derived from RNA viruses. Although RIG-I is also known to have a role in the antiviral response to DNA viruses, physiological RNA species recognized by RIG-I during infection with a DNA virus are largely unknown. Using next-generation RNA sequencing (RNAseq), we found that host-derived RNAs, most prominently 5S ribosomal RNA pseudogene 141 (RNA5SP141), bound to RIG-I during infection with herpes simplex virus 1 (HSV-1). Infection with HSV-1 induced relocalization of RNA5SP141 from the nucleus to the cytoplasm, and virus-induced shutoff of host protein synthesis downregulated the abundance of RNA5SP141-interacting proteins, which allowed RNA5SP141 to bind RIG-I and induce the expression of type I interferons. Silencing of RNA5SP141 strongly dampened the antiviral response to HSV-1 and the related virus Epstein-Barr virus (EBV), as well as influenza A virus (IAV). Our findings reveal that antiviral immunity can be triggered by host RNAs that are unshielded following depletion of their respective binding proteins by the virus.

Oxidative damage of 18S and 5S ribosomal RNA in digestive gland of mussels exposed to trace metals

Numerous studies have shown the ability of trace metals to accumulate in marine organisms and cause oxidative stress that leads to perturbations in many important intracellular processes, including protein synthesis. This study is mainly focused on the exploration of structural changes, like base modifications, scissions, and conformational changes, caused in 18S and 5S ribosomal RNA (rRNA) isolated from the mussel Mytilus galloprovincialis exposed to 40μg/L Cu, 30μg/L Hg, or 100μg/L Cd, for 5 or 15days. 18S rRNA and 5S rRNA are components of the small and large ribosomal subunit, respectively, found in complex with ribosomal proteins, translation factors and other auxiliary components (metal ions, toxins etc). 18S rRNA plays crucial roles in all stages of protein synthesis, while 5S rRNA serves as a master signal transducer between several functional regions of 28S rRNA. Therefore, structural changes in these ribosomal constituents could affect the basic functions of ribosomes and hence the normal metabolism of cells. Especially, 18S rRNA along with ribosomal proteins forms the decoding centre that ensures the correct codon-anticodon pairing. As exemplified by ELISA, primer extension analysis and DMS footprinting analysis, each metal caused oxidative damage to rRNA, depending on the nature of metal ion and the duration of exposure. Interestingly, exposure of mussels to Cu or Hg caused structural alterations in 5S rRNA, localized in paired regions and within loops A, B, C, and E, leading to a continuous progressive loss of the 5S RNA structural integrity. In contrast, structural impairments of 5S rRNA in mussels exposed to Cd were accumulating for the initial 5days, and then progressively decreased to almost the normal level by day 15, probably due to the parallel elevation of metallothionein content that depletes the pools of free Cd. Regions of interest in 18S rRNA, such as the decoding centre, sites implicated in the binding of tRNAs (A- and P-sites) or translation factors, and areas related to translation fidelity, were found to undergo significant metal-induced conformational alterations, leading either to loosening of their structure or to more compact folding. These modifications were associated with parallel alterations in the translation process at multiple levels, a fact suggesting that structural perturbations in ribosomes, caused by metals, pose significant hurdles in translational efficiency and fidelity.

The Maize Imprinted Gene Floury3 Encodes a PLATZ Protein Required for tRNA and 5S rRNA Transcription through Interaction with RNA Polymerase III

Maize (Zea mays) floury3 (fl3) is a classic semidominant negative mutant that exhibits severe defects in the endosperm but fl3 plants otherwise appear normal. We cloned the fl3 gene and determined that it encodes a PLATZ (plant AT-rich sequence and zinc binding) protein. The mutation in fl3 resulted in an Asn-to-His replacement in the conserved PLATZ domain, creating a dominant allele. Fl3 is specifically expressed in starchy endosperm cells and regulated by genomic imprinting, which leads to the suppressed expression of fl3 when transmitted through the male, perhaps as a consequence the semidominant behavior. Yeast two-hybrid screening and bimolecular luciferase complementation experiments revealed that FL3 interacts with the RNA polymerase III subunit 53 (RPC53) and transcription factor class C 1 (TFC1), two critical factors of the RNA polymerase III (RNAPIII) transcription complex. In the fl3 endosperm, the levels of many tRNAs and 5S rRNA that are transcribed by RNAPIII are significantly reduced, suggesting that the incorrectly folded fl3 protein may impair the function of RNAPIII. The transcriptome is dramatically altered in fl3 mutants, in which the downregulated genes are primarily enriched in pathways related to translation, ribosome, misfolded protein responses, and nutrient reservoir activity. Collectively, these changes may lead to defects in endosperm development and storage reserve filling in fl3 seeds.

Organization and evolution of four differentially amplified tandem repeats in the Cucumis hystrix genome

Three subtelomeric satellites and one interstitial 5S rDNA were characterized in Cucumis hystrix, and the pericentromeric signals of two C. hystrix subtelomeric satellites along C. sativus chromosomes supported the hypothesis of chromosome fusion in Cucumis. Tandem repeats are chromosome structural fractions consisting of highly repetitive sequences organized in large tandem arrays in most eukaryotes. Differentiation of tandem repeats directly affects the chromosome structure, which contributes to species formation and evolution. Cucumis hystrix (2n = 2x = 24) is the only wild Cucumis species grouped into the same subgenus with C. sativus (2n = 2x = 14), hence its phylogenetic position confers a vital role for C. hystrix to understand the chromosome evolution in Cucumis. However, our knowledge of C. hystrix tandem repeats is insufficient for a detailed understanding of the chromosome evolution in Cucumis. Based on de novo tandem repeat characterization using bioinformatics and in situ hybridization (ISH), we identified and characterized four differentially amplified tandem repeats, Cucumis hystrix satellite 1-3 (CuhySat1-CuhySat3) located at the subtelomeric regions of all chromosomes, and Cucumis hystrix 5S (Cuhy5S) located at the interstitial regions of one single chromosome pair. Comparative ISH mapping using CuhySat1-3 and Cuhy5S revealed high homology of tandem repeats between C. hystrix and C. sativus. Intriguingly, we found signal distribution variations of CuhySat2 and CuhySat3 on C. sativus chromosomes. In comparison to their subtelomeric signal distribution on C. hystrix chromosomes, CuhySat3 showed a pericentromeric signal distribution and CuhySat2 showed both subtelomeric and pericentromeric signal distributions on C. sativus chromosomes. This detailed characterization of four C. hystrix tandem repeats significantly widens our knowledge of the C. hystrix chromosome structure, and the observed signal distribution variations will be helpful for understanding the chromosome evolution of Cucumis.

Ribosomal DNA copy number amplification and loss in human cancers is linked to tumor genetic context, nucleolus activity, and proliferation

Ribosomal RNAs (rRNAs) are transcribed from two multicopy DNA arrays: the 5S ribosomal DNA (rDNA) array residing in a single human autosome and the 45S rDNA array residing in five human autosomes. The arrays are among the most variable segments of the genome, exhibit concerted copy number variation (cCNV), encode essential components of the ribosome, and modulate global gene expression. Here we combined whole genome data from >700 tumors and paired normal tissues to provide a portrait of rDNA variation in human tissues and cancers of diverse mutational signatures, including stomach and lung adenocarcinomas, ovarian cancers, and others of the TCGA panel. We show that cancers undergo coupled 5S rDNA array expansion and 45S rDNA loss that is accompanied by increased estimates of proliferation rate and nucleolar activity. These somatic changes in rDNA CN occur in a background of over 10-fold naturally occurring rDNA CN variation across individuals and cCNV of 5S-45S arrays in some but not all tissues. Analysis of genetic context revealed associations between cancer rDNA CN amplification or loss and the presence of specific somatic alterations, including somatic SNPs and copy number gain/losses in protein coding genes across the cancer genome. For instance, somatic inactivation of the tumor suppressor gene TP53 emerged with a strong association with coupled 5S expansion / 45S loss in several cancers. Our results uncover frequent and contrasting changes in the 5S and 45S rDNA along rapidly proliferating cell lineages with high nucleolar activity. We suggest that 5S rDNA amplification facilitates increased proliferation, nucleolar activity, and ribosomal synthesis in cancer, whereas 45S rDNA loss emerges as a byproduct of transcription-replication conflict in rapidly replicating tumor cells. The observations raise the prospects of using the rDNA arrays as re-emerging targets for the design of novel strategies in cancer therapy.

The 45S and 5S ribosomal DNA (rDNA) arrays contain hundreds of rDNA copies, with substantial variability across individuals in human populations. Although physically unlinked, the arrays also exhibit joint variation across individual genotypes. However, whether this co-variation in copy number (CN) is universally observed across all tissues is unknown. It also remains unknown if rDNA CN might vary across tissues and in cancer lineages. Here we showed that most cancers undergo coupled 5S rDNA array amplification and 45S rDNA loss, and concerted 5S-45S CN variation in some but not all tissues. The coupled 5S amplification and 45S loss is associated with the presence of certain somatic genetic alterations, as well as increased estimates of cancerous cell proliferation rate and nucleolar activity. Our research uncovers frequent and contrasting changes in rDNA CN in cancers of diverse tissue origin and associated with diverse mutational contexts of tumor suppressors and oncogenes. The observations raise the prospects of using the rDNA arrays as re-emerging targets in novel strategies for cancer therapy.

Exploiting repetitive sequences and BAC clones in Festuca pratensis karyotyping

The Festuca genus is thought to be the most numerous genus of the Poaceae family. One of the most agronomically important forage grasses, Festuca pratensis Huds. is treated as a model plant to study the molecular mechanisms associated with tolerance to winter stresses, including frost. However, the precise mapping of the genes governing stress tolerance in this species is difficult as its karyotype remains unrecognized. Only two F. pratensis chromosomes with 35S and 5S rDNA sequences can be easily identified, but its remaining chromosomes have not been distinguished to date. Here, two libraries derived from F. pratensis nuclear DNA with various contents of repetitive DNA sequences were used as sources of molecular probes for fluorescent in situ hybridisation (FISH), a BAC library and a library representing sequences most frequently present in the F. pratensis genome. Using FISH, six groups of DNA sequences were revealed in chromosomes on the basis of their signal position, including dispersed-like sequences, chromosome painting-like sequences, centromeric-like sequences, knob-like sequences, a group without hybridization signals, and single locus-like sequences. The last group was exploited to develop cytogenetic maps of diploid and tetraploid F. pratensis, which are presented here for the first time and provide a remarkable progress in karyotype characterization.

Cytomixis in tobacco microsporogenesis: are there any genome parts predisposed to migration?

Cytomixis is a poorly studied process of nuclear migration between plant cells, discovered in microsporogenesis of several hundreds of plant species. The chromosomes that migrate between tobacco microsporocytes have been for the first time identified using fluorescence in situ hybridization (FISH), and the question whether cytomixis is a random or a targeted process is answered. The distribution of four repetitive sequences used for identifying the tobacco chromosomes-NTRS, 5S rDNA, GRS, and HSR60-has been examined in the migrating chromatin, and the micronuclei formed after cytomixis. The distribution of tobacco S and T genomes has been analyzed in the cytomictic chromatin using genomic in situ hybridization (GISH). As has been shown (χ 2 test), the labeled DNA probes marking the listed sequences in tobacco genome are observed in the micronuclei formed after cytomixis with the probability not exceeding the theoretically expected value if cytomixis considered as a random process. Thus, it is shown that cytomixis is not a targeted process, and the chromosomes migrate between microsporocytes in a random manner.

TRE5-A retrotransposition profiling reveals putative RNA polymerase III transcription complex binding sites on the Dictyostelium extrachromosomal rDNA element

The amoeba Dictyostelium discoideum has a haploid genome in which two thirds of the DNA encodes proteins. Consequently, the space available for selfish mobile elements to expand without excess damage to the host genome is limited. The non-long terminal repeat retrotransposon TRE5-A maintains an active population in the D. discoideum genome and apparently adapted to this gene-dense environment by targeting positions ~47 bp upstream of tRNA genes that are devoid of protein-coding regions. Because only ~24% of tRNA genes are associated with a TRE5-A element in the reference genome, we evaluated whether TRE5-A retrotransposition is limited to this subset of tRNA genes. We determined that a tagged TRE5-A element (TRE5-A^bsr) integrated at 384 of 405 tRNA genes, suggesting that expansion of the current natural TRE5-A population is not limited by the availability of targets. We further observed that TRE5-A^bsr targets the ribosomal 5S gene on the multicopy extrachromosomal DNA element that carries the ribosomal RNA genes, indicating that TRE5-A integration may extend to the entire RNA polymerase III (Pol III) transcriptome. We determined that both natural TRE5-A and cloned TRE5-A^bsr retrotranspose to locations on the extrachromosomal rDNA element that contain tRNA gene-typical A/B box promoter motifs without displaying any other tRNA gene context. Based on previous data suggesting that TRE5-A targets tRNA genes by locating Pol III transcription complexes, we propose that A/B box loci reflect Pol III transcription complex assembly sites that possess a function in the biology of the extrachromosomal rDNA element.

Linking maternal and somatic 5S rRNA types with different sequence-specific non-LTR retrotransposons

5S rRNA is a ribosomal core component, transcribed from many gene copies organized in genomic repeats. Some eukaryotic species have two 5S rRNA types defined by their predominant expression in oogenesis or adult tissue. Our next-generation sequencing study on zebrafish egg, embryo, and adult tissue identified maternal-type 5S rRNA that is exclusively accumulated during oogenesis, replaced throughout the embryogenesis by a somatic-type, and thus virtually absent in adult somatic tissue. The maternal-type 5S rDNA contains several thousands of gene copies on chromosome 4 in tandem repeats with small intergenic regions, whereas the somatic-type is present in only 12 gene copies on chromosome 18 with large intergenic regions. The nine-nucleotide variation between the two 5S rRNA types likely affects TFIII binding and riboprotein L5 binding, probably leading to storage of maternal-type rRNA. Remarkably, these sequence differences are located exactly at the sequence-specific target site for genome integration by the 5S rRNA-specific Mutsu retrotransposon family. Thus, we could define maternal- and somatic-type MutsuDr subfamilies. Furthermore, we identified four additional maternal-type and two new somatic-type MutsuDr subfamilies, each with their own target sequence. This target-site specificity, frequently intact maternal-type retrotransposon elements, plus specific presence of Mutsu retrotransposon RNA and piRNA in egg and adult tissue, suggest an involvement of retrotransposons in achieving the differential copy number of the two types of 5S rDNA loci.

Cytogenetic features of rRNA genes across land plants: analysis of the Plant rDNA database

The online resource http://www.plantrdnadatabase.com/ stores information on the number, chromosomal locations and structure of the 5S and 18S-5.8S-26S (35S) ribosomal DNAs (rDNA) in plants. This resource was exploited to study relationships between rDNA locus number, distribution, the occurrence of linked (L-type) and separated (S-type) 5S and 35S rDNA units, chromosome number, genome size and ploidy level. The analyses presented summarise current knowledge on rDNA locus numbers and distribution in plants. We analysed 2949 karyotypes, from 1791 species and 86 plant families, and performed ancestral character state reconstructions. The ancestral karyotype (2n = 16) has two terminal 35S sites and two interstitial 5S sites, while the median (2n = 24) presents four terminal 35S sites and three interstitial 5S sites. Whilst 86.57% of karyotypes show S-type organisation (ancestral condition), the L-type arrangement has arisen independently several times during plant evolution. A non-terminal position of 35S rDNA was found in about 25% of single-locus karyotypes, suggesting that terminal locations are not essential for functionality and expression. Single-locus karyotypes are very common, even in polyploids. In this regard, polyploidy is followed by subsequent locus loss. This results in a decrease in locus number per monoploid genome, forming part of the diploidisation process returning polyploids to a diploid-like state over time.

The Evolution of Vicia ramuliflora (Fabaceae) at Tetraploid and Diploid Levels Revealed with FISH and RAPD

Vicia ramuliflora L. is a widely distributed species in Eurasia with high economic value. For past 200 years, it has evolved a tetraploid cytotype and new subspecies at the diploid level. Based on taxonomy, cytogeography and other lines of evidence, previous studies have provided valuable information about the evolution of V. ramuliflora ploidy level, but due to the limited resolution of traditional methods, important questions remain. In this study, fluorescence in situ hybridization (FISH) and random amplified polymorphic DNA (RAPD) were used to analyze the evolution of V. ramuliflora at the diploid and tetraploid levels. Our aim was to reveal the genomic constitution and parents of the tetraploid V. ramuliflora and the relationships among diploid V. ramuliflora populations. Our study showed that the tetraploid cytotype of V. ramuliflora at Changbai Mountains (M) has identical 18S and 5S rDNA distribution patterns with the diploid Hengdaohezi population (B) and the diploid Dailing population (H). However, UPGMA clustering, Neighbor-Joining clustering and principal coordinates analysis based on RAPD showed that the tetraploid cytotype (M) has more close relationships with Qianshan diploid population T. Based on our results and the fact that interspecific hybridization among Vicia species is very difficult, we think that the tetraploid V. ramuliflora is an autotetraploid and its genomic origin still needs further study. In addition, our study also found that Qianshan diploid population (T) had evolved distinct new traits compared with other diploid populations, which hints that V. ramuliflora evolved further at diploid level. We suggest that diploid population T be re-classified as a new subspecies.

Evolutional dynamics of 45S and 5S ribosomal DNA in ancient allohexaploid Atropa belladonna

BACKGROUND

Polyploid hybrids represent a rich natural resource to study molecular evolution of plant genes and genomes. Here, we applied a combination of karyological and molecular methods to investigate chromosomal structure, molecular organization and evolution of ribosomal DNA (rDNA) in nightshade, Atropa belladonna (fam. Solanaceae), one of the oldest known allohexaploids among flowering plants. Because of their abundance and specific molecular organization (evolutionarily conserved coding regions linked to variable intergenic spacers, IGS), 45S and 5S rDNA are widely used in plant taxonomic and evolutionary studies.

RESULTS

Molecular cloning and nucleotide sequencing of A. belladonna 45S rDNA repeats revealed a general structure characteristic of other Solanaceae species, and a very high sequence similarity of two length variants, with the only difference in number of short IGS subrepeats. These results combined with the detection of three pairs of 45S rDNA loci on separate chromosomes, presumably inherited from both tetraploid and diploid ancestor species, example intensive sequence homogenization that led to substitution/elimination of rDNA repeats of one parent. Chromosome silver-staining revealed that only four out of six 45S rDNA sites are frequently transcriptionally active, demonstrating nucleolar dominance. For 5S rDNA, three size variants of repeats were detected, with the major class represented by repeats containing all functional IGS elements required for transcription, the intermediate size repeats containing partially deleted IGS sequences, and the short 5S repeats containing severe defects both in the IGS and coding sequences. While shorter variants demonstrate increased rate of based substitution, probably in their transition into pseudogenes, the functional 5S rDNA variants are nearly identical at the sequence level, pointing to their origin from a single parental species. Localization of the 5S rDNA genes on two chromosome pairs further supports uniparental inheritance from the tetraploid progenitor.

CONCLUSIONS

The obtained molecular, cytogenetic and phylogenetic data demonstrate complex evolutionary dynamics of rDNA loci in allohexaploid species of Atropa belladonna. The high level of sequence unification revealed in 45S and 5S rDNA loci of this ancient hybrid species have been seemingly achieved by different molecular mechanisms.

Date Palm Sex Differentiation Based on Fluorescence In Situ Hybridization (FISH)

In situ hybridization (ISH) is used to visualize defined DNA sequences in cellular preparations by hybridization of complementary probe sequences. Recently, the fluorescence in situ hybridization (FISH) technique has become a powerful and useful tool for the direct detection of specific DNA fragments in the genome. Ribosomal DNA genes (45S and 5S rDNA) are commonly used as markers for the physical mapping of plant chromosomes to analyze genomic organization. Here we describe cytological-based markers to differentiate date palm gender through localization of 45S and 5S rDNA markers on date palm chromosomes using FISH.

Assessing the 5S ribosomal RNA heterogeneity in Arabidopsis thaliana using short RNA next generation sequencing data

In eukaryotes, ribosomal 5S rRNAs are products of multigene families organized within clusters of tandemly repeated units. Accumulation of genomic data obtained from a variety of organisms demonstrated that the potential 5S rRNA coding sequences show a large number of variants, often incompatible with folding into a correct secondary structure. Here, we present results of an analysis of a large set of short RNA sequences generated by the next generation sequencing techniques, to address the problem of heterogeneity of the 5S rRNA transcripts in Arabidopsis and identification of potentially functional rRNA-derived fragments.