Identification of novel non-coding RNAs as potential antisense regulators in the archaeon Sulfolobus solfataricus

Unity among the diverse RNA-guided CRISPR-Cas interference mechanisms

CRISPR-Cas (clustered regularly interspaced short palindromic repeats-CRISPR-associated) systems are adaptive immune systems that protect bacteria and archaea from invading mobile genetic elements (MGEs). The Cas protein-CRISPR RNA (crRNA) complex uses complementarity of the crRNA "guide" region to specifically recognize the invader genome. CRISPR effectors that perform targeted destruction of the foreign genome have emerged independently as multi-subunit protein complexes (Class 1 systems) and as single multi-domain proteins (Class 2). These different CRISPR-Cas systems can cleave RNA, DNA, and protein in an RNA-guided manner to eliminate the invader, and in some cases, they initiate programmed cell death/dormancy. The versatile mechanisms of the different CRISPR-Cas systems to target and destroy nucleic acids have been adapted to develop various programmable-RNA-guided tools and have revolutionized the development of fast, accurate, and accessible genomic applications. In this review, we present the structure and interference mechanisms of different CRISPR-Cas systems and an analysis of their unified features. The three types of Class 1 systems (I, III, and IV) have a conserved right-handed helical filamentous structure that provides a backbone for sequence-specific targeting while using unique proteins with distinct mechanisms to destroy the invader. Similarly, all three Class 2 types (II, V, and VI) have a bilobed architecture that binds the RNA-DNA/RNA hybrid and uses different nuclease domains to cleave invading MGEs. Additionally, we highlight the mechanistic similarities of CRISPR-Cas enzymes with other RNA-cleaving enzymes and briefly present the evolutionary routes of the different CRISPR-Cas systems.

Complicated target recognition by archaeal box C/D guide RNAs

Box C/D RNAs guide the site-specific formation of 2'-O-methylated nucleotides (Nm) of RNAs in eukaryotes and archaea. Although C/D RNAs have been profiled in several archaea, their targets have not been experimentally determined. Here, we mapped Nm in rRNAs, tRNAs, and abundant small RNAs (sRNAs) and profiled C/D RNAs in the crenarchaeon Sulfolobus islandicus. The targets of C/D RNAs were assigned by analysis of base-pairing interactions, in vitro modification assays, and gene deletion experiments, revealing a complicated landscape of C/D RNA-target interactions. C/D RNAs widely use dual antisense elements to target adjacent sites in rRNAs, enhancing modification at weakly bound sites. Two consecutive sites can be guided with the same antisense element upstream of box D or D', a phenomenon known as double-specificity that is exclusive to internal box D' in eukaryotic C/D RNAs. Several C/D RNAs guide modification at a single non-canonical site. This study reveals the global landscape of RNA-guided 2'-O-methylation in an archaeon and unexpected targeting rules employed by C/D RNA.

Bacterial tRNA landscape revisited

The updated Wobble Hypothesis reasonably explains why some 40 tRNA species are sufficient to decode the 61 amino acid codons of the Universal Genetic Code. However, we still have no clue why eubacteria lack tRNA isoacceptors with ANN anticodons, whereas eukaryotes universally lack eight GNN anticodons, only one of which is also absent in bacteria. Direct tRNA sequencing could resolve the patterns of nucleoside modification that had been driving the divergent evolution in prokaryotes and eukaryotes, but this task will require the development of AI-supported base-callers that can recognize modified nucleosides without any subsequent analytical verification. Our knowledge of the bacterial tRNA landscape is moreover broadened by the recent discovery of antisense tRNAs and tRNA-derived fragments that should be examined in their roles for gene expression, translation, bacterial physiology or metabolism.

CRISPRs in the human genome are differentially expressed between malignant and normal adjacent to tumor tissue

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) have been identified in bacteria, archaea and mitochondria of plants, but not in eukaryotes. Here, we report the discovery of 12,572 putative CRISPRs randomly distributed across the human chromosomes, which we termed hCRISPRs. By using available transcriptome datasets, we demonstrate that hCRISPRs are distinctively expressed as small non-coding RNAs (sncRNAs) in cell lines and human tissues. Moreover, expression patterns thereof enabled us to distinguish normal from malignant tissues. In prostate cancer, we confirmed the differential hCRISPR expression between normal adjacent and malignant primary prostate tissue by RT-qPCR and demonstrate that the SHERLOCK and DETECTR dipstick tools are suitable to detect these sncRNAs. We anticipate that the discovery of CRISPRs in the human genome can be further exploited for diagnostic purposes in cancer and other medical conditions, which certainly will lead to the development of point-of-care tests based on the differential expression of the hCRISPRs.

CRISPR elements in the human genome are expressed in both healthy tissues and tumors but with distinct patterns, representing a potential biomarker for cancer.

Direct RNA nanopore sequencing of Pseudomonas aeruginosa clone C transcriptomes

The transcriptomes of Pseudomonas aeruginosa clone C isolates NN2 and SG17M during the mid-exponential and early stationary phase of planktonic growth were evaluated by direct RNA sequencing on the nanopore platform and compared with established short-read cDNA sequencing on the Illumina platform. Fifty to ninety percent of the sense RNAs turned out to be rRNA molecules followed by similar proportions of mRNA transcripts and non-coding RNAs. Both platforms detected similar proportions of uncharged tRNAs and 29 yet undescribed antisense tRNAs. For example, the rarest arginine codon was paired with the most abundant tRNA^Arg, and the tRNA^Arg gene is missing for the most frequent arginine codon. More than 90% of the antisense RNA molecules were complementary to a coding sequence. The antisense RNAs were evenly distributed in the genomes. Direct RNA sequencing identified more than 4,000 distinct non-overlapping antisense RNAs during exponential and stationary growth. Besides highly expressed small antisense RNAs less than 200 bases in size, a population of longer antisense RNAs was sequenced that covered a broad range of a few hundred to thousands of bases and could be complementary to a contig of several genes. In summary, direct RNA sequencing identified yet undescribed RNA molecules and an unexpected composition of the pools of tRNAs, sense and antisense RNAs. IMPORTANCE Genome-wide gene expression of bacteria is commonly studied by high-throughput sequencing of size-selected cDNA fragment libraries of reverse-transcribed RNA preparations. However, the depletion of ribosomal RNAs, enzymatic reverse transcription and the fragmentation, size selection and amplification during library preparation lead to inevitable losses of information about the initial composition of the RNA pool. We demonstrate that direct RNA sequencing on the nanopore platform can overcome these limitations. Nanopore sequencing of total RNA yielded novel insights into the Pseudomonas aeruginosa transcriptome that - if replicated in other species - will change our view of the bacterial RNA world. The discovery of sense - antisense pairs of tmRNA, tRNAs and mRNAs indicates a further and unknown level of gene regulation in bacteria.

Viral recombination systems limit CRISPR-Cas targeting through the generation of escape mutations

CRISPR-Cas systems provide immunity to bacteria by programing Cas nucleases with RNA guides that recognize and cleave infecting viral genomes. Bacteria and their viruses each encode recombination systems that could repair the cleaved viral DNA. However, it is unknown whether and how these systems can affect CRISPR immunity. Bacteriophage λ uses the Red system (gam-exo-bet) to promote recombination between related phages. Here, we show that λ Red also mediates evasion of CRISPR-Cas targeting. Gam inhibits the host E. coli RecBCD recombination system, allowing recombination and repair of the cleaved DNA by phage Exo-Beta, which promotes the generation of mutations within the CRISPR target sequence. Red recombination is strikingly more efficient than the host's RecBCD-RecA in the production of large numbers of phages that escape CRISPR targeting. These results reveal a role for Red-like systems in the protection of bacteriophages against sequence-specific nucleases, which may facilitate their spread across viral genomes.

The biology of thermoacidophilic archaea from the order Sulfolobales

Thermoacidophilic archaea belonging to the order Sulfolobales thrive in extreme biotopes, such as sulfuric hot springs and ore deposits. These microorganisms have been model systems for understanding life in extreme environments, as well as for probing the evolution of both molecular genetic processes and central metabolic pathways. Thermoacidophiles, such as the Sulfolobales, use typical microbial responses to persist in hot acid (e.g. motility, stress response, biofilm formation), albeit with some unusual twists. They also exhibit unique physiological features, including iron and sulfur chemolithoautotrophy, that differentiate them from much of the microbial world. Although first discovered >50 years ago, it was not until recently that genome sequence data and facile genetic tools have been developed for species in the Sulfolobales. These advances have not only opened up ways to further probe novel features of these microbes but also paved the way for their potential biotechnological applications. Discussed here are the nuances of the thermoacidophilic lifestyle of the Sulfolobales, including their evolutionary placement, cell biology, survival strategies, genetic tools, metabolic processes and physiological attributes together with how these characteristics make thermoacidophiles ideal platforms for specialized industrial processes.

CRISPR Genome Editing Technology and its Application in Genetic Diseases: A Review

Gene therapy has been a long lasting goal for scientists, and there are many optimal methods and tools to correct disease-causing mutations in humans. Recently, the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) technology has been progressively adopted for the assessment a treatment of human diseases, including thalassemia, Parkinson's disease, cystic fibrosis, glaucoma, Huntington's disease, and Human Immunodeficiency Virus/Acquired Immunodeficiency Syndrome (HIV/AIDS). CRISPR sequences belong to the bacterial immune system, which includes the nuclease Cas enzyme and an RNA sequence. The RNA sequence is unique and pathogen-specific, and identifies and binds to the DNA of invasive viruses, allowing the nuclease Cas enzyme to cut the identified DNA and destroy the invasive viruses. This feature provides the possibility to edit mutations in the DNA sequence of live cells by replacing a specific targeted RNA sequence with the RNA sequence in the CRISPR system. Previous studies have reported the improvement steps in confrontation with human diseases caused by single-nucleotide mutations using this system. In this review, we first introduce CRISPR and its functions and then elaborate on the use of CRISPR in the treatment of human diseases.

Type III-A CRISPR immunity promotes mutagenesis of staphylococci

Horizontal gene transfer and mutation are the two major drivers of microbial evolution that enable bacteria to adapt to fluctuating environmental stressors¹. Clustered, regularly interspaced, short palindromic repeats (CRISPR) systems use RNA-guided nucleases to direct sequence-specific destruction of the genomes of mobile genetic elements that mediate horizontal gene transfer, such as conjugative plasmids² and bacteriophages³, thus limiting the extent to which bacteria can evolve by this mechanism. A subset of CRISPR systems also exhibit non-specific degradation of DNA^4,5; however, whether and how this feature affects the host has not yet been examined. Here we show that the non-specific DNase activity of the staphylococcal type III-A CRISPR-Cas system increases mutations in the host and accelerates the generation of antibiotic resistance in Staphylococcus aureus and Staphylococcus epidermidis. These mutations require the induction of the SOS response to DNA damage and display a distinct pattern. Our results demonstrate that by differentially affecting both mechanisms that generate genetic diversity, type III-A CRISPR systems can modulate the evolution of the bacterial host.

History, evolution and classification of CRISPR-Cas associated systems

This chapter provides a detailed description of the history of CRISPR-Cas and its evolution into one of the most efficient genome-editing strategies. The chapter begins by providing information on early findings that were critical in deciphering the role of CRISPR-Cas associated systems in prokaryotes. It then describes how CRISPR-Cas had been evolved into an efficient genome-editing strategy. In the subsequent section, latest developments in the genome-editing approaches based on CRISPR-Cas are discussed. The chapter ends with the recent classification and possible evolution of CRISPR-Cas systems.

Post-Transcriptional Modifications of Conserved Nucleotides in the T-Loop of tRNA: A Tale of Functional Convergent Evolution

The high conservation of nucleotides of the T-loop, including their chemical identity, are hallmarks of tRNAs from organisms belonging to the three Domains of Life. These structural characteristics allow the T-loop to adopt a peculiar intraloop conformation able to interact specifically with other conserved residues of the D-loop, which ultimately folds the mature tRNA in a unique functional canonical L-shaped architecture. Paradoxically, despite the high conservation of modified nucleotides in the T-loop, enzymes catalyzing their formation depend mostly on the considered organism, attesting for an independent but convergent evolution of the post-transcriptional modification processes. The driving force behind this is the preservation of a native conformation of the tRNA elbow that underlies the various interactions of tRNA molecules with different cellular components.

Comparative patterns of modified nucleotides in individual tRNA species from a mesophilic and two thermophilic archaea

To improve and complete our knowledge of archaeal tRNA modification patterns, we have identified and compared the modification pattern (type and location) in tRNAs of three very different archaeal species, Methanococcus maripaludis (a mesophilic methanogen), Pyrococcus furiosus (a hyperthermophile thermococcale), and Sulfolobus acidocaldarius (an acidophilic thermophilic sulfolobale). Most abundant isoacceptor tRNAs (79 in total) for each of the 20 amino acids were isolated by two-dimensional gel electrophoresis followed by in-gel RNase digestions. The resulting oligonucleotide fragments were separated by nanoLC and their nucleotide content analyzed by mass spectrometry (MS/MS). Analysis of total modified nucleosides obtained from complete digestion of bulk tRNAs was also performed. Distinct base- and/or ribose-methylations, cytidine acetylations, and thiolated pyrimidines were identified, some at new positions in tRNAs. Novel, some tentatively identified, modifications were also found. The least diversified modification landscape is observed in the mesophilic Methanococcus maripaludis and the most complex one in Sulfolobus acidocaldarius Notable observations are the frequent occurrence of ac⁴C nucleotides in thermophilic archaeal tRNAs, the presence of m⁷G at positions 1 and 10 in Pyrococcus furiosus tRNAs, and the use of wyosine derivatives at position 37 of tRNAs, especially those decoding U1- and C1-starting codons. These results complete those already obtained by others with sets of archaeal tRNAs from Methanocaldococcus jannaschii and Haloferax volcanii.

iCLIP analysis of RNA substrates of the archaeal exosome

Background

The archaeal exosome is an exoribonucleolytic multiprotein complex, which degrades single-stranded RNA in 3′ to 5′ direction phosphorolytically. In a reverse reaction, it can add A-rich tails to the 3′-end of RNA. The catalytic center of the exosome is in the aRrp41 subunit of its hexameric core. Its RNA-binding subunits aRrp4 and aDnaG confer poly(A) preference to the complex. The archaeal exosome was intensely characterized in vitro, but still little is known about its interaction with natural substrates in the cell, particularly because analysis of the transcriptome-wide interaction of an exoribonuclease with RNA is challenging.

Results

To determine binding sites of the exosome to RNA on a global scale, we performed individual-nucleotide resolution UV crosslinking and immunoprecipitation (iCLIP) analysis with antibodies directed against aRrp4 and aRrp41 of the chrenarchaeon Sulfolobus solfataricus. A relatively high proportion (17–19%) of the obtained cDNA reads could not be mapped to the genome. Instead, they corresponded to adenine-rich RNA tails, which are post-transcriptionally synthesized by the exosome, and to circular RNAs (circRNAs). We identified novel circRNAs corresponding to 5′ parts of two homologous, transposase-related mRNAs. To detect preferred substrates of the exosome, the iCLIP reads were compared to the transcript abundance using RNA-Seq data. Among the strongly enriched exosome substrates were RNAs antisense to tRNAs, overlapping 3′-UTRs and RNAs containing poly(A) stretches. The majority of the read counts and crosslink sites mapped in mRNAs. Furthermore, unexpected crosslink sites clustering at 5′-ends of RNAs was detected.

Conclusions

In this study, RNA targets of an exoribonuclease were analyzed by iCLIP. The data documents the role of the archaeal exosome as an exoribonuclease and RNA-tailing enzyme interacting with all RNA classes, and underlines its role in mRNA turnover, which is important for adaptation of prokaryotic cells to changing environmental conditions. The clustering of crosslink sites near 5′-ends of genes suggests simultaneous binding of both RNA ends by the S. solfataricus exosome. This may serve to prevent translation of mRNAs dedicated to degradation in 3′-5′ direction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07200-x.

Heavily Armed Ancestors: CRISPR Immunity and Applications in Archaea with a Comparative Analysis of CRISPR Types in Sulfolobales

Prokaryotes are constantly coping with attacks by viruses in their natural environments and therefore have evolved an impressive array of defense systems. Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) is an adaptive immune system found in the majority of archaea and about half of bacteria which stores pieces of infecting viral DNA as spacers in genomic CRISPR arrays to reuse them for specific virus destruction upon a second wave of infection. In detail, small CRISPR RNAs (crRNAs) are transcribed from CRISPR arrays and incorporated into type-specific CRISPR effector complexes which further degrade foreign nucleic acids complementary to the crRNA. This review gives an overview of CRISPR immunity to newcomers in the field and an update on CRISPR literature in archaea by comparing the functional mechanisms and abundances of the diverse CRISPR types. A bigger fraction is dedicated to the versatile and prevalent CRISPR type III systems, as tremendous progress has been made recently using archaeal models in discerning the controlled molecular mechanisms of their unique tripartite mode of action including RNA interference, DNA interference and the unique cyclic-oligoadenylate signaling that induces promiscuous RNA shredding by CARF-domain ribonucleases. The second half of the review spotlights CRISPR in archaea outlining seminal in vivo and in vitro studies in model organisms of the euryarchaeal and crenarchaeal phyla, including the application of CRISPR-Cas for genome editing and gene silencing. In the last section, a special focus is laid on members of the crenarchaeal hyperthermophilic order Sulfolobales by presenting a thorough comparative analysis about the distribution and abundance of CRISPR-Cas systems, including arrays and spacers as well as CRISPR-accessory proteins in all 53 genomes available to date. Interestingly, we find that CRISPR type III and the DNA-degrading CRISPR type I complexes co-exist in more than two thirds of these genomes. Furthermore, we identified ring nuclease candidates in all but two genomes and found that they generally co-exist with the above-mentioned CARF domain ribonucleases Csx1/Csm6. These observations, together with published literature allowed us to draft a working model of how CRISPR-Cas systems and accessory proteins cross talk to establish native CRISPR anti-virus immunity in a Sulfolobales cell.

The CARF Protein MM_0565 Affects Transcription of the Casposon-Encoded cas1-solo Gene in Methanosarcina mazei Gö1

Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) loci are found in bacterial and archaeal genomes where they provide the molecular machinery for acquisition of immunity against foreign DNA. In addition to the cas genes fundamentally required for CRISPR activity, a second class of genes is associated with the CRISPR loci, of which many have no reported function in CRISPR-mediated immunity. Here, we characterize MM_0565 associated to the type I-B CRISPR-locus of Methanosarcina mazei Gö1. We show that purified MM_0565 composed of a CRISPR-Cas Associated Rossmann Fold (CARF) and a winged helix-turn-helix domain forms a dimer in solution; in vivo, the dimeric MM_0565 is strongly stabilized under high salt stress. While direct effects on CRISPR-Cas transcription were not detected by genetic approaches, specific binding of MM_0565 to the leader region of both CRISPR-Cas systems was observed by microscale thermophoresis and electromobility shift assays. Moreover, overexpression of MM_0565 strongly induced transcription of the cas1-solo gene located in the recently reported casposon, the gene product of which shows high similarity to classical Cas1 proteins. Based on our findings, and taking the absence of the expressed CRISPR locus-encoded Cas1 protein into account, we hypothesize that MM_0565 might modulate the activity of the CRISPR systems on different levels.

Detection of CRISPR adaptation

Prokaryotic adaptive immunity is built when short DNA fragments called spacers are acquired into CRISPR (clustered regularly interspaced short palindromic repeats) arrays. CRISPR adaptation is a multistep process which comprises selection, generation, and incorporation of prespacers into arrays. Once adapted, spacers provide immunity through the recognition of complementary nucleic acid sequences, channeling them for destruction. To prevent deleterious autoimmunity, CRISPR adaptation must therefore be a highly regulated and infrequent process, at least in the absence of genetic invaders. Over the years, ingenious methods to study CRISPR adaptation have been developed. In this paper, we discuss and compare methods that detect CRISPR adaptation and its intermediates in vivo and propose suppressing PCR as a simple modification of a popular assay to monitor spacer acquisition with increased sensitivity.

Transcriptional profiling of long noncoding RNAs associated with leaf-color mutation in Ginkgo biloba L

BACKGROUND

Long noncoding RNAs (lncRNAs) play an important role in diverse biological processes and have been widely studied in recent years. However, the roles of lncRNAs in leaf pigment formation in ginkgo (Ginkgo biloba L.) remain poorly understood.

RESULTS

In this study, lncRNA libraries for mutant yellow-leaf and normal green-leaf ginkgo trees were constructed via high-throughput sequencing. A total of 2044 lncRNAs were obtained with an average length of 702 nt and typically harbored 2 exons. We identified 238 differentially expressed lncRNAs (DELs), 32 DELs and 49 differentially expressed mRNAs (DEGs) that constituted coexpression networks. We also found that 48 cis-acting DELs regulated 72 target genes, and 31 trans-acting DELs regulated 31 different target genes, which provides a new perspective for the regulation of the leaf-color mutation. Due to the crucial regulatory roles of lncRNAs in a wide range of biological processes, we conducted in-depth studies on the DELs and their targets and found that the chloroplast thylakoid membrane subcategory and the photosynthesis pathways (ko00195) were most enriched, suggesting their potential roles in leaf coloration mechanisms. In addition, our correlation analysis indicates that eight DELs and 68 transcription factors (TFs) might be involved in interaction networks.

CONCLUSIONS

This study has enriched the knowledge concerning lncRNAs and provides new insights into the function of lncRNAs in leaf-color mutations, which will benefit future selective breeding of ginkgo.

sRNA OsiA Stabilizes Catalase mRNA during Oxidative Stress Response of Deincoccus radiodurans R1

Deinococcus radiodurans adapts to challenging environments by modulating gene expression in response to oxidative stress. Recently, bacterial small noncoding RNAs (sRNAs) have been presumed to participate in the transcriptional or translational regulation of stress-responsive genes. We found 24 sRNAs that may be involved in the oxidative stress response of D. radiodurans by deep RNA sequencing. Moreover, a typical stress-inducible sRNA, IGR_3053, named OsiA, was predicted to bind to the mRNA of katA, katE, and sodC by the bioinformatics method. An osiA knockout of D. radiodurans displayed increased sensitivity to H₂O₂ and the decreased catalase activity and total antioxidant activity, suggesting that OsiA probably serves as a regulator in the adaptation to oxidative environments. Further microscale thermophoresis results demonstrated that OsiA can directly bind to the mRNA of katA, sodC, and katE. The stability test result of katA mRNA showed that its half-life was 2 min in the osiA mutant compared with 5 min in the wildtype(wt) strain. Our results indicated that OsiA can enhance the stability of katA mRNA and the activity of KatA and consequently the oxidation resistance of D.radiodurans. We are the first one to explore the super-strong oxidative stress resistance of D.radiodurans at the level of post-transcriptional regulation, and found a new pathway that provides a new explanation for the long-term adaptability of D.radiodurans in extreme environments.

A regulatory RNA is involved in RNA duplex formation and biofilm regulation in Sulfolobus acidocaldarius

Non-coding RNAs (ncRNA) are involved in essential biological processes in all three domains of life. The regulatory potential of ncRNAs in Archaea is, however, not fully explored. In this study, RNA-seq analyses identified a set of 29 ncRNA transcripts in the hyperthermophilic archaeon Sulfolobus acidocaldarius that were differentially expressed in response to biofilm formation. The most abundant ncRNA of this set was found to be resistant to RNase R treatment (RNase R resistant RNA, RrrR(+)) due to duplex formation with a reverse complementary RNA (RrrR(−)). The deletion of the RrrR(+) gene resulted in significantly impaired biofilm formation, while its overproduction increased biofilm yield. RrrR(+) was found to act as an antisense RNA against the mRNA of a hypothetical membrane protein. The RrrR(+) transcript was shown to be stabilized by the presence of the RrrR(−) strand in S. acidocaldarius cell extracts. The accumulation of these RrrR duplexes correlates with an apparent absence of dsRNA degrading RNase III domains in archaeal proteins.

Next Generation DNA-Seq and Differential RNA-Seq Allow Re-annotation of the Pyrococcus furiosus DSM 3638 Genome and Provide Insights Into Archaeal Antisense Transcription

Pyrococcus furiosus DSM 3638 is a model organism for hyperthermophilic archaea with an optimal growth temperature near 100°C. The genome was sequenced about 18 years ago. However, some publications suggest that in contrast to other Pyrococcus species, the genome of P. furiosus DSM 3638 is prone to genomic rearrangements. Therefore, we re-sequenced the genome using third generation sequencing techniques. The new de novo assembled genome is 1,889,914 bp in size and exhibits high sequence identity to the published sequence. However, two major deviations were detected: (1) The genome is 18,342 bp smaller than the NCBI reference genome due to a recently described deletion. (2) The region between PF0349 and PF0388 is inverted most likely due an assembly problem for the original sequence. In addition, numerous minor variations, ranging from single nucleotide exchanges, deletions or insertions were identified. The total number of insertion sequence (IS) elements is also reduced from 30 to 24 in the new sequence. Re-sequencing of a 2-year-old “lab culture” using Nanopore sequencing confirmed the overall stability of the P. furiosus DSM 3638 genome even under normal lab conditions without taking any special care. To improve genome annotation, the updated DNA sequence was combined with an RNA sequencing approach. Here, RNAs from eight different growth conditions were pooled to increase the number of detected transcripts. Furthermore, a differential RNA-Seq approach was employed for the identification of transcription start sites (TSSs). In total, 2515 TSSs were detected and classified into 834 primary (pTSS), 797 antisense (aTSS), 739 internal and 145 secondary TSSs. Our analysis of the upstream regions revealed a well conserved archaeal promoter structure. Interrogation of the distances between pTSSs and aTSSs revealed a significant number of antisense transcripts, which are a result of bidirectional transcription from the same TATA box. This mechanism of antisense transcript production could be further confirmed by in vitro transcription experiments. We assume that bidirectional transcription gives rise to non-functional antisense RNAs and that this is a widespread phenomenon in archaea due to the architecture of the TATA element and the symmetric structure of the TATA-binding protein.

Characterization of the transcriptome of Haloferax volcanii, grown under four different conditions, with mixed RNA-Seq

Haloferax volcanii is a well-established model species for haloarchaea. Small scale RNomics and bioinformatics predictions were used to identify small non-coding RNAs (sRNAs), and deletion mutants revealed that sRNAs have important regulatory functions. A recent dRNA-Seq study was used to characterize the primary transcriptome. Unexpectedly, it was revealed that, under optimal conditions, H. volcanii contains more non-coding sRNAs than protein-encoding mRNAs. However, the dRNA-Seq approach did not contain any length information. Therefore, a mixed RNA-Seq approach was used to determine transcript length and to identify additional transcripts, which are not present under optimal conditions. In total, 50 million paired end reads of 150 nt length were obtained. 1861 protein-coding RNAs (cdRNAs) were detected, which encoded 3092 proteins. This nearly doubled the coverage of cdRNAs, compared to the previous dRNA-Seq study. About 2/3 of the cdRNAs were monocistronic, and 1/3 covered more than one gene. In addition, 1635 non-coding sRNAs were identified. The highest fraction of non-coding RNAs were cis antisense RNAs (asRNAs). Analysis of the length distribution revealed that sRNAs have a median length of about 150 nt. Based on the RNA-Seq and dRNA-Seq results, genes were chosen to exemplify characteristics of the H. volcanii transcriptome by Northern blot analyses, e.g. 1) the transcript patterns of gene clusters can be straightforward, but also very complex, 2) many transcripts differ in expression level under the four analyzed conditions, 3) some genes are transcribed into RNA isoforms of different length, which can be differentially regulated, 4) transcripts with very long 5'-UTRs and with very long 3'-UTRs exist, and 5) about 30% of all cdRNAs have overlapping 3'-ends, which indicates, together with the asRNAs, that H. volcanii makes ample use of sense-antisense interactions. Taken together, this RNA-Seq study, together with a previous dRNA-Seq study, enabled an unprecedented view on the H. volcanii transcriptome.

The Primary Antisense Transcriptome of Halobacterium salinarum NRC-1

Antisense RNAs (asRNAs) are present in diverse organisms and play important roles in gene regulation. In this work, we mapped the primary antisense transcriptome in the halophilic archaeon Halobacterium salinarum NRC-1. By reanalyzing publicly available data, we mapped antisense transcription start sites (aTSSs) and inferred the probable 3′ ends of these transcripts. We analyzed the resulting asRNAs according to the size, location, function of genes on the opposite strand, expression levels and conservation. We show that at least 21% of the genes contain asRNAs in H. salinarum. Most of these asRNAs are expressed at low levels. They are located antisense to genes related to distinctive characteristics of H. salinarum, such as bacteriorhodopsin, gas vesicles, transposases and other important biological processes such as translation. We provide evidence to support asRNAs in type II toxin–antitoxin systems in archaea. We also analyzed public Ribosome profiling (Ribo-seq) data and found that ~10% of the asRNAs are ribosome-associated non-coding RNAs (rancRNAs), with asRNAs from transposases overrepresented. Using a comparative transcriptomics approach, we found that ~19% of the asRNAs annotated in H. salinarum belong to genes with an ortholog in Haloferax volcanii, in which an aTSS could be identified with positional equivalence. This shows that most asRNAs are not conserved between these halophilic archaea.

Insights into RNA-processing pathways and associated RNA-degrading enzymes in Archaea

RNA-processing pathways are at the centre of regulation of gene expression. All RNA transcripts undergo multiple maturation steps in addition to covalent chemical modifications to become functional in the cell. This includes destroying unnecessary or defective cellular RNAs. In Archaea, information on mechanisms by which RNA species reach their mature forms and associated RNA-modifying enzymes are still fragmentary. To date, most archaeal actors and pathways have been proposed in light of information gathered from Bacteria and Eukarya. In this context, this review provides a state of the art overview of archaeal endoribonucleases and exoribonucleases that cleave and trim RNA species and also of the key small archaeal proteins that bind RNAs. Furthermore, synthetic up-to-date views of processing and biogenesis pathways of archaeal transfer and ribosomal RNAs as well as of maturation of stable small non-coding RNAs such as CRISPR RNAs, small C/D and H/ACA box guide RNAs, and other emerging classes of small RNAs are described. Finally, prospective post-transcriptional mechanisms to control archaeal messenger RNA quality and quantity are discussed.

Nonmutational mechanism of inheritance in the Archaeon Sulfolobus solfataricus

Epigenetic phenomena have not yet been reported in archaea, which are presumed to use a classical genetic process of heritability. Here, analysis of independent lineages of Sulfolobus solfataricus evolved for enhanced fitness implicated a non-Mendelian basis for trait inheritance. The evolved strains, called super acid-resistant Crenarchaeota (SARC), acquired traits of extreme acid resistance and genome stability relative to their wild-type parental lines. Acid resistance was heritable because it was retained regardless of extensive passage without selection. Despite the hereditary pattern, in one strain, it was impossible for these SARC traits to result from mutation because its resequenced genome had no mutation. All strains also had conserved, heritable transcriptomes implicated in acid resistance. In addition, they had improved genome stability with absent or greatly decreased mutation and transposition relative to a passaged control. A mechanism that would confer these traits without DNA sequence alteration could involve posttranslationally modified archaeal chromatin proteins. To test this idea, homologous recombination with isogenic DNA was used to perturb native chromatin structure. Recombination at up-regulated loci from the heritable SARC transcriptome reduced acid resistance and gene expression in the majority of recombinants. In contrast, recombination at a control locus that was not part of the heritable transcriptome changed neither acid resistance nor gene expression. Variation in the amount of phenotypic and expression changes across individuals was consistent with Rad54-dependent chromatin remodeling that dictated crossover location and branch migration. These data support an epigenetic model implicating chromatin structure as a contributor to heritable traits.

Small RNAs of Haloferax mediterranei: Identification and Potential Involvement in Nitrogen Metabolism

Small RNAs have been studied in detail in domains Bacteria and Eukarya but, in the case of the domain Archaea, the knowledge is scarce and the physiological function of these small RNAs (sRNAs) is still uncertain. To extend the knowledge of sRNAs in the domain Archaea and their possible role in the regulation of the nitrogen assimilation metabolism in haloarchaea, Haloferax mediterranei has been used as a model microorganism. The bioinformatic approach has allowed for the prediction of 295 putative sRNAs genes in the genome of H. mediterranei, 88 of which have been verified by means of RNA-Sequencing (RNA-Seq). The secondary structure of these sRNAs and their possible targets have been identified. Curiously, some of them present as possible target genes relating to nitrogen assimilation, such as glutamate dehydrogenase and the nitrogen regulatory PII protein. Analysis of RNA-Seq data has also revealed differences in the expression pattern of 16 sRNAs according to the nitrogen source. Consequently, RNomic and bioinformatic approaches used in this work have allowed for the identification of new sRNAs in H. mediterranei, some of which show different expression patterns depending on the nitrogen source. This suggests that these sRNAs could be involved in the regulation of nitrogen assimilation and can constitute an important gene regulatory network.

sRNA41 affects ribosome binding sites within polycistronic mRNAs in Methanosarcina mazei Gö1

Several noncoding RNAs potentially involved in nitrogen (N)-regulation have been detected in Methanosarcina mazei, however, targets have been identified only for one of them. Here, we report on the function of sRNA₄₁ , highly expressed under N-sufficiency. Comprising 120 nucleotides, sRNA₄₁ shows high sequence and structural conservation within draft genomes of numerous Methanosarcina species. In silico target prediction revealed several potential targets, including genes of two homologous operons encoding for acetyl-CoA-decarbonylase/synthase complexes (ACDS) representing highly probable target candidates. A highly conserved single stranded region of sRNA₄₁ was predicted to mask six independent ribosome binding sites of these two polycistronic mRNAs and was verified in vitro by microscale thermophoresis. Proteome analysis of the respective sRNA₄₁ -deletion mutant showed increased protein expression of both ACDS complexes in the absence of sRNA₄₁ , whereas no effect on transcript levels was detected, arguing for sRNA₄₁ -mediated post-transcriptional fine-tuning of ACDS expression. We hypothesize that the physiological advantage of downregulating sRNA₄₁ under N-limiting conditions is the resulting increase of ACDS protein levels. This provides sufficient amounts of amino acids for nitrogenase synthesis as well as reducing equivalents and energy for N₂ -fixation, thus linking the carbon and N-metabolism.

CRISPR History: Discovery, Characterization, and Prosperity

CRISPR research is a very young research field since it was only 10years ago when the system was found to confer antiviral defense. Nevertheless, there has been an explosion of publications in CRISPR research in the past 5years. The research was started with the comparative genomics of microbial genomes early this century, which revealed the prevalence of clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) in bacteria and archaea. Series of hypotheses were made based on bioinformatics analyses and tested experimentally within a few years after the CRISPR acronym was coined. These findings have not only led to the discovery of the unique antiviral system and the involved molecular mechanisms, but also to the development of CRISPR technology with various well-developed applications, such as genome editing in all three domains of life. Currently, widespread research efforts in multiple research disciplines have constantly yielded new insights into molecular mechanisms of CRISPR antiviral immunity, and new applications in scientific research and biomedical applications. Retrospectively, it is worth pointing out that close interdisciplinary interactions have fostered series of discoveries in the CRISPR research and worked as the driving force in the fast developing research field.

RNA sequencing uncovers antisense RNAs and novel small RNAs in Streptococcus pyogenes

Streptococcus pyogenes is a human pathogen responsible for a wide spectrum of diseases ranging from mild to life-threatening infections. During the infectious process, the temporal and spatial expression of pathogenicity factors is tightly controlled by a complex network of protein and RNA regulators acting in response to various environmental signals. Here, we focus on the class of small RNA regulators (sRNAs) and present the first complete analysis of sRNA sequencing data in S. pyogenes. In the SF370 clinical isolate (M1 serotype), we identified 197 and 428 putative regulatory RNAs by visual inspection and bioinformatics screening of the sequencing data, respectively. Only 35 from the 197 candidates identified by visual screening were assigned a predicted function (T-boxes, ribosomal protein leaders, characterized riboswitches or sRNAs), indicating how little is known about sRNA regulation in S. pyogenes. By comparing our list of predicted sRNAs with previous S. pyogenes sRNA screens using bioinformatics or microarrays, 92 novel sRNAs were revealed, including antisense RNAs that are for the first time shown to be expressed in this pathogen. We experimentally validated the expression of 30 novel sRNAs and antisense RNAs. We show that the expression profile of 9 sRNAs including 2 predicted regulatory elements is affected by the endoribonucleases RNase III and/or RNase Y, highlighting the critical role of these enzymes in sRNA regulation.

ArnS, a kinase involved in starvation-induced archaellum expression

Organisms have evolved motility organelles that allow them to move to favourable habitats. Cells integrate environmental stimuli into intracellular signals to motility machineries to direct this migration. Many motility organelles are complex surface appendages that have evolved a tight, hierarchical regulation of expression. In the crenearchaeon Sulfolobus acidocaldarius, biosynthesis of the archaellum is regulated by regulatory network proteins that control expression of archaellum components in a phosphorylation-dependent manner. A major trigger for archaellum expression is nutrient starvation, but although some components are known, the regulatory cascade triggered by starvation is poorly understood. In this work, the starvation-induced Ser/Thr protein kinase ArnS (Saci_1181) which is located proximally to the archaellum operon was identified. Deletion of arnS results in reduced motility, though the archaellum is properly assembled. Therefore, our experimental and modelling results indicate that ArnS plays an essential role in the precisely controlled expression of archaellum components during starvation-induced motility in Sulfolobus acidocaldarius. Furthermore they combined in vivo experiments and mathematical models to describe for the first time in archaea the dynamics of key regulators of archaellum expression.

Major and minor crRNA annealing sites facilitate low stringency DNA protospacer binding prior to Type I-A CRISPR-Cas interference in Sulfolobus

The stringency of crRNA-protospacer DNA base pair matching required for effective CRISPR-Cas interference is relatively low in crenarchaeal Sulfolobus species in contrast to that required in some bacteria. To understand its biological significance we studied crRNA-protospacer interactions in Sulfolobus islandicus REY15A which carries multiple, and functionally diverse, interference complexes. A range of mismatches were introduced into a vector-borne protospacer that was identical to spacer 1 of CRISPR locus 2, with a cognate CCN PAM sequence. Two important crRNA annealing regions were identified on the 39 bp protospacer, a strong primary site centered on nucleotides 3 - 7 and a weaker secondary site at nucleotides 21 - 25. Multiple mismatches introduced into remaining protospacer regions did not seriously impair interference. Extending the study to different protospacers demonstrated that the efficacy of the secondary site was greatest for protospacers with higher G+C contents. In addition, the interference effects were assigned specifically to the type I-A dsDNA-targeting module by repeating the experiments with mutated protospacer constructs that were transformed into an S. islandicus mutant lacking type III-Bα and III-Bβ interference gene cassettes, which showed similar interference levels to those of the wild-type strain. Parallels are drawn to the involvement of 2 annealing sites for microRNAs on some eukaryal mRNAs which provide enhanced binding capacity and specificity. A biological rationale for the relatively low crRNA-protospacer base pairing stringency among the Sulfolobales is considered.

CRISPR technologies for bacterial systems: Current achievements and future directions

Throughout the decades of its history, the advances in bacteria-based bio-industries have coincided with great leaps in strain engineering technologies. Recently unveiled clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated proteins (Cas) systems are now revolutionizing biotechnology as well as biology. Diverse technologies have been derived from CRISPR/Cas systems in bacteria, yet the applications unfortunately have not been actively employed in bacteria as extensively as in eukaryotic organisms. A recent trend of engineering less explored strains in industrial microbiology-metabolic engineering, synthetic biology, and other related disciplines-is demanding facile yet robust tools, and various CRISPR technologies have potential to cater to the demands. Here, we briefly review the science in CRISPR/Cas systems and the milestone inventions that enabled numerous CRISPR technologies. Next, we describe CRISPR/Cas-derived technologies for bacterial strain development, including genome editing and gene expression regulation applications. Then, other CRISPR technologies possessing great potential for industrial applications are described, including typing and tracking of bacterial strains, virome identification, vaccination of bacteria, and advanced antimicrobial approaches. For each application, we note our suggestions for additional improvements as well. In the same context, replication of CRISPR/Cas-based chromosome imaging technologies developed originally in eukaryotic systems is introduced with its potential impact on studying bacterial chromosomal dynamics. Also, the current patent status of CRISPR technologies is reviewed. Finally, we provide some insights to the future of CRISPR technologies for bacterial systems by proposing complementary techniques to be developed for the use of CRISPR technologies in even wider range of applications.

Box C/D sRNA stem ends act as stabilizing anchors for box C/D di-sRNPs

Ribosomal RNA (rRNA) modifications are essential for ribosome function in all cellular organisms. Box C/D small (nucleolar) ribonucleoproteins [s(no)RNPs] catalyze 2′-O-methylation, one rRNA modification type in Eukarya and Archaea. Negatively stained electron microscopy (EM) models of archaeal box C/D sRNPs have demonstrated the dimeric sRNP (di-sRNP) architecture, which has been corroborated by nuclear magnetic resonance (NMR) studies. Due to limitations of the structural techniques, the orientation of the box C/D sRNAs has remained unclear. Here, we have used cryo-EM to elucidate the sRNA orientation in a M. jannaschii box C/D di-sRNP. The cryo-EM reconstruction suggests a parallel orientation of the two sRNAs. Biochemical and structural analyses of sRNPs assembled with mutant sRNAs indicate a potential interaction between the sRNA stem ends. Our results suggest that the parallel arrangement of the sRNAs juxtaposes their stem ends into close proximity to allow for a stabilizing interaction that helps maintain the di-sRNP architecture.

Diverse CRISPR-Cas responses and dramatic cellular DNA changes and cell death in pKEF9-conjugated Sulfolobus species

The Sulfolobales host a unique family of crenarchaeal conjugative plasmids some of which undergo complex rearrangements intracellularly. Here we examined the conjugation cycle of pKEF9 in the recipient strain Sulfolobus islandicus REY15A. The plasmid conjugated and replicated rapidly generating high average copy numbers which led to strong growth retardation that was coincident with activation of CRISPR-Cas adaptation. Simultaneously, intracellular DNA was extensively degraded and this also occurred in a conjugated Δcas6 mutant lacking a CRISPR-Cas immune response. Furthermore, the integrated forms of pKEF9 in the donor Sulfolobus solfataricus P1 and recipient host were specifically corrupted by transposable orfB elements, indicative of a dual mechanism for inactivating free and integrated forms of the plasmid. In addition, the CRISPR locus of pKEF9 was progressively deleted when conjugated into the recipient strain. Factors influencing activation of CRISPR-Cas adaptation in the recipient strain are considered, including the first evidence for a possible priming effect in Sulfolobus. The 3-Mbp genome sequence of the donor P1 strain is presented.

Transcriptome changes in STSV2-infected Sulfolobus islandicus REY15A undergoing continuous CRISPR spacer acquisition

A transcriptome study was performed on Sulfolobus islandicus REY15A actively undergoing CRISPR spacer acquisition from the crenarchaeal monocaudavirus STSV2 in rich and basal media over a 6 day period. Spacer acquisition preceded strong host growth retardation, altered transcriptional activity of four different CRISPR-Cas modules and changes in viral copy numbers, and with significant differences in the two media. Transcript levels of proteins involved in the cell cycle were reduced, whereas those of DNA replication, DNA repair, transcriptional regulation and some antitoxin-toxin pairs and transposases were unchanged or enhanced. Antisense RNAs were implicated in the transcriptional regulation of adaptation and interference modules of the type I-A CRISPR-Cas system, and evidence was found for the occurrence of functional co-ordination between the single CRISPR-Cas adaptation module and the functionally diverse interference modules.

Controlling mRNA stability and translation with the CRISPR endoribonuclease Csy4

The bacterial CRISPR endoribonuclease Csy4 has recently been described as a potential RNA processing tool. Csy4 recognizes substrate RNA through a specific 28-nt hairpin sequence and cleaves at the 3' end of the stem. To further explore applicability in mammalian cells, we introduced this hairpin at various locations in mRNAs derived from reporter transgenes and systematically evaluated the effects of Csy4-mediated processing on transgene expression. Placing the hairpin in the 5' UTR or immediately after the start codon resulted in efficient degradation of target mRNA by Csy4 and knockdown of transgene expression by 20- to 40-fold. When the hairpin was incorporated in the 3' UTR prior to the poly(A) signal, the mRNA was cleaved, but only a modest decrease in transgene expression (∼2.5-fold) was observed. In the absence of a poly(A) tail, Csy4 rescued the target mRNA substrate from degradation, resulting in protein expression, which suggests that the cleaved mRNA was successfully translated. In contrast, neither catalytically inactive (H29A) nor binding-deficient (R115A/R119A) Csy4 mutants were able to exert any of the effects described above. Generation of a similar 3' end by RNase P-mediated cleavage was unable to rescue transgene expression independent of Csy4. These results support the idea that the selective generation of the Csy4/hairpin complex resulting from cleavage of target mRNA might serve as a functional poly(A)/poly(A) binding protein (PABP) surrogate, stabilizing the mRNA and supporting translation. Although the exact mechanism(s) remain to be determined, our studies expand the potential utility of CRISPR nucleases as tools for controlling mRNA stability and translation.

Strand-specific community RNA-seq reveals prevalent and dynamic antisense transcription in human gut microbiota

Metagenomics and other meta-omics approaches (including metatranscriptomics) provide insights into the composition and function of microbial communities living in different environments or animal hosts. Metatranscriptomics research provides an unprecedented opportunity to examine gene regulation for many microbial species simultaneously, and more importantly, for the majority that are unculturable microbial species, in their natural environments (or hosts). Current analyses of metatranscriptomic datasets focus on the detection of gene expression levels and the study of the relationship between changes of gene expression and changes of environment. As a demonstration of utilizing metatranscriptomics beyond these common analyses, we developed a computational and statistical procedure to analyze the antisense transcripts in strand-specific metatranscriptomic datasets. Antisense RNAs encoded on the DNA strand opposite a gene’s CDS have the potential to form extensive base-pairing interactions with the corresponding sense RNA, and can have important regulatory functions. Most studies of antisense RNAs in bacteria are rather recent, are mostly based on transcriptome analysis, and have been applied mainly to single bacterial species. Application of our approaches to human gut-associated metatranscriptomic datasets allowed us to survey antisense transcription for a large number of bacterial species associated with human beings. The ratio of protein coding genes with antisense transcription ranges from 0 to 35.8% (median = 10.0%) among 47 species. Our results show that antisense transcription is dynamic, varying between human individuals. Functional enrichment analysis revealed a preference of certain gene functions for antisense transcription, and transposase genes are among the most prominent ones (but we also observed antisense transcription in bacterial house-keeping genes).

CRISPR-Cas: New Tools for Genetic Manipulations from Bacterial Immunity Systems

Prokaryotic CRISPR-Cas loci encode proteins that function as an adaptive immune system against infectious viruses and plasmids. Immunity is mediated by Cas nucleases and small RNA guides, which specify a cleavage site within the genome of the invader. In type II CRISPR-Cas systems, the RNA-guided Cas9 nuclease cleaves the DNA. Cas9 can be reprogrammed to create double-strand DNA breaks in the genomes of a variety of organisms, from bacteria to human cells. Repair of Cas9 lesions by homologous recombination or nonhomologous end joining mechanisms can lead to the introduction of specific nucleotide substitutions or indel mutations, respectively. Furthermore, a nuclease-null Cas9 has been developed to regulate endogenous gene expression and to label genomic loci in living cells. Targeted genome editing and gene regulation mediated by Cas9 are easy to program, scale, and multiplex, allowing researchers to decipher the causal link between genetic and phenotypic variation. In this review, we describe the most notable applications of Cas9 in basic biology, translational medicine, synthetic biology, biotechnology, and other fields.

Biogenesis pathways of RNA guides in archaeal and bacterial CRISPR-Cas adaptive immunity

CRISPR-Cas is an RNA-mediated adaptive immune system that defends bacteria and archaea against mobile genetic elements. Short mature CRISPR RNAs (crRNAs) are key elements in the interference step of the immune pathway. A CRISPR array composed of a series of repeats interspaced by spacer sequences acquired from invading mobile genomes is transcribed as a precursor crRNA (pre-crRNA) molecule. This pre-crRNA undergoes one or two maturation steps to generate the mature crRNAs that guide CRISPR-associated (Cas) protein(s) to cognate invading genomes for their destruction. Different types of CRISPR-Cas systems have evolved distinct crRNA biogenesis pathways that implicate highly sophisticated processing mechanisms. In Types I and III CRISPR-Cas systems, a specific endoribonuclease of the Cas6 family, either standalone or in a complex with other Cas proteins, cleaves the pre-crRNA within the repeat regions. In Type II systems, the trans-acting small RNA (tracrRNA) base pairs with each repeat of the pre-crRNA to form a dual-RNA that is cleaved by the housekeeping RNase III in the presence of the protein Cas9. In this review, we present a detailed comparative analysis of pre-crRNA recognition and cleavage mechanisms involved in the biogenesis of guide crRNAs in the three CRISPR-Cas types.

This review presents a detailed comparative analysis of pre-crRNA recognition and cleavage mechanisms involved in the biogenesis of guide crRNAs in the different bacterial and archaeal CRISPR-Cas immune systems.

The Heptameric SmAP1 and SmAP2 Proteins of the Crenarchaeon Sulfolobus Solfataricus Bind to Common and Distinct RNA Targets

Sm and Sm-like proteins represent an evolutionarily conserved family with key roles in RNA metabolism. Sm-based regulation is diverse and can range in scope from eukaryotic mRNA splicing to bacterial quorum sensing, with at least one step in these processes being mediated by an RNA-associated molecular assembly built on Sm proteins. Despite the availability of several 3D-structures of Sm-like archaeal proteins (SmAPs), their function has remained elusive. The aim of this study was to shed light on the function of SmAP1 and SmAP2 of the crenarchaeon Sulfolobus solfataricus (Sso). Using co-purification followed by RNA_Seq different classes of non-coding RNAs and mRNAs were identified that co-purified either with both paralogues or solely with Sso-SmAP1 or Sso-SmAP2. The large number of associated intron-containing tRNAs and tRNA/rRNA modifying RNAs may suggest a role of the two Sso-SmAPs in tRNA/rRNA processing. Moreover, the 3D structure of Sso-SmAP2 was elucidated. Like Sso-SmAP1, Sso-SmAP2 forms homoheptamers. The binding of both proteins to distinct RNA substrates is discussed in terms of surface conservation, structural differences in the RNA binding sites and differences in the electrostatic surface potential of the two Sso-SmAP proteins. Taken together, this study may hint to common and different functions of both Sso-SmAPs in Sso RNA metabolism.

Evolution of small guide RNA genes in hyperthermophilic archaea

Profiling the RNA production in hyperthermophilic archaea revealed an abundance of small RNA-guided processes near the upper temperature limit of life. Archaea utilize the base-pairing ability of RNA guide sequences to target ribosomal RNAs, transfer RNAs, messenger RNAs, and viral genomes. Cellular processes that are guided by small RNAs include the modification of RNA molecules, trans-splicing, gene regulation, and RNA and DNA degradation. Here, a brief overview of our knowledge on small guide RNA genes in archaeal genomes is provided and examples of their putative roles in genome evolution are described.

Non-protein coding RNA genes as the novel diagnostic markers for the discrimination of Salmonella species using PCR

Salmonellosis, a communicable disease caused by members of the Salmonella species, transmitted to humans through contaminated food or water. It is of paramount importance, to generate accurate detection methods for discriminating the various Salmonella species that cause severe infection in humans, including S. Typhi and S. Paratyphi A. Here, we formulated a strategy of detection and differentiation of salmonellosis by a multiplex polymerase chain reaction assay using S. Typhi non-protein coding RNA (sRNA) genes. With the designed sequences that specifically detect sRNA genes from S. Typhi and S. Paratyphi A, a detection limit of up to 10 pg was achieved. Moreover, in a stool-seeding experiment with S. Typhi and S. Paratyphi A, we have attained a respective detection limit of 15 and 1.5 CFU/mL. The designed strategy using sRNA genes shown here is comparatively sensitive and specific, suitable for clinical diagnosis and disease surveillance, and sRNAs represent an excellent molecular target for infectious disease.

CRISPR-Cas Adaptive Immune Systems of the Sulfolobales: Unravelling Their Complexity and Diversity

The Sulfolobales have provided good model organisms for studying CRISPR-Cas systems of the crenarchaeal kingdom of the archaea. These organisms are infected by a wide range of exceptional archaea-specific viruses and conjugative plasmids, and their CRISPR-Cas systems generally exhibit extensive structural and functional diversity. They carry large and multiple CRISPR loci and often multiple copies of diverse Type I and Type III interference modules as well as more homogeneous adaptation modules. These acidothermophilic organisms have recently provided seminal insights into both the adaptation process, the diverse modes of interference, and their modes of regulation. The functions of the adaptation and interference modules tend to be loosely coupled and the stringency of the crRNA-DNA sequence matching during DNA interference is relatively low, in contrast to some more streamlined CRISPR-Cas systems of bacteria. Despite this, there is evidence for a complex and differential regulation of expression of the diverse functional modules in response to viral infection. Recent work also supports critical roles for non-core Cas proteins, especially during Type III-directed interference, and this is consistent with these proteins tending to coevolve with core Cas proteins. Various novel aspects of CRISPR-Cas systems of the Sulfolobales are considered including an alternative spacer acquisition mechanism, reversible spacer acquisition, the formation and significance of antisense CRISPR RNAs, and a novel mechanism for avoidance of CRISPR-Cas defense. Finally, questions regarding the basis for the complexity, diversity, and apparent redundancy, of the intracellular CRISPR-Cas systems are discussed.

Hot and crispy: CRISPR-Cas systems in the hyperthermophile Sulfolobus solfataricus

The CRISPR (clustered regularly interspaced short palindromic repeats) and Cas (CRISPR-associated) genes are widely spread in bacteria and archaea, representing an intracellular defence system against invading viruses and plasmids. In the system, fragments from foreign DNA are captured and integrated into the host genome at the CRISPR locus. The locus is transcribed and the resulting RNAs are processed by Cas6 into small crRNAs (CRISPR RNAs) that guide a variety of effector complexes to degrade the invading genetic elements. Many bacteria and archaea have one major type of effector complex. However, Sulfolobus solfataricus strain P2 has six CRISPR loci with two families of repeats, four cas6 genes and three different types of effector complex. These features make S. solfataricus an important model for studying CRISPR-Cas systems. In the present article, we review our current understanding of crRNA biogenesis and its effector complexes, subtype I-A and subtype III-B, in S. solfataricus. We also discuss the differences in terms of mechanisms between the subtype III-B systems in S. solfataricus and Pyrococcus furiosus.

Primary transcriptome map of the hyperthermophilic archaeon Thermococcus kodakarensis

Background

Prokaryotes have relatively small genomes, densely-packed with protein-encoding sequences. RNA sequencing has, however, revealed surprisingly complex transcriptomes and here we report the transcripts present in the model hyperthermophilic Archaeon, Thermococcus kodakarensis, under different physiological conditions.

Results

Sequencing cDNA libraries, generated from RNA isolated from cells under different growth and metabolic conditions has identified >2,700 sites of transcription initiation, established a genome-wide map of transcripts, and consensus sequences for transcription initiation and post-transcription regulatory elements. The primary transcription start sites (TSS) upstream of 1,254 annotated genes, plus 644 primary TSS and their promoters within genes, are identified. Most mRNAs have a 5'-untranslated region (5'-UTR) 10 to 50 nt long (median = 16 nt), but ~20% have 5'-UTRs from 50 to 300 nt long and ~14% are leaderless. Approximately 50% of mRNAs contain a consensus ribosome binding sequence. The results identify TSS for 1,018 antisense transcripts, most with sequences complementary to either the 5'- or 3'-region of a sense mRNA, and confirm the presence of transcripts from all three CRISPR loci, the RNase P and 7S RNAs, all tRNAs and rRNAs and 69 predicted snoRNAs. Two putative riboswitch RNAs were present in growing but not in stationary phase cells. The procedure used is designed to identify TSS but, assuming that the number of cDNA reads correlates with transcript abundance, the results also provide a semi-quantitative documentation of the differences in T. kodakarensis genome expression under different growth conditions and confirm previous observations of substrate-dependent specific gene expression. Many previously unanticipated small RNAs have been identified, some with relative low GC contents (≤50%) and sequences that do not fold readily into base-paired secondary structures, contrary to the classical expectations for non-coding RNAs in a hyperthermophile.

Conclusion

The results identify >2,700 TSS, including almost all of the primary sites of transcription initiation upstream of annotated genes, plus many secondary sites, sites within genes and sites resulting in antisense transcripts. The T. kodakarensis genome is small (~2.1 Mbp) and tightly packed with protein-encoding genes, but the transcriptomes established also contain many non-coding RNAs and predict extensive RNA-based regulation in this model Archaeon.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-684) contains supplementary material, which is available to authorized users.