Histone methyltransferase TXR1 is required for both H3 and H3.3 lysine 27 methylation in the well-known ciliated protist Tetrahymena thermophila

Comprehensive genome annotation of the model ciliate Tetrahymena thermophila by in-depth epigenetic and transcriptomic profiling

The ciliate Tetrahymena thermophila is a well-established unicellular model eukaryote, contributing significantly to foundational biological discoveries. Despite its acknowledged importance, current studies on Tetrahymena biology face challenges due to gene annotation inaccuracy, particularly the notable absence of untranslated regions (UTRs). To comprehensively annotate the Tetrahymena macronuclear genome, we collected extensive transcriptomic data spanning various cell stages. To ascertain transcript orientation and transcription start/end sites, we incorporated data on epigenetic marks displaying enrichment towards the 5' end of gene bodies, including H3 lysine 4 tri-methylation (H3K4me3), histone variant H2A.Z, nucleosome positioning and N6-methyldeoxyadenine (6mA). Cap-seq data was subsequently applied to validate the accuracy of identified transcription start sites. Additionally, we integrated Nanopore direct RNA sequencing (DRS), strand-specific RNA sequencing (RNA-seq) and assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) data. Using a newly developed bioinformatic pipeline, coupled with manual curation and experimental validation, our work yielded substantial improvements to the current gene models, including the addition of 2,481 new genes, updates to 23,936 existing genes, and the incorporation of 8,339 alternatively spliced isoforms. Furthermore, novel UTR information was annotated for 26,687 high-confidence genes. Intriguingly, 20% of protein-coding genes were identified to have natural antisense transcripts characterized by high diversity in alternative splicing, thus offering insights into understanding transcriptional regulation. Our work will enhance the utility of Tetrahymena as a robust genetic toolkit for advancing biological research, and provides a promising framework for genome annotation in other eukaryotes.

Dynamic DNA N 6-adenine methylation (6mA) governs the encystment process, showcased in the unicellular eukaryote Pseudocohnilembus persalinus

The formation of resting cysts commonly found in unicellular eukaryotes is a complex and highly regulated survival strategy against environmental stress that involves drastic physiological and biochemical changes. Although most studies have focused on the morphology and structure of cysts, little is known about the molecular mechanisms that control this process. Recent studies indicate that DNA N 6-adenine methylation (6mA) could be dynamically changing in response to external stimuli; however, its potential role in the regulation of cyst formation remains unknown. We used the ciliate Pseudocohnilembus persalinus, which can be easily induced to form cysts to investigate the dynamic pattern of 6mA in trophonts and cysts. Single-molecule real-time (SMRT) sequencing reveals high levels of 6mA in trophonts that decrease in cysts, along with a conversion of symmetric 6mA to asymmetric 6mA. Further analysis shows that 6mA, a mark of active transcription, is involved in altering the expression of encystment-related genes through changes in 6mA levels and 6mA symmetric-to-asymmetric conversion. Most importantly, we show that reducing 6mA levels by knocking down the DNA 6mA methyltransferase PpAMT1 accelerates cyst formation. Taken together, we characterize the genome-wide 6mA landscape in P. persalinus and provide insights into the role of 6mA in gene regulation under environmental stress in eukaryotes. We propose that 6mA acts as a mark of active transcription to regulate the encystment process along with symmetric-to-asymmetric conversion, providing important information for understanding the molecular response to environmental cues from the perspective of 6mA modification.

The Role of the TSK/TONSL-H3.1 Pathway in Maintaining Genome Stability in Multicellular Eukaryotes

Replication-dependent histone H3.1 and replication-independent histone H3.3 are nearly identical proteins in most multicellular eukaryotes. The N-terminal tails of these H3 variants, where the majority of histone post-translational modifications are made, typically differ by only one amino acid. Despite extensive sequence similarity with H3.3, the H3.1 variant has been hypothesized to play unique roles in cells, as it is specifically expressed and inserted into chromatin during DNA replication. However, identifying a function that is unique to H3.1 during replication has remained elusive. In this review, we discuss recent findings regarding the involvement of the H3.1 variant in regulating the TSK/TONSL-mediated resolution of stalled or broken replication forks. Uncovering this new function for the H3.1 variant has been made possible by the identification of the first proteins containing domains that can selectively bind or modify the H3.1 variant. The functional characterization of H3-variant-specific readers and writers reveals another layer of chromatin-based information regulating transcription, DNA replication, and DNA repair.

Identification and utilization of a mutated 60S ribosomal subunit coding gene as an effective and cost-efficient selection marker for Tetrahymena genetic manipulation

Since the onset of molecular biology, the ciliate Tetrahymena thermophila has been one of the most convenient single-celled model eukaryotes for genetics, biochemistry, and cell biology. Particularly, thanks to the availability of several different selection markers, it is possible to knock out or knock in genes at multiple genetic loci simultaneously in Tetrahymena, which makes it an excellent model ciliate for tackling complex regulatory mechanisms. Despite these selection markers are efficient for genetic manipulation, the costly drugs used for selection have highlighted the urgent demand for an additional cost-efficient and effective selection marker. Here, we found that a mutated 60S ribosomal subunit component, RPL36A, confers T. thermophila with cycloheximide resistance. On top of that, we developed a cycloheximide cassette and explored suitable transformation and selection conditions. Using the new cassette, we obtained both knockout and knock-in strains successfully at a relatively low cost. This study also provided the first evidence that a cycloheximide resistance gene can be engineered as a selection marker to completely delete a gene from the highly-polyploid somatic nucleus in Tetrahymena.

An Optimized and Versatile Counter-Flow Centrifugal Elutriation Workflow to Obtain Synchronized Eukaryotic Cells

Cell synchronization is a powerful tool to understand cell cycle events and its regulatory mechanisms. Counter-flow centrifugal elutriation (CCE) is a more generally desirable method to synchronize cells because it does not significantly alter cell behavior and/or cell cycle progression, however, adjusting specific parameters in a cell type/equipment-dependent manner can be challenging. In this paper, we used the unicellular eukaryotic model organism, Tetrahymena thermophila as a testing system for optimizing CCE workflow. Firstly, flow cytometry conditions were identified that reduced nuclei adhesion and improved the assessment of cell cycle stage. We then systematically examined how to achieve the optimal conditions for three critical factors affecting the outcome of CCE, including loading flow rate, collection flow rate and collection volume. Using our optimized workflow, we obtained a large population of highly synchronous G1-phase Tetrahymena as measured by 5-ethynyl-2'-deoxyuridine (EdU) incorporation into nascent DNA strands, bulk DNA content changes by flow cytometry, and cell cycle progression by light microscopy. This detailed protocol can be easily adapted to synchronize other eukaryotic cells.

A distinct class of eukaryotic MT-A70 methyltransferases maintain symmetric DNA N6-adenine methylation at the ApT dinucleotides as an epigenetic mark associated with transcription

Rediscovered as a potential eukaryotic epigenetic mark, DNA N⁶-adenine methylation (6mA) varies across species in abundance and its relationships with transcription. Here we characterize AMT1—representing a distinct MT-A70 family methyltransferase—in the ciliate Tetrahymena thermophila. AMT1 loss-of-function leads to severe defects in growth and development. Single Molecule, Real-Time (SMRT) sequencing reveals that AMT1 is required for the bulk of 6mA and all symmetric methylation at the ApT dinucleotides. The detection of hemi-methylated ApT sites suggests a semi-conservative mechanism for maintaining symmetric methylation. AMT1 affects expression of many genes; in particular, RAB46, encoding a Rab family GTPase involved in contractile vacuole function, is likely a direct target. The distribution of 6mA resembles H3K4 methylation and H2A.Z, two conserved epigenetic marks associated with RNA polymerase II transcription. Furthermore, strong 6mA and nucleosome positioning in wild-type cells is attenuated in ΔAMT1 cells. Our results support that AMT1-catalyzed 6mA is an integral part of the transcription-associated epigenetic landscape. AMT1 homologues are generally found in protists and basal fungi featuring ApT hyper-methylation associated with transcription, which are missing in animals, plants, and true fungi. This dichotomy of 6mA functions and the underlying molecular mechanisms may have implications in eukaryotic diversification.

The completed macronuclear genome of a model ciliate Tetrahymena thermophila and its application in genome scrambling and copy number analyses

The ciliate Tetrahymena thermophila has been a powerful model system for molecular and cellular biology. However, some investigations have been limited due to the incomplete closure and sequencing of the macronuclear genome assembly, which for many years has been stalled at 1,158 scaffolds, with large sections of unknown sequences (available in Tetrahymena Genome Database, TGD, http://ciliate.org/ ). Here we completed the first chromosome-level Tetrahymena macronuclear genome assembly, with approximately 300× long Single Molecule, Real-Time reads of the wild-type SB210 cells-the reference strain for the initial macronuclear genome sequencing project. All 181 chromosomes were capped with two telomeres and gaps were entirely closed. The completed genome shows significant improvements over the current assembly (TGD 2014) in both chromosome structure and sequence integrity. The majority of previously identified gene models shown in TGD were retained, with the addition of 36 new genes and 883 genes with modified gene models. The new genome and annotation were incorporated into TGD. This new genome allows for pursuit in some underexplored areas that were far more challenging previously; two of them, genome scrambling and chromosomal copy number, were investigated in this study. We expect that the completed macronuclear genome will facilitate many studies in Tetrahymena biology, as well as multiple lines of research in other eukaryotes.

Morphology and Molecular Phylogeny of Two Freshwater Oligotrich Ciliates (Protozoa, Ciliophora, Oligotrichia), Pelagostrombidium fallax (Zacharias, 1895) Krainer, 1991 and Limnostrombidium viride (Stein, 1867) Krainer, 1995, with Brief Notes on Stomatogenesis

The morphology of two oligotrich ciliates, Pelagostrombidium fallax (Zacharias, 1895) Krainer, 1991 and Limnostrombidium viride (Stein, 1867) Krainer, 1995, collected from a freshwater pond in northern China, was studied based on live observation and protargol staining. Currently, undescribed features in the girdle kinety in P. fallax are (a) sparsely spaced single argyrophilic basal bodies in the shoulder region and (b) a U-shape formed below the buccal peristome. An improved diagnosis for P. fallax is supplied, based on previous and present morphological descriptions. In addition, certain ontogenetic stages of P. fallax are reported for the first time. During division, two new embryonic bodies are successively generated de novo, the first one goes to the proter, the second one to the opisthe; the new canal derives from the old canal. The morphology of L. viride is redescribed in detail; the Chinese population is highly consistent with populations in Europe. The SSU rRNA gene of P. fallax was sequenced for the first time. Phylogenetic analyses, based on SSU rRNA gene sequence data, reveal that P. fallax and L. viride cluster with Strombidium species while the result of an AU test did not reject the possibility of monophyly of the family Pelagostrombidiidae.

Genome analyses of the new model protist Euplotes vannus focusing on genome rearrangement and resistance to environmental stressors

As a model organism for studies of cell and environmental biology, the free-living and cosmopolitan ciliate Euplotes vannus shows intriguing features like dual genome architecture (i.e., separate germline and somatic nuclei in each cell/organism), "gene-sized" chromosomes, stop codon reassignment, programmed ribosomal frameshifting (PRF) and strong resistance to environmental stressors. However, the molecular mechanisms that account for these remarkable traits remain largely unknown. Here we report a combined analysis of de novo assembled high-quality macronuclear (MAC; i.e., somatic) and partial micronuclear (MIC; i.e., germline) genome sequences for E. vannus, and transcriptome profiling data under varying conditions. The results demonstrate that: (a) the MAC genome contains more than 25,000 complete "gene-sized" nanochromosomes (~85 Mb haploid genome size) with the N50 ~2.7 kb; (b) although there is a high frequency of frameshifting at stop codons UAA and UAG, we did not observe impaired transcript abundance as a result of PRF in this species as has been reported for other euplotids; (c) the sequence motif 5'-TA-3' is conserved at nearly all internally-eliminated sequence (IES) boundaries in the MIC genome, and chromosome breakage sites (CBSs) are duplicated and retained in the MAC genome; (d) by profiling the weighted correlation network of genes in the MAC under different environmental stressors, including nutrient scarcity, extreme temperature, salinity and the presence of ammonia, we identified gene clusters that respond to these external physical or chemical stimulations, and (e) we observed a dramatic increase in HSP70 gene transcription under salinity and chemical stresses but surprisingly, not under temperature changes; we link this temperature-resistance to the evolved loss of temperature stress-sensitive elements in regulatory regions. Together with the genome resources generated in this study, which are available online at Euplotes vannus Genome Database (http://evan.ciliate.org), these data provide molecular evidence for understanding the unique biology of highly adaptable microorganisms.

Cyclin Cyc2p is required for micronuclear bouquet formation in Tetrahymena thermophila

Meiotic bouquet formation (known as crescent formation in Tetrahymena thermophila) is indispensable for homologous pairing and recombination, but the regulatory mechanism of bouquet formation remains largely unknown. As a conjugation specific cyclin gene, CYC2 knockout mutants failed to form an elongated crescent structure and aborted meiosis progress in T. thermophila. γ-H2A.X staining revealed fewer micronuclear DNA double-strand breaks (DSBs) in cyc2Δ cells than in wild-type cells. Furthermore, cyc2Δ cells still failed to form a crescent structure even though DSBs were induced by exogenous agents, indicating that a lack of DSBs was not completely responsible for failure to enter the crescent stage. Tubulin staining showed that impaired perinuclear microtubule structure may contribute to the blockage in micronuclear elongation. At the same time, expression of microtubule-associated kinesin genes, KIN11 and KIN141, was significantly downregulated in cyc2Δ cells. Moreover, micronuclear specific accumulation of heterochromatin marker trimethylated H3K23 abnormally increased in the cyc2Δ mutants. Together, these results show that cyclin Cyc2p is required for micronuclear bouquet formation via controlling microtubule-directed nuclear elongation in Tetrahymena.

Further analyses of variation of ribosome DNA copy number and polymorphism in ciliates provide insights relevant to studies of both molecular ecology and phylogeny

Sequence-based approaches, such as analyses of ribosome DNA (rDNA) clone libraries and high-throughput amplicon sequencing, have been used extensively to infer evolutionary relationships and elucidate the biodiversity in microbial communities. However, recent studies demonstrate both rDNA copy number variation and intra-individual (intra-genomic) sequence variation in many organisms, which challenges the application of the rDNA-based surveys. In ciliates, an ecologically important clade of microbial eukaryotes, rDNA copy number and sequence variation are rarely studied. In the present study, we estimate the intraindividual small subunit rDNA (SSU rDNA) copy number and sequence variation in a wide range of taxa covering nine classes and 18 orders of the phylum Ciliophora. Our studies reveal that: (i) intra-individual sequence variation of SSU rDNA is ubiquitous in all groups of ciliates detected and the polymorphic level varies among taxa; (ii) there is a most common version of SSU rDNA sequence in each cell that is highly predominant and may represent the germline micronuclear template; (iii) compared with the most common version, other variant sequences differ in only 1-3 nucleotides, likely generated during macronuclear (somatic) amplification; (iv) the intra-cell sequence variation is unlikely to impact phylogenetic analyses; (v) the rDNA copy number in ciliates is highly variable, ranging from 10³ to 10⁶, with the highest record in Stentor roeselii. Overall, these analyses indicate the need for careful consideration of SSU rDNA variation in analyses of the role of ciliates in ecosystems.

Time-course analysis of nuclear events during conjugation in the marine ciliate Euplotes vannus and comparison with other ciliates (Protozoa, Ciliophora)

Ciliates represent a morphologically and genetically distinct group of single-celled eukaryotes that segregate germline and somatic functions into two types of nuclei and exhibit complex cytogenetic events during the sexual process of conjugation, which is under the control of the so-called "mating type systems". Studying conjugation in ciliates may provide insight into our understanding of the origins and evolution of sex and fertilization. In the present work, we studied in detail the sexual process of conjugation using the model species Euplotes vannus, and compared these nuclear events with those occurring in other ciliates. Our results indicate that in E. vannus: 1) conjugation requires about 75 hours to complete: the longest step is the development of the new macronucleus (ca. 64h), followed by the nuclear division of meiosis I (5h); the mitotic divisions usually take only 2h; 2) there are three prezygotic divisions (mitosis and meiosis I and II), and two of the eight resulting nuclei become pronuclei; 3) after the exchange and fusion of the pronuclei, two postzygotic divisions occur; two of the four products differentiate into the new micronucleus and macronucleus, respectively, and the parental macronucleus degenerates completely; 4) comparison of the nuclear events during conjugation in different ciliates reveals that there are generally three prezygotic divisions while the number of postzygotic divisions is highly variable. These results can serve as reference to investigate the mating type system operating in this species and to analyze genes involved in the different steps of the sexual process.

Enhancer RNA and NFκB-dependent P300 regulation of ADAMDEC1

We observed increased expression of ADAMDEC1 RNA in monocytes from patients with systemic lupus erythematosus. The precise role of ADAMDEC1 is uncertain and uniquely among metalloproteinases it utilizes a zinc-coordinating aspartic acid residue which allows it to escape inhibition by tissue inhibitor of metalloprotease-3 (TIMP-3). A closely related gene encodes the protein ADAM28, which is not up-regulated in lupus. We leveraged the ability to look at both gene's promoters and enhancers simultaneously. ADAMDEC1 was up-regulated by LPS while ADAM28 was not upregulated in the short term. We identified MAP kinases and NFκB as critical cell pathways regulating the expression of ADAMDEC1. These same pathways were implicated in driving the expression of the ADAMDEC1 upstream enhancer RNAs. We demonstrated that binding of the enhancer RNAs produced from the upstream enhancer were critically important and that p300 bound to both the RNA from the enhancer and the DNA at the enhancer. P300 binding to the enhancer was dependent on NFκB. These data define the critical pathways regulating the expression of ADAMDEC1 and extend our knowledge of the roles of enhancer RNAs and mechanistically links p300 and enhancer RNAs.

Insights into an Extensively Fragmented Eukaryotic Genome: De Novo Genome Sequencing of the Multinuclear Ciliate Uroleptopsis citrina

Ciliated protists are a large group of single-celled eukaryotes with separate germline and somatic nuclei in each cell. The somatic genome is developed from the zygotic nucleus through a series of chromosomal rearrangements, including fragmentation, DNA elimination, de novo telomere addition, and DNA amplification. This unique feature makes them perfect models for research in genome biology and evolution. However, genomic research of ciliates has been limited to a few species, owing to problems with DNA contamination and obstacles in cultivation. Here, we introduce a method combining telomere-primer PCR amplification and high-throughput sequencing, which can reduce DNA contamination and obtain genomic data efficiently. Based on this method, we report a draft somatic genome of a multimacronuclear ciliate, Uroleptopsis citrina. 1) The telomeric sequence in U. citrina is confirmed to be C4A4C4A4C4 by directly blunt-end cloning. 2) Genomic analysis of the resulting chromosomes shows a "one-gene one-chromosome" pattern, with a small number of multiple-gene chromosomes. 3) Amino acid usage is analyzed, and reassignment of stop codons is confirmed. 4) Chromosomal analysis shows an obvious asymmetrical GC skew and high bias between A and T in the subtelomeric regions of the sense-strand, with the detection of an 11-bp high AT motif region in the 3' subtelomeric region. 5) The subtelomeric sequence also has an obvious 40 nt strand oscillation of nucleotide ratio. 6) In the 5' subtelomeric region of the coding strand, the distribution of potential TATA-box regions is illustrated, which accumulate between 30 and 50 nt. This work provides a valuable reference for genomic research and furthers our understanding of the dynamic nature of unicellular eukaryotic genomes.

Redescription of a Hymenostome Ciliate, Tetrahymena setosa (Protozoa, Ciliophora) Notes on its Molecular Phylogeny

In recent years, Tetrahymena species have been used as model organisms for research in a wide range of fields, highlighting the need for a fuller understanding of the taxonomy of this group. It is in this context that this paper uses living observation and silver staining methods to investigate the morphology and infraciliature of one Tetrahymena species, T. setosa (Schewiakoff 1892 Verh. Naturh. Med. Ver. Heidelb., 4:544) McCoy (1975) Acta Protozool., 14:253; the senior subjective synonym of T. setifera Holz and Corliss (1956) J. Protozool., 3:112; isolated from a freshwater pond in Harbin, north-eastern China. This organism can be distinguished from other described Tetrahymena species mainly by its single caudal cilium, which is about twice the length of the somatic ciliature. While the Harbin isolate appears similar to the population described by Holz and Corliss (1956) J. Protozool., 3:112, an improved diagnosis for T. setosa is given based on the previous descriptions and the Harbin population. In summary, this species can be recognized mainly by the combination of the following characters: body in vivo approximately 40 μm × 25 μm, 21-26 somatic kineties, one to four contractile vacuole pores associated with meridians 6-11 and a single caudal cilium. The small subunit ribosomal (SSU) rRNA gene and the cox1 gene sequences of Harbin population are also characterized in order to corroborate that the isolated species branches in phylogenetic trees as a T. setosa species. The phylogenetic analysis also indicated that sequences of populations of Tetrahymena species should be published with detailed morphological identifications.

EARLY FLOWERING IN SHORT DAYS (EFS) regulates the seed size in Arabidopsis

Post-transcriptional modifications, including histone modifications and DNA methylation, alter the chromatin landscape to regulate gene expression, thus control various cellular processes in plants. EARLY FLOWERING IN SHORT DAYS (EFS) is the major contributor for H3K36 methylation in Arabidopsis and is important for plant development. Here, we find that EFS is expressed in different stages of embryo morphogenesis, and the efs mutant produces larger embryo that results in enlarged seeds. Further analysis reveals that an imprinted gene MOP9.5 is hypomethylated at the promoter region and its expression is derepressed in efs mutant. MOP9.5 promoter is marked by various epigenetic modifications, and we find that following the increase of H3K36me3, H3K27me3 and H3K9me2 levels are reduced in efs mutant. This data indicates an antagonistic regulation between H3K36me3 and DNA methylation, and/or H3K27me3 at MOP9.5. Our results further show that both maternal and paternal EFS alleles are responsible for the seed size regulation, which unraveled a novel function of EFS in plant development.

N6-adenine DNA methylation is associated with the linker DNA of H2A.Z-containing well-positioned nucleosomes in Pol II-transcribed genes in Tetrahymena

DNA N6-methyladenine (6mA) is newly rediscovered as a potential epigenetic mark across a more diverse range of eukaryotes than previously realized. As a unicellular model organism, Tetrahymena thermophila is among the first eukaryotes reported to contain 6mA modification. However, lack of comprehensive information about 6mA distribution hinders further investigations into its function and regulatory mechanism. In this study, we provide the first genome-wide, base pair-resolution map of 6mA in Tetrahymena by applying single-molecule real-time (SMRT) sequencing. We provide evidence that 6mA occurs mostly in the AT motif of the linker DNA regions. More strikingly, these linker DNA regions with 6mA are usually flanked by well-positioned nucleosomes and/or H2A.Z-containing nucleosomes. We also find that 6mA is exclusively associated with RNA polymerase II (Pol II)-transcribed genes, but is not an unambiguous mark for active transcription. These results support that 6mA is an integral part of the chromatin landscape shaped by adenosine triphosphate (ATP)-dependent chromatin remodeling and transcription.

A Contribution to the Morphology and Phylogeny of Chlamydodon, with Three New Species from China (Ciliophora, Cyrtophoria)

Three new cyrtophorian ciliates isolated from coastal areas of China were described based on morphological and genetic data. The Chlamydodon mnemosyne-like species Chlamydodon similis sp. n. differs from its congeners mainly by its number of somatic kineties. Chlamydodon oligochaetus sp. n. is distinguished from its congeners mainly by having fewer somatic kineties, and/or an elongated body shape. Chlamydodon crassidens sp. n. is characterized mainly by an inverted triangular body shape, a posteriorly interrupted cross-striated band (5-6 μm wide), and a large cytostome. Moreover, we provided small-subunit (SSU) rDNA sequences of C. similis sp. n. and C. oligochaetus sp. n. Maximum likelihood (ML) and Bayesian inference (BI) consistently placed C. similis sp. n. as a sister to C. paramnemosyne, but showed different branching position of C. oligochaetus sp. n., which may be due to a low taxon sampling in the Chlamydodontidae and/or an insufficient resolution of the marker gene at species level.