Coevolution of tRNA intron motifs and tRNA endonuclease architecture in Archaea

The Origin of Translation: Bridging the Nucleotides and Peptides

Extant biology uses RNA to record genetic information and proteins to execute biochemical functions. Nucleotides are translated into amino acids via transfer RNA in the central dogma. tRNA is essential in translation as it connects the codon and the cognate amino acid. To reveal how the translation emerged in the prebiotic context, we start with the structure and dissection of tRNA, followed by the theory and hypothesis of tRNA and amino acid recognition. Last, we review how amino acids assemble on the tRNA and further form peptides. Understanding the origin of life will also promote our knowledge of artificial living systems.

Archaeal tRNA-Splicing Endonuclease as an Effector for RNA Recombination and Novel Trans-Splicing Pathways in Eukaryotes

We have characterized a homodimeric tRNA endonuclease from the euryarchaeota Ferroplasma acidarmanus (FERAC), a facultative anaerobe which can grow at temperatures ranging from 35 to 42 °C. This enzyme, contrary to the eukaryal tRNA endonucleases and the homotetrameric Methanocaldococcus jannaschii (METJA) homologs, is able to cleave minimal BHB (bulge–helix–bulge) substrates at 30 °C. The expression of this enzyme in Schizosaccharomyces pombe (SCHPO) enables the use of its properties as effectors by inserting BHB motif introns into hairpin loops normally seen in mRNA transcripts. In addition, the FERAC endonuclease can create proteins with new functionalities through the recombination of protein domains.

Characterization of the Methanomicrobial Archaeal RNase Zs for Processing the CCA-Containing tRNA Precursors

RNase Z is a widely distributed and usually essential endoribonuclease involved in the 3′-end maturation of transfer RNAs (tRNAs). A CCA triplet that is needed for tRNA aminoacylation in protein translation is added by a nucleotidyl-transferase after the 3′-end processing by RNase Z. However, a considerable proportion of the archaeal pre-tRNAs genetically encode a CCA motif, while the enzymatic characteristics of the archaeal RNase (aRNase) Zs in processing CCA-containing pre-tRNAs remain unclear. This study intensively characterized two methanomicrobial aRNase Zs, the Methanolobus psychrophilus mpy-RNase Z and the Methanococcus maripaludis mmp-RNase Z, particularly focusing on the properties of processing the CCA-containing pre-tRNAs, and in parallel comparison with a bacterial bsu-RNase Z from Bacillus subtilis. Kinetic analysis found that Co²⁺ supplementation enhanced the cleavage efficiency of mpy-RNase Z, mmp-RNase Z, and bsu-RNase Z for 1400-, 2990-, and 34-fold, respectively, and Co²⁺ is even more indispensable to the aRNase Zs than to bsu-RNase Z. Mg²⁺ also elevated the initial cleavage velocity (V₀) of bsu-RNase Z for 60.5-fold. The two aRNase Zs exhibited indiscriminate efficiencies in processing CCA-containing vs. CCA-less pre-tRNAs. However, V₀ of bsu-RNase Z was markedly reduced for 1520-fold by the CCA motif present in pre-tRNAs under Mg²⁺ supplementation, but only 5.8-fold reduced under Co²⁺ supplementation, suggesting Co²⁺ could ameliorate the CCA motif inhibition on bsu-RNase Z. By 3′-RACE, we determined that the aRNase Zs cleaved just downstream the discriminator nucleotide and the CCA triplet in CCA-less and CCA-containing pre-tRNAs, thus exposing the 3′-end for linking CCA and the genetically encoded CCA triplet, respectively. The aRNase Zs, but not bsu-RNase Z, were also able to process the intron-embedded archaeal pre-tRNAs, and even process pre-tRNAs that lack the D, T, or anticodon arm, but strictly required the acceptor stem. In summary, the two methanomicrobial aRNase Zs use cobalt as a metal ligand and process a broad spectrum of pre-tRNAs, and the characteristics would extend our understandings on aRNase Zs.

Comprehensive analysis of the pre-ribosomal RNA maturation pathway in a methanoarchaeon exposes the conserved circularization and linearization mode in archaea

The ribosomal RNA (rRNA) genes are generally organized as an operon and cotranscribed into a polycistronic precursor; therefore, processing and maturation of pre-rRNAs are essential for ribosome biogenesis. However, rRNA maturation pathways of archaea, particularly of methanoarchaea, are scarcely known. Here, we thoroughly elucidated the maturation pathway of the rRNA operon (16S-tRNA^Ala-23S-tRNA^Cys-5S) in Methanolobus psychrophilus, one representative of methanoarchaea. Enzymatic assay demonstrated that EndA, a tRNA splicing endoribonuclease, cleaved bulge-helix-bulge (BHB) motifs buried in the processing stems of pre-16S and pre-23S rRNAs. Northern blot and quantitative PCR detected splicing-coupled circularization of pre-16S and pre-23S rRNAs, which accounted for 2% and 12% of the corresponding rRNAs, respectively. Importantly, endoribonuclease Nob1 was determined to linearize circular pre-16S rRNA at the mature 3' end so to expose the anti-Shine-Dalgarno sequence, while circular pre-23S rRNA was linearized at the mature 5' end by an unknown endoribonuclease. The resultant 5' and 3' extension in linearized pre-16S and pre-23S rRNAs were finally matured through 5'-3' and 3'-5' exoribonucleolytic trimming, respectively. Additionally, a novel processing pathway of endoribonucleolysis coupled with exoribonucleolysis was identified for the pre-5S rRNA maturation in this methanogen, which could be also conserved in most methanogenic euryarchaea. Based on evaluating the phylogenetic conservation of the key elements that are involved in circularization and linearization of pre-rRNA maturation, we predict that the rRNA maturation mode revealed here could be prevalent among archaea.

Identified Hybrid tRNA Structure Genes in Archaeal Genome

Background

In Archaea, previous studies have revealed the presence of multiple intron-containing tRNAs and split tRNAs. The full unexpurgated analysis of archaeal tRNA genes remains a challenging task in the field of bioinformatics, because of the presence of various types of hidden tRNA genes in archaea. Here, we suggested a computational method that searched for widely separated genes encoding tRNA halves to generate suppressive variants of missing tRNAs.

Objectives

The exploration of tRNA genes from a genome with varying hypotheses, among all three domain of life (eukaryotes, bacteria and archaea), has been rapidly identified in different ways in the field of bioinformatics. Like eukaryotic tRNA genes, it has been established that two separated regions of the coding sequence of a tRNA gene are essential and sufficient for promotion of transcription. Our objective is to find out the two essential regions in the genome sequence which comprises two halves of the hidden tRNAs.

Material and Methods

Considering the existence of split tRNA genes widely separated throughout the genome, we developed our tRNA search algorithm to predict such separated tRNA genes by searching both a conserved terminal 5'- and 3'-motif of tRNA in agreement with the split hypothesis on the basis of cloverleaf prediction and precise insilico determination of bulge-helix-bulge secondary structure at the splice sites.

Results

By a comprehensive search for all kinds of missing tRNA genes, we have constructed hybrid tRNA genes containing one essential region from tDNA (XYZ) and the other from tDNA (ABC), both from same species in the archaea. We have also found, this type of hybrid tRNA genes are identified in the different species of the archaea (XYZ ASN, ARG and MET; ABC ASP,SER, ARG and PRO).These hybrid split tRNA share a common structural motif called bulge-helix-bulge (BHB) a more relaxed bulge-helix loop (BHL), at the leader exon boundary and suggested to be evolutionary interrelated.

Conclusions

Analysis of the complete genome sequences of Metallosphaera sedula DSM 5348, Desulfurococcus kamchatkensis 1221n and Ignicoccus hospitalis KIN4/I in archaea by our algorithm revealed that a number of hybrid tRNAs are constructed from different tDNAs . Asymmetric combination of 5' and 3' tRNA halves may have generated the diversity of tRNA molecules. Our study of hybrid tRNA genes will provide a new molecular basis for upcoming tRNA studies.

Recent Insights Into the Structure, Function, and Evolution of the RNA-Splicing Endonucleases

RNA-splicing endonuclease (EndA) cleaves out introns from archaeal and eukaryotic precursor (pre)-tRNA and is essential for tRNA maturation. In archaeal EndA, the molecular mechanisms underlying complex assembly, substrate recognition, and catalysis have been well understood. Recently, certain studies have reported novel findings including the identification of new subunit types in archaeal EndA structures, providing insights into the mechanism underlying broad substrate specificity. Further, metagenomics analyses have enabled the acquisition of numerous DNA sequences of EndAs and intron-containing pre-tRNAs from various species, providing information regarding the co-evolution of substrate specificity of archaeal EndAs and tRNA genetic diversity, and the evolutionary pathway of archaeal and eukaryotic EndAs. Although the complex structure of the heterothermic form of eukaryotic EndAs is unknown, previous reports regarding their functions indicated that mutations in human EndA cause neurological disorders including pontocerebellar hypoplasia and progressive microcephaly, and yeast EndA significantly cleaves mitochondria-localized mRNA encoding cytochrome b mRNA processing 1 (Cpb1) for mRNA maturation. This mini-review summarizes the aforementioned results, discusses their implications, and offers my personal opinion regarding future directions for the analysis of the structure and function of EndAs.

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.

Unique Archaeal Small RNAs

Advances in genome-wide sequence technologies allow for detailed insights into the complexity of RNA landscapes of organisms from all three domains of life. Recent analyses of archaeal transcriptomes identified interaction and regulation networks of noncoding RNAs in this understudied domain. Here, we review current knowledge of small, noncoding RNAs with important functions for the archaeal lifestyle, which often requires adaptation to extreme environments. One focus is RNA metabolism at elevated temperatures in hyperthermophilic archaea, which reveals elevated amounts of RNA-guided RNA modification and virus defense strategies. Genome rearrangement events result in unique fragmentation patterns of noncoding RNA genes that require elaborate maturation pathways to yield functional transcripts. RNA-binding proteins, e.g., L7Ae and LSm, are important for many posttranscriptional control functions of RNA molecules in archaeal cells. We also discuss recent insights into the regulatory potential of their noncoding RNA partners.

The RNA-splicing endonuclease from the euryarchaeaon Methanopyrus kandleri is a heterotetramer with constrained substrate specificity

Four different types (α4, α'2, (αβ)2 and ϵ2) of RNA-splicing endonucleases (EndAs) for RNA processing are known to exist in the Archaea. Only the (αβ)2 and ϵ2 types can cleave non-canonical introns in precursor (pre)-tRNA. Both enzyme types possess an insert associated with a specific loop, allowing broad substrate specificity in the catalytic α units. Here, the hyperthermophilic euryarchaeon Methanopyrus kandleri (MKA) was predicted to harbor an (αβ)2-type EndA lacking the specific loop. To characterize MKA EndA enzymatic activity, we constructed a fusion protein derived from MKA α and β subunits (fMKA EndA). In vitro assessment demonstrated complete removal of the canonical bulge-helix-bulge (BHB) intron structure from MKA pre-tRNAAsn. However, removal of the relaxed BHB structure in MKA pre-tRNAGlu was inefficient compared to crenarchaeal (αβ)2 EndA, and the ability to process the relaxed intron within mini-helix RNA was not detected. fMKA EndA X-ray structure revealed a shape similar to that of other EndA types, with no specific loop. Mapping of EndA types and their specific loops and the tRNA gene diversity among various Archaea suggest that MKA EndA is evolutionarily related to other (αβ)2-type EndAs found in the Thaumarchaeota, Crenarchaeota and Aigarchaeota but uniquely represents constrained substrate specificity.

Engineering and expressing circular RNAs via tRNA splicing

Circular (circ)RNAs have recently become a subject of great biologic interest. It is now clear that they represent a diverse and abundant class of RNAs with regulated expression and evolutionarily conserved functions. There are several mechanisms by which RNA circularization can occur in vivo. Here, we focus on the biogenesis of tRNA intronic circular RNAs (tricRNAs) in archaea and animals, and we detail their use as research tools for orthogonal, directed circRNA expression in vivo.

Transcription Regulation in Archaea

The known diversity of metabolic strategies and physiological adaptations of archaeal species to extreme environments is extraordinary. Accurate and responsive mechanisms to ensure that gene expression patterns match the needs of the cell necessitate regulatory strategies that control the activities and output of the archaeal transcription apparatus. Archaea are reliant on a single RNA polymerase for all transcription, and many of the known regulatory mechanisms employed for archaeal transcription mimic strategies also employed for eukaryotic and bacterial species. Novel mechanisms of transcription regulation have become apparent by increasingly sophisticated in vivo and in vitro investigations of archaeal species. This review emphasizes recent progress in understanding archaeal transcription regulatory mechanisms and highlights insights gained from studies of the influence of archaeal chromatin on transcription.

Eukaryotic tRNA paradox

tRNAs are widely believed to segregate into two classes, I and II. Computational analysis of eukaryotic tRNA entries in Genomic tRNA Database, however, leads to new, albeit paradoxical, presence of more than a thousand class-I tRNAs with uncharacteristic long variable arms (V-arms), like in class-II. Out of 62,202 tRNAs from 69 eukaryotes, as many as 1431 class-I tRNAs have these novel extended V-arms, and we refer to them as paradoxical tRNAs (pxtRNAs). A great majority of these 1431 pxtRNA genes are located in intergenic regions, about 18% embedded in introns of genes or ESTs, and just one in 3'UTR. A check on the conservations of 2D and 3D base pairs for each position of these pxtRNAs reveals a few variations, but they seem to have almost all the known features (already known identity and conserved elements of tRNA). Analyses of the A-Box and B-Box of these pxtRNA genes in eukaryotes display salient deviations from the previously annotated conserved features of the standard promoters, whereas the transcription termination signals are just canonical and non-canonical runs of thymidine, similar to the ones in standard tRNA genes. There is just one such pxtRNA(ProAGG) gene in the entire human genome, and the availability of data allows epigenetic analysis of this human pxtRNA(ProAGG) in three different cell lines, H1 hESC, K562, and NHEK, to assess the level of its expression. Histone acetylation and methylation of this lone pxtRNA(ProAGG) gene in human differ from that of the nine standard human tRNA(ProAGG) genes. The V-arm nucleotide sequences and their secondary structures in pxtRNA differ from that of class-II tRNA. Considering these differences, hypotheses of alternative splicing, non-canonical intron and gene transfer are examined to partially improve the Cove scores of these pxtRNAs and to critically question their antecedence and novelty.

Disrupted tRNA Genes and tRNA Fragments: A Perspective on tRNA Gene Evolution

Transfer RNAs (tRNAs) are small non-coding RNAs with lengths of approximately 70-100 nt. They are directly involved in protein synthesis by carrying amino acids to the ribosome. In this sense, tRNAs are key molecules that connect the RNA world and the protein world. Thus, study of the evolution of tRNA molecules may reveal the processes that led to the establishment of the central dogma: genetic information flows from DNA to RNA to protein. Thanks to the development of DNA sequencers in this century, we have determined a huge number of nucleotide sequences from complete genomes as well as from transcriptomes in many species. Recent analyses of these large data sets have shown that particular tRNA genes, especially in Archaea, are disrupted in unique ways: some tRNA genes contain multiple introns and some are split genes. Even tRNA molecules themselves are fragmented post-transcriptionally in many species. These fragmented small RNAs are known as tRNA-derived fragments (tRFs). In this review, I summarize the progress of research into the disrupted tRNA genes and the tRFs, and propose a possible model for the molecular evolution of tRNAs based on the concept of the combination of fragmented tRNA halves.

Handling tRNA introns, archaeal way and eukaryotic way

Introns are found in various tRNA genes in all the three kingdoms of life. Especially, archaeal and eukaryotic genomes are good sources of tRNA introns that are removed by proteinaceous splicing machinery. Most intron-containing tRNA genes both in archaea and eukaryotes possess an intron at a so-called canonical position, one nucleotide 3′ to their anticodon, while recent bioinformatics have revealed unusual types of tRNA introns and their derivatives especially in archaeal genomes. Gain and loss of tRNA introns during various stages of evolution are obvious both in archaea and eukaryotes from analyses of comparative genomics. The splicing of tRNA molecules has been studied extensively from biochemical and cell biological points of view, and such analyses of eukaryotic systems provided interesting findings in the past years. Here, I summarize recent progresses in the analyses of tRNA introns and the splicing process, and try to clarify new and old questions to be solved in the next stages.

tRNA gene diversity in the three domains of life

Transfer RNA (tRNA) is widely known for its key role in decoding mRNA into protein. Despite their necessity and relatively short nucleotide sequences, a large diversity of gene structures and RNA secondary structures of pre-tRNAs and mature tRNAs have recently been discovered in the three domains of life. Growing evidences of disrupted tRNA genes in the genomes of Archaea reveals unique gene structures such as, intron-containing tRNA, split tRNA, and permuted tRNA. Coding sequence for these tRNAs are either separated with introns, fragmented, or permuted at the genome level. Although evolutionary scenario behind the tRNA gene disruption is still unclear, diversity of tRNA structure seems to be co-evolved with their processing enzyme, so-called RNA splicing endonuclease. Metazoan mitochondrial tRNAs (mtRNAs) are known for their unique lack of either one or two arms from the typical tRNA cloverleaf structure, while still maintaining functionality. Recently identified nematode-specific V-arm containing tRNAs (nev-tRNAs) possess long variable arms that are specific to eukaryotic class II tRNA^Ser and tRNA^Leu but also decode class I tRNA codons. Moreover, many tRNA-like sequences have been found in the genomes of different organisms and viruses. Thus, this review is aimed to cover the latest knowledge on tRNA gene diversity and further recapitulate the evolutionary and biological aspects that caused such uniqueness.

Circularly permuted tRNA genes: their expression and implications for their physiological relevance and development

A number of genome analyses and searches using programs that focus on the RNA-specific bulge-helix-bulge (BHB) motif have uncovered a wide variety of disrupted tRNA genes. The results of these analyses have shown that genetic information encoding functional RNAs is described in the genome cryptically and is retrieved using various strategies. One such strategy is represented by circularly permuted tRNA genes, in which the sequences encoding the 5′-half and 3′-half of the specific tRNA are separated and inverted on the genome. Biochemical analyses have defined a processing pathway in which the termini of tRNA precursors (pre-tRNAs) are ligated to form a characteristic circular RNA intermediate, which is then cleaved at the acceptor-stem to generate the typical cloverleaf structure with functional termini. The sequences adjacent to the processing site located between the 3′-half and the 5′-half of pre-tRNAs potentially form a BHB motif, which is the dominant recognition site for the tRNA-intron splicing endonuclease, suggesting that circularization of pre-tRNAs depends on the splicing machinery. Some permuted tRNAs contain a BHB-mediated intron in their 5′- or 3′-half, meaning that removal of an intron, as well as swapping of the 5′- and 3′-halves, are required during maturation of their pre-tRNAs. To date, 34 permuted tRNA genes have been identified from six species of unicellular algae and one archaeon. Although their physiological significance and mechanism of development remain unclear, the splicing system of BHB motifs seems to have played a key role in the formation of permuted tRNA genes. In this review, current knowledge of circularly permuted tRNA genes is presented and some unanswered questions regarding these species are discussed.

Bacterial group I introns: mobile RNA catalysts

Group I introns are intervening sequences that have invaded tRNA, rRNA and protein coding genes in bacteria and their phages. The ability of group I introns to self-splice from their host transcripts, by acting as ribozymes, potentially renders their insertion into genes phenotypically neutral. Some group I introns are mobile genetic elements due to encoded homing endonuclease genes that function in DNA-based mobility pathways to promote spread to intronless alleles. Group I introns have a limited distribution among bacteria and the current assumption is that they are benign selfish elements, although some introns and homing endonucleases are a source of genetic novelty as they have been co-opted by host genomes to provide regulatory functions. Questions regarding the origin and maintenance of group I introns among the bacteria and phages are also addressed.

Identification of highly-disrupted tRNA genes in nuclear genome of the red alga, Cyanidioschyzon merolae 10D

The limited locations of tRNA introns are crucial for eukaryal tRNA-splicing endonuclease recognition. However, our analysis of the nuclear genome of an early-diverged red alga, Cyanidioschyzon merolae, demonstrated the first evidence of nuclear-encoded tRNA genes that contain ectopic and/or multiple introns. Some genes exhibited both intronic and permuted structures in which the 3′-half of the tRNA coding sequence lies upstream of the 5′-half, and an intron is inserted into either half. These highly disrupted tRNA genes, which account for 63% of all nuclear tRNA genes, are expressed via the orderly and sequential processing of bulge-helix-bulge (BHB) motifs at intron-exon junctions and termini of permuted tRNA precursors, probably by a C. merolae tRNA-splicing endonuclease with an unidentified subunit architecture. The results revealed a considerable diversity in eukaryal tRNA intron properties and endonuclease architectures, which will help to elucidate the acquisition mechanism of the BHB-mediated disrupted tRNA genes.

C/D box sRNA, CRISPR RNA and tRNA processing in an archaeon with a minimal fragmented genome

The analysis of deep sequencing data allows for a genome-wide overview of all the small RNA molecules (the 'sRNome') that are present in a single organism. In the present paper, we review the processing of CRISPR (clustered regularly interspaced short palindromic repeats) RNA, C/D box sRNA (small non-coding RNA) and tRNA in Nanoarchaeum equitans. The minimal and fragmented genome of this tiny archaeon permits a sequencing depth that enables the identification of processing intermediates in the study of RNA processing pathways. These intermediates include circular C/D box sRNA molecules and tRNA half precursors.

Avatar pre-tRNAs help elucidate the properties of tRNA-splicing endonucleases that produce tRNA from permuted genes

Unusual tRNA genes, found in some algae, have their mature terminal 3' portion in front of their 5' portion in the genome. The transcripts from such genes must be cleaved by a pre-tRNA endonuclease to form a functional tRNA. We present a mechanism for the generation of "corrected" tRNAs from such a "permuted" pre-tRNA configuration. We used two avatar (av) or model pre-tRNAs and two splicing endonucleases with distinct mechanisms of recognition of the pre-tRNA. The splicing results are compatible with an evolutionary route in which permuted genes result from a duplication event followed by DNA rearrangement. The model pre-tRNAs permit description of the features that a transcript, derived from a rearranged duplicated gene, must have to give rise to functional tRNA. The two tRNA endonucleases are a eukaryal enzyme that normally acts in a mature domain-dependent mode and an archaeal enzyme that acts in a mature domain-independent mode. Both av pre-tRNAs are able to fold into two conformations: 1 and 2. We find that only conformation 2 can yield a corrected functional tRNA. This result is consistent with contemporary algae representing snapshots of different evolutionary stages, with duplicated genes preceding recombinatorial events generating a permutated gene. In a scenario elucidated by the use of the av pre-tRNAs, algal permuted tRNA genes could have further lost one of two mature domains, eliminating steric problems for the algal tRNA endonuclease, which remains a typical eukaryal enzyme capable of correcting the permuted transcript to a functional tRNA.

X-ray structure of the fourth type of archaeal tRNA splicing endonuclease: insights into the evolution of a novel three-unit composition and a unique loop involved in broad substrate specificity

Cleavage of introns from precursor transfer RNAs (tRNAs) by tRNA splicing endonuclease (EndA) is essential for tRNA maturation in Archaea and Eukarya. In the past, archaeal EndAs were classified into three types (α′₂, α₄ and α₂β₂) according to subunit composition. Recently, we have identified a fourth type of archaeal EndA from an uncultivated archaeon Candidatus Micrarchaeum acidiphilum, referred to as ARMAN-2, which is deeply branched within Euryarchaea. The ARMAN-2 EndA forms an ε₂ homodimer and has broad substrate specificity like the α₂β₂ type EndAs found in Crenarchaea and Nanoarchaea. However, the precise architecture of ARMAN-2 EndA was unknown. Here, we report the crystal structure of the ε₂ homodimer of ARMAN-2 EndA. The structure reveals that the ε protomer is separated into three novel units (α^N, α and β^C) fused by two distinct linkers, although the overall structure of ARMAN-2 EndA is similar to those of the other three types of archaeal EndAs. Structural comparison and mutational analyses reveal that an ARMAN-2 type-specific loop (ASL) is involved in the broad substrate specificity and that K161 in the ASL functions as the RNA recognition site. These findings suggest that the broad substrate specificities of ε₂ and α₂β₂ EndAs were separately acquired through different evolutionary processes.

RNA processing in the minimal organism Nanoarchaeum equitans

BACKGROUND

The minimal genome of the tiny, hyperthermophilic archaeon Nanoarchaeum equitans contains several fragmented genes and revealed unusual RNA processing pathways. These include the maturation of tRNA molecules via the trans-splicing of tRNA halves and genomic rearrangements to compensate for the absence of RNase P.

RESULTS

Here, the RNA processing events in the N. equitans cell are analyzed using RNA-Seq deep sequencing methodology. All tRNA half precursor and tRNA termini were determined and support the tRNA trans-splicing model. The processing of CRISPR RNAs from two CRISPR clusters was verified. Twenty-seven C/D box small RNAs (sRNAs) and a H/ACA box sRNA were identified. The C/D box sRNAs were found to flank split genes, to form dicistronic tRNA-sRNA precursors and to be encoded within the tRNAMet intron.

CONCLUSIONS

The presented data provide an overview of the production and usage of small RNAs in a cell that has to survive with a highly reduced genome. N. equitans lost many essential metabolic pathways but maintains highly active CRISPR/Cas and rRNA modification systems that appear to play an important role in genome fragmentation.

Diversity and roles of (t)RNA ligases

The discovery of discontiguous tRNA genes triggered studies dissecting the process of tRNA splicing. As a result, we have gained detailed mechanistic knowledge on enzymatic removal of tRNA introns catalyzed by endonuclease and ligase proteins. In addition to the elucidation of tRNA processing, these studies facilitated the discovery of additional functions of RNA ligases such as RNA repair and non-conventional mRNA splicing events. Recently, the identification of a new type of RNA ligases in bacteria, archaea, and humans closed a long-standing gap in the field of tRNA processing. This review summarizes past and recent findings in the field of tRNA splicing with a focus on RNA ligation as it preferentially occurs in archaea and humans. In addition to providing an integrated view of the types and phyletic distribution of RNA ligase proteins known to date, this survey also aims at highlighting known and potential accessory biological functions of RNA ligases.

Genomic heterogeneity in a natural archaeal population suggests a model of tRNA gene disruption

Understanding the mechanistic basis of the disruption of tRNA genes, as manifested in the intron-containing and split tRNAs found in Archaea, will provide considerable insight into the evolution of the tRNA molecule. However, the evolutionary processes underlying these disruptions have not yet been identified. Previously, a composite genome of the deep-branching archaeon Caldiarchaeum subterraneum was reconstructed from a community genomic library prepared from a C. subterraneum–dominated microbial mat. Here, exploration of tRNA genes from the library reveals that there are at least three types of heterogeneity at the tRNA^Thr(GGU) gene locus in the Caldiarchaeum population. All three involve intronic gain and splitting of the tRNA gene. Of two fosmid clones found that encode tRNA^Thr(GGU), one (tRNA^Thr-I) contains a single intron, whereas another (tRNA^Thr-II) contains two introns. Notably, in the clone possessing tRNA^Thr-II, a 5′ fragment of the tRNA^Thr-I (tRNA^Thr-F) gene was observed 1.8-kb upstream of tRNA^Thr-II. The composite genome contains both tRNA^Thr-II and tRNA^Thr-F, although the loci are >500 kb apart. Given that the 1.8-kb sequence flanked by tRNA^Thr-F and tRNA^Thr-II is predicted to encode a DNA recombinase and occurs in six regions of the composite genome, it may be a transposable element. Furthermore, its dinucleotide composition is most similar to that of the pNOB8-type plasmid, which is known to integrate into archaeal tRNA genes. Based on these results, we propose that the gain of the tRNA intron and the scattering of the tRNA fragment occurred within a short time frame via the integration and recombination of a mobile genetic element.

A novel three-unit tRNA splicing endonuclease found in ultrasmall Archaea possesses broad substrate specificity

tRNA splicing endonucleases, essential enzymes found in Archaea and Eukaryotes, are involved in the processing of pre-tRNA molecules. In Archaea, three types of splicing endonuclease [homotetrameric: α₄, homodimeric: α₂, and heterotetrameric: (αβ)₂] have been identified, each representing different substrate specificity during the tRNA intron cleavage. Here, we discovered a fourth type of archaeal tRNA splicing endonuclease (ε₂) in the genome of the acidophilic archaeon Candidatus Micrarchaeum acidiphilum, referred to as ARMAN-2 and its closely related species, ARMAN-1. The enzyme consists of two duplicated catalytic units and one structural unit encoded on a single gene, representing a novel three-unit architecture. Homodimeric formation was confirmed by cross-linking assay, and site-directed mutagenesis determined that the conserved L10-pocket interaction between catalytic and structural unit is necessary for the assembly. A tRNA splicing assay reveal that ε₂ endonuclease cleaves both canonical and non-canonical bulge–helix–bulge motifs, similar to that of (αβ)₂ endonuclease. Unlike other ARMAN and Euryarchaeota, tRNAs found in ARMAN-2 are highly disrupted by introns at various positions, which again resemble the properties of archaeal species with (αβ)₂ endonuclease. Thus, the discovery of ε₂ endonuclease in an archaeon deeply branched within Euryarchaeota represents a new example of the coevolution of tRNA and their processing enzymes.

Cleavage of intron from the standard or non-standard position of the precursor tRNA by the splicing endonuclease of Aeropyrum pernix, a hyper-thermophilic Crenarchaeon, involves a novel RNA recognition site in the Crenarchaea specific loop

In Crenarchaea, several tRNA genes are predicted to express precursor-tRNAs (pre-tRNAs) with canonical or non-canonical introns at various positions. We initially focused on the tRNA(Thr) species of hyperthermophilic crenarchaeon, Aeropyrum pernix (APE) and found that in the living APE cells three tRNA(Thr) species were transcribed and subsequently matured to functional tRNAs. During maturation, introns in two of them were cleaved from standard and non-standard positions. Biochemical studies revealed that the APE splicing endonuclease (APE-EndA) removed both types of introns, including the non-canonical introns, without any nucleotide modification. To clarify the underlying reasons for broad substrate specificity of APE-EndA, we determined the crystal structure of wild-type APE-EndA and subsequently compared its structure with that of Archaeaoglobus fulgidus (AFU)-EndA, which has narrow substrate specificity. Remarkably, structural comparison revealed that APE-EndA possesses a Crenarchaea specific loop (CSL). Introduction of CSL into AFU-EndA enhanced its intron-cleaving activity irrespective of the position or motif of the intron. Thus, our biochemical and crystallographic analyses of the chimera-EndA demonstrated that the CSL is responsible for the broad substrate specificity of APE-EndA. Furthermore, mutagenesis studies revealed that Lys44 in CSL functions as the RNA recognition site.

Evolution of introns in the archaeal world

The self-splicing group I introns are removed by an autocatalytic mechanism that involves a series of transesterification reactions. They require RNA binding proteins to act as chaperones to correctly fold the RNA into an active intermediate structure in vivo. Pre-tRNA introns in Bacteria and in higher eukaryote plastids are typical examples of self-splicing group I introns. By contrast, two striking features characterize RNA splicing in the archaeal world. First, self-splicing group I introns cannot be found, to this date, in that kingdom. Second, the RNA splicing scenario in Archaea is uniform: All introns, whether in pre-tRNA or elsewhere, are removed by tRNA splicing endonucleases. We suggest that in Archaea, the protein recruited for splicing is the preexisting tRNA splicing endonuclease and that this enzyme, together with the ligase, takes over the task of intron removal in a more efficient fashion than the ribozyme. The extinction of group I introns in Archaea would then be a consequence of recruitment of the tRNA splicing endonuclease. We deal here with comparative genome analysis, focusing specifically on the integration of introns into genes coding for 23S rRNA molecules, and how this newly acquired intron has to be removed to regenerate a functional RNA molecule. We show that all known oligomeric structures of the endonuclease can recognize and cleave a ribosomal intron, even when the endonuclease derives from a strain lacking rRNA introns. The persistence of group I introns in mitochondria and chloroplasts would be explained by the inaccessibility of these introns to the endonuclease.

A conserved lysine residue in the crenarchaea-specific loop is important for the crenarchaeal splicing endonuclease activity

In Archaea, splicing endonuclease (EndA) recognizes and cleaves precursor RNAs to remove introns. Currently, EndAs are classified into three families according to their subunit structures: homotetramer, homodimer, and heterotetramer. The crenarchaeal heterotetrameric EndAs can be further classified into two subfamilies based on the size of the structural subunit. Subfamily A possesses a structural subunit similar in size to the catalytic subunit, whereas subfamily B possesses a structural subunit significantly smaller than the catalytic subunit. Previously, we solved the crystal structure of an EndA from Pyrobaculum aerophilum. The endonuclease was classified into subfamily B, and the structure revealed that the enzyme lacks an N-terminal subdomain in the structural subunit. However, no structural information is available for crenarchaeal heterotetrameric EndAs that are predicted to belong to subfamily A. Here, we report the crystal structure of the EndA from Aeropyrum pernix, which is predicted to belong to subfamily A. The enzyme possesses the N-terminal subdomain in the structural subunit, revealing that the two subfamilies of heterotetrameric EndAs are structurally distinct. EndA from A. pernix also possesses an extra loop region that is characteristic of crenarchaeal EndAs. Our mutational study revealed that the conserved lysine residue in the loop is important for endonuclease activity. Furthermore, the sequence characteristics of the loops and the positions towards the substrate RNA according to a docking model prompted us to propose that crenarchaea-specific loops and an extra amino acid sequence at the catalytic loop of nanoarchaeal EndA are derived by independent convergent evolution and function for recognizing noncanonical bulge-helix-bulge motif RNAs as substrates.

Large-scale tRNA intron transposition in the archaeal order Thermoproteales represents a novel mechanism of intron gain

Recently, diverse arrangements of transfer RNA (tRNA) genes have been found in the domain Archaea, in which the tRNA is interrupted by a maximum of three introns or is even fragmented into two or three genes. Whereas most of the eukaryotic tRNA introns are inserted strictly at the canonical nucleotide position (37/38), archaeal intron-containing tRNAs have a wide diversity of small tRNA introns, which differ in their numbers and locations. This feature is especially pronounced in the archaeal order Thermoproteales. In this study, we performed a comprehensive sequence comparison of 286 tRNA introns and their genes in seven Thermoproteales species to clarify how these introns have emerged and diversified during tRNA gene evolution. We identified 46 intron groups containing sets of highly similar sequences (>70%) and showed that 16 of them contain sequences from evolutionarily distinct tRNA genes. The phylogeny of these 16 intron groups indicates that transposition events have occurred at least seven times throughout the evolution of Thermoproteales. These findings suggest that frequent intron transposition occurs among the tRNA genes of Thermoproteales. Further computational analysis revealed limited insertion positions and corresponding amino acid types of tRNA genes. This has arisen because the bulge-helix-bulge splicing motif is required at the newly transposed position if the pre-tRNA is to be correctly processed. These results clearly demonstrate a newly identified mechanism that facilitates the late gain of short introns at various noncanonical positions in archaeal tRNAs.

Transfer RNA processing in archaea: unusual pathways and enzymes

Transfer RNA (tRNA) molecules are highly conserved in length, sequence and structure in order to be functional in the ribosome. However, mostly in archaea, the short genes encoding tRNAs can be found disrupted, fragmented, with permutations or with non-functional mutations of conserved nucleotides. Here, we give an overview of recently discovered tRNA maturation pathways that require intricate processing steps to finally generate the standard tRNA from these unusual tRNA genes.

Permuted tRNA genes in the nuclear and nucleomorph genomes of photosynthetic eukaryotes

Transfer RNA (tRNA) is a central genetic element in the decoding of genome information for all of Earth's life forms. Nevertheless, there are a great number of missing tRNAs that have been left without examination, especially in microbial genomes. Two tRNA gene families remarkable in their structure and expression mechanism have been reported: split and permuted tRNAs. Split tRNAs in archaea are encoded on the genome as two or three fragmented genes and then processed into single tRNA molecules. Permuted tRNAs are organized with the 5' and 3' halves of the gene positioned in reverse on the genome and hitherto have been found only in one tiny red alga. Here we reveal a wide-ranging distribution of permuted tRNA genes in the genomes of photosynthetic eukaryotes. This includes in the smallest eukaryotic genome known to date, the nucleomorph genome of the chlorarachniophyte alga Bigelowiella natans. Comparison between cDNA and genomic DNA sequences of two nucleomorph-encoded tRNA(Ser) genes confirms that precursors are circularized and processed into mature tRNA molecules in vivo. In the tRNA(Ser)(AGA), adenine at the wobble position of the codon is likely modified to inosine to expand capacity of the codon recognition. We also show the presence of permuted tRNAs in the ultrasmall free-living green algae Ostreococcus and Micromonas, which are closely related to the B. natans nucleomorph. Conserved intron/leader sequence structures in the intron-containing and permuted tRNAs suggest the ancient origin of the splicing machinery in the common ancestor of eukaryotes and archaea. Meanwhile, a wide but patchy distribution of permuted tRNA genes in the photosynthetic eukaryotes implies that extant permuted tRNAs might have emerged multiple times. Taken together, our data demonstrate that permuted tRNA is an evolutionarily conserved and fundamental element in tiny eukaryotic genomes.

Processing of multiple-intron-containing pretRNA

Computational studies predict the simultaneous presence of two and even three introns in certain crenarchaeal tRNA genes. In these multiple-intron-containing pretRNAs, the introns are nested one inside the other and the pretRNA folds into a conformation that is anticipated to allow splicing of the last intron only after splicing the others. A set of operations, each consisting of two cleavages and one ligation, therefore needs to be carried out sequentially. PretRNAs containing multiple introns are predicted to fold, forming bulge-helix-bulge (BHB) and BHB-like motifs. The tRNA splicing endonuclease should recognize these motifs. To test this hypothetical scenario, we used the homotetrameric enzyme from Methanocaldococcus jannaschii (METJA) and the heterotetrameric enzyme from Sulfolobus solfataricus (SULSO). On the basis of our previous studies, the METJA enzyme should cleave only the BHB structure motif, while the SULSO enzyme can in addition cleave variant substrate structures, like the bulge-helix-loop (BHL). We show here that the processing of multiple-intron-containing pretRNA can be observed in vitro.

Disrupted tRNA gene diversity and possible evolutionary scenarios

The following unusual tRNAs have recently been discovered in the genomes of Archaea and primitive Eukaryota: multiple-intron-containing tRNAs, which have more than one intron; split tRNAs, which are produced from two pieces of RNA transcribed from separate genes; tri-split tRNAs, which are produced from three separate genes; and permuted tRNA, in which the 5' and 3' halves are encoded with permuted orientations within a single gene. All these disrupted tRNA genes can form mature contiguous tRNA, which is aminoacylated after processing by cis or trans splicing. The discovery of such tRNA disruptions has raised the question of when and why these complex tRNA processing pathways emerged during the evolution of life. Many previous reports have noted that tRNA genes contain a single intron in the anticodon loop region, a feature common throughout all three domains of life, suggesting an ancient trait of the last universal common ancestor. In this context, these unique tRNA disruptions recently found only in Archaea and primitive Eukaryota provide new insight into the origin and evolution of tRNA genes, encouraging further research in this field. In this paper, we summarize the phylogeny, structure, and processing machinery of all known types of disrupted tRNAs and discuss possible evolutionary scenarios for these tRNA genes.

Messenger RNA processing in Methanocaldococcus (Methanococcus) jannaschii

Messenger RNA (mRNA) processing plays important roles in gene expression in all domains of life. A number of cases of mRNA cleavage have been documented in Archaea, but available data are fragmentary. We have examined RNAs present in Methanocaldococcus (Methanococcus) jannaschii for evidence of RNA processing upstream of protein-coding genes. Of 123 regions covered by the data, 31 were found to be processed, with 30 including a cleavage site 12-16 nucleotides upstream of the corresponding translation start site. Analyses with 3'-RACE (rapid amplification of cDNA ends) and 5'-RACE indicate that the processing is endonucleolytic. Analyses of the sequences surrounding the processing sites for functional sites, sequence motifs, or potential RNA secondary structure elements did not reveal any recurring features except for an AUG translation start codon and (in most cases) a ribosome binding site. These properties differ from those of all previously described mRNA processing systems. Our data suggest that the processing alters the representation of various genes in the RNA pool and therefore, may play a significant role in defining the balance of proteins in the cell.

Gain and loss of an intron in a protein-coding gene in Archaea: the case of an archaeal RNA pseudouridine synthase gene

Background

We previously found the first examples of splicing of archaeal pre-mRNAs for homologs of the eukaryotic CBF5 protein (also known as dyskerin in humans) in Aeropyrum pernix, Sulfolobus solfataricus, S. tokodaii, and S. acidocaldarirus, and also showed that crenarchaeal species in orders Desulfurococcales and Sulfolobales, except for Hyperthermus butylicus, Pyrodictium occultum, Pyrolobus fumarii, and Ignicoccus islandicus, contain the (putative) cbf5 intron. However, the exact timing of the intron insertion was not determined and verification of the putative secondary loss of the intron in some lineages was not performed.

Results

In the present study, we determined approximately two-thirds of the entire coding region of crenarchaeal Cbf5 sequences from 43 species. A phylogenetic analysis of our data and information from the available genome sequences suggested that the (putative) cbf5 intron existed in the common ancestor of the orders Desulfurococcales and Sulfolobales and that probably at least two independent lineages in the order Desulfurococcales lost the (putative) intron.

Conclusion

This finding is the first observation of a lineage-specific loss of a pre-mRNA intron in Archaea. As the insertion or deletion of introns in protein-coding genes in Archaea has not yet been seriously considered, our finding suggests the possible difficulty of accurately and completely predicting protein-coding genes in Archaea.

Crystal structure and assembly of the functional Nanoarchaeum equitans tRNA splicing endonuclease

The RNA splicing and processing endonuclease from Nanoarchaeum equitans (NEQ) belongs to the recently identified (αβ)₂ family of splicing endonucleases that require two different subunits for splicing activity. N. equitans splicing endonuclease comprises the catalytic subunit (NEQ205) and the structural subunit (NEQ261). Here, we report the crystal structure of the functional NEQ enzyme at 2.1 Å containing both subunits, as well as that of the NEQ261 subunit alone at 2.2 Å. The functional enzyme resembles previously known α₂ and α₄ endonucleases but forms a heterotetramer: a dimer of two heterodimers of the catalytic subunit (NEQ205) and the structural subunit (NEQ261). Surprisingly, NEQ261 alone forms a homodimer, similar to the previously known homodimer of the catalytic subunit. The homodimers of isolated subunits are inhibitory to heterodimerization as illustrated by a covalently linked catalytic homodimer that had no RNA cleavage activity upon mixing with the structural subunit. Detailed structural comparison reveals a more favorable hetero- than homodimerization interface, thereby suggesting a possible regulation mechanism of enzyme assembly through available subunits. Finally, the uniquely flexible active site of the NEQ endonuclease provides a possible explanation for its broader substrate specificity.

Functional importance of crenarchaea-specific extra-loop revealed by an X-ray structure of a heterotetrameric crenarchaeal splicing endonuclease

Archaeal splicing endonucleases (EndAs) are currently classified into three groups. Two groups require a single subunit protein to form a homodimer or homotetramer. The third group requires two nonidentical protein components for the activity. To elucidate the molecular architecture of the two-subunit EndA system, we studied a crenarchaeal splicing endonuclease from Pyrobaculum aerophilum. In the present study, we solved a crystal structure of the enzyme at 1.7-Å resolution. The enzyme adopts a heterotetrameric form composed of two catalytic and two structural subunits. By connecting the structural and the catalytic subunits of the heterotetrameric EndA, we could convert the enzyme to a homodimer that maintains the broad substrate specificity that is one of the characteristics of heterotetrameric EndA. Meanwhile, a deletion of six amino acids in a Crenarchaea-specific loop abolished the endonuclease activity even on a substrate with canonical BHB motif. These results indicate that the subunit architecture is not a major factor responsible for the difference of substrate specificity between single- and two-subunit EndA systems. Rather, the structural basis for the broad substrate specificity is built into the crenarchaeal splicing endonuclease itself.

Characterization of a heat-stable enzyme possessing GTP-dependent RNA ligase activity from a hyperthermophilic archaeon, Pyrococcus furiosus

Using an expression protein library of a hyperthermophilic archaeon, Pyrococcus furiosus, we identified a gene (PF0027) that encodes a protein with heat-stable cyclic nucleotide phosphodiesterase (CPDase) activity. The PF0027 gene encoded a 21-kDa protein and an amino acid sequence that showed approximately 27% identity to that of the 2'-5' tRNA ligase protein, ligT (20 kDa), from Escherichia coli. We found that the purified PF0027 protein possessed GTP-dependent RNA ligase activity and that synthetic tRNA halves bearing 2',3'-cyclic phosphate and 5'-OH termini were substrates for the ligation reaction in vitro. GTP hydrolysis was not required for the reaction, and GTPgammaS enhanced the tRNA ligation activity of PF0027 protein, suggesting that the ligation step is regulated by a novel mechanism. In comparison to the strong CPDase activity of the PF0027 protein, the RNA ligase activity itself was quite weak, and the ligation product was unstable during in vitro reaction. Finally, we used NMR to determine the solution structure of the PF0027 protein and discuss the implications of our results in understanding the role of the PF0027 protein.

Tri-split tRNA is a transfer RNA made from 3 transcripts that provides insight into the evolution of fragmented tRNAs in archaea

Transfer RNA (tRNA) is essential for decoding the genome sequence into proteins. In Archaea, previous studies have revealed unique multiple intron-containing tRNAs and tRNAs that are encoded on 2 separate genes, so-called split tRNAs. Here, we discovered 10 fragmented tRNA genes in the complete genome of the hyperthermoacidophilic Archaeon Caldivirga maquilingensis that are individually transcribed and further trans-spliced to generate all of the missing tRNAs encoding glycine, alanine, and glutamate. Notably, the 3 mature tRNA(Gly)'s with synonymous codons are created from 1 constitutive 3' half transcript and 4 alternatively switching transcripts, representing tRNA made from a total of 3 transcripts named a "tri-split tRNA." Expression and nucleotide sequences of 10 split tRNA genes and their joined tRNA products were experimentally verified. The intervening sequences of split tRNA have high identity to tRNA intron sequences located at the same positions in intron-containing tRNAs in related Thermoproteales species. This suggests that an evolutionary relationship between intron-containing and split tRNAs exists. Our findings demonstrate the first example of split tRNA genes in a free-living organism and a unique tri-split tRNA gene that provides further insight into the evolution of fragmented tRNAs.

Comprehensive analysis of archaeal tRNA genes reveals rapid increase of tRNA introns in the order thermoproteales

The analysis of archaeal tRNA genes is becoming more important to evaluate the origin and evolution of tRNA molecule. Even with the recent accumulation of complete genomes of numerous archaeal species, several tRNA genes are still required for a full complement of the codon table. We conducted comprehensive screening of tRNA genes from 47 archaeal genomes by using a combination of different types of tRNA prediction programs and extracted a total of 2,143 reliable tRNA gene candidates including 437 intron-containing tRNA genes, which covered more than 99.9% of the codon tables in Archaea. Previously, the content of intron-containing tRNA genes in Archaea was estimated to be approximately 15% of the whole tRNA genes, and most of the introns were known to be located at canonical positions (nucleotide position between 37 and 38) of precursor tRNA (pre-tRNA). Surprisingly, we observed marked enrichment of tRNA introns in five species of the archaeal order Thermoproteales; about 70% of tRNA gene candidates were found to be intron-containing tRNA genes, half of which contained multiple introns, and the introns were located at various noncanonical positions. Sequence similarity analysis revealed that approximately half of the tRNA introns found at Thermoproteales-specific intron locations were highly conserved among several tRNA genes. Intriguingly, identical tRNA intron sequences were found within different types of tRNA genes that completely lacked exon sequence similarity, suggesting that the tRNA introns in Thermoproteales could have been gained via intron insertion events at a later stage of tRNA evolution. Moreover, although the CCA sequence at the 3' terminal of pre-tRNA is added by a CCA-adding enzyme after gene transcription in Archaea, most of the tRNA genes containing highly conserved introns already encode the CCA sequence at their 3' terminal. Based on these results, we propose possible models explaining the rapid increase of tRNA introns as a result of intron insertion events via retrotransposition of pre-tRNAs. The sequences and secondary structures of the tRNA genes and their bulge-helix-bulge motifs were registered in SPLITSdb (http://splits.iab.keio.ac.jp/splitsdb/), a novel and comprehensive database for archaeal tRNA genes.

Structural determinants characteristic to AARS subclasses and tRNA-splicing endonuclease in eukaryotes

We compare and analyse the whole set of cytoplasmic, nonorganellar transfer RNA genes from 22 eukaryal genomes. Grouping this whole set into sets of isoacceptors we have elucidated structural elements that are characteristic to individual isoacceptor tRNAs within the subclasses of aminoacyl tRNA synthetases. Further, we have observed structural motifs straddling the exon-intron boundaries, which includes selective occurrence of both symmetric- 3-4-3 and asymmetric-3-4-(4, 5) bulge-helix-bulge-like structural motifs. Among all the tRNA isoacceptors, Ile, Leu, Ser, Pro, Met, Arg, and Tyr harbor BHB-like secondary structures at exon-intron boundaries. The structural signatories at the exon-intron boundaries appear to contribute to the specificity of splice site recognition.

Cis- and trans-splicing of mRNAs mediated by tRNA sequences in eukaryotic cells

The formation of chimeric mRNAs is a strategy used by human cells to increase the complexity of their proteome, as revealed by the ENCODE project. Here, we use Saccharomyces cerevisiae to show a way by which trans-spliced mRNAs can be generated. We demonstrate that a pretRNA inserted into a premRNA context directs the splicing reaction precisely to the sites of the tRNA intron. A suppressor pretRNA gene was inserted, in cis, into the sequence encoding the third cytoplasmic loop of the Ste2 or Ste3 G protein-coupled receptor. The hybrid RNAs are spliced at the specific pretRNA splicing sites, releasing both functional tRNAs that suppress nonsense mutations and translatable mRNAs that activate the signal transduction pathway. The RNA molecules extracted from yeast cells were amplified by RT-PCR, and their sequences were determined, confirming the identity of the splice junctions. We then constructed two fusions between the premRNA sequence (STE2 or STE3) and the 5'- or 3'-pretRNA half, so that the two hybrid RNAs can associate with each other, in trans, through their tRNA halves. Splicing occurs at the predicted pretRNA sites, producing a chimeric STE3-STE2 receptor mRNA. RNA trans-splicing mediated by tRNA sequences, therefore, is a mechanism capable of producing new kinds of RNAs, which could code for novel proteins.

Sequence evidence in the archaeal genomes that tRNAs emerged through the combination of ancestral genes as 5' and 3' tRNA halves

The discovery of separate 5′ and 3′ halves of transfer RNA (tRNA) molecules—so-called split tRNA—in the archaeal parasite Nanoarchaeum equitans made us wonder whether ancestral tRNA was encoded on 1 or 2 genes. We performed a comprehensive phylogenetic analysis of tRNAs in 45 archaeal species to explore the relationship between the three types of tRNAs (nonintronic, intronic and split). We classified 1953 mature tRNA sequences into 22 clusters. All split tRNAs have shown phylogenetic relationships with other tRNAs possessing the same anticodon. We also mimicked split tRNA by artificially separating the tRNA sequences of 7 primitive archaeal species at the anticodon and analyzed the sequence similarity and diversity of the 5′ and 3′ tRNA halves. Network analysis revealed specific characteristics of and topological differences between the 5′ and 3′ tRNA halves: the 5′ half sequences were categorized into 6 distinct groups with a sequence similarity of >80%, while the 3′ half sequences were categorized into 9 groups with a higher sequence similarity of >88%, suggesting different evolutionary backgrounds of the 2 halves. Furthermore, the combinations of 5′ and 3′ halves corresponded with the variation of amino acids in the codon table. We found not only universally conserved combinations of 5′–3′ tRNA halves in tRNA^iMet, tRNA^Thr, tRNA^Ile, tRNA^Gly, tRNA^Gln, tRNA^Glu, tRNA^Asp, tRNA^Lys, tRNA^Arg and tRNA^Leu but also phylum-specific combinations in tRNA^Pro, tRNA^Ala, and tRNA^Trp. Our results support the idea that tRNA emerged through the combination of separate genes and explain the sequence diversity that arose during archaeal tRNA evolution.

Identification and characterization of a tRNA decoding the rare AUA codon in Haloarcula marismortui

Annotation of the complete genome of the extreme halophilic archaeon Haloarcula marismortui does not include a tRNA for translation of AUA, the rare codon for isoleucine. This is a situation typical for most archaeal genomes sequenced to date. Based on computational analysis, it has been proposed recently that a single intron-containing tRNA gene produces two very similar but functionally different tRNAs by means of alternative splicing; a UGG-decoding tRNA(TrpCCA) and an AUA-decoding tRNA(IleUAU). Through analysis of tRNAs from H. marismortui, we have confirmed the presence of tRNA(TrpCCA), but found no evidence for the presence of tRNA(IleUAU). Instead, we have shown that a tRNA, currently annotated as elongator methionine tRNA and containing CAU as the anticodon, is aminoacylated with isoleucine in vivo and that this tRNA represents the missing isoleucine tRNA. Interestingly, this tRNA carries a base modification of C34 in the anticodon different from the well-known lysidine found in eubacteria, which switches the amino acid identity of the tRNA from methionine to isoleucine and its decoding specificity from AUG to AUA. The methods described in this work for the identification of individual tRNAs present in H. marismortui provide the tools necessary for experimentally confirming the presence of any tRNA in a cell and, thereby, to test computational predictions of tRNA genes.

Structural and mutational analysis of tRNA intron-splicing endonuclease from Thermoplasma acidophilum DSM 1728: catalytic mechanism of tRNA intron-splicing endonucleases

In archaea, RNA endonucleases that act specifically on RNA with bulge-helix-bulge motifs play the main role in the recognition and excision of introns, while the eukaryal enzymes use a measuring mechanism to determine the positions of the universally positioned splice sites relative to the conserved domain of pre-tRNA. Two crystallographic structures of tRNA intron-splicing endonuclease from Thermoplasma acidophilum DSM 1728 (EndA(Ta)) have been solved to 2.5-A and 2.7-A resolution by molecular replacement, using the 2.7-A resolution data as the initial model and the single-wavelength anomalous-dispersion phasing method using selenomethionine as anomalous signals, respectively. The models show that EndA(Ta) is a homodimer and that it has overall folding similar to that of other archaeal tRNA endonucleases. From structural and mutational analyses of H236A, Y229F, and K265I in vitro, we have demonstrated that they play critical roles in recognizing the splice site and in cleaving the pre-tRNA substrate.

The dawn of dominance by the mature domain in tRNA splicing

The relationship between enzyme architecture and substrate specificity among archaeal pre-tRNA splicing endonucleases has been investigated more deeply, by using biochemical assays and model building. The enzyme from Archeoglobus fulgidus (AF) is particularly interesting: it cleaves the bulge-helix-bulge target without requiring the mature tRNA domain, but, when the target is a bulge-helix-loop, the mature domain is required. A model of AF based on its electrostatic potential shows three polar patches interacting with the pre-tRNA substrate. A simple deletion mutant of the AF endonuclease lacking two of the three polar patches no longer cleaves the bulge-helix-loop substrate with or without the mature domain. This single deletion shows a possible path for the evolution of eukaryal splicing endonucleases from the archaeal enzyme.

Three-dimensional structure determined for a subunit of human tRNA splicing endonuclease (Sen15) reveals a novel dimeric fold

Splicing of eukaryal intron-containing tRNAs requires the action of the heterotetrameric splicing endonuclease, which is composed of two catalytic subunits, Sen34 and Sen2, and two structural subunits, Sen15 and Sen54. Here we report the solution structure of the human tRNA splicing endonuclease subunit HsSen15. To facilitate the structure determination, we removed the disordered 35 N-terminal and 14 C-terminal residues of the full-length protein to produce HsSen15(36-157). The structure of HsSen15(36-157), the first for a subunit of a eukaryal splicing endonuclease, revealed that the protein possesses a novel homodimeric fold. Each monomer consists of three alpha-helices and a mixed antiparallel/parallel beta-sheet, arranged in a topology similar to that of the C-terminal domain of Methanocaldococcus jannaschii endonuclease. The dimeric interface is dominated by a beta-barrel structure, formed by face-to-face packing of two, three-stranded beta-sheets. Each of the beta-sheets results from reciprocal parallel pairing of one beta-strand from one subunit with two other beta-strands from the symmetric subunit. The structural model provides insights into the functional assembly of the human tRNA splicing endonuclease.