Deficiency of the tRNATyr:Psi 35-synthase aPus7 in Archaea of the Sulfolobales order might be rescued by the H/ACA sRNA-guided machinery

Landscape of RNA pseudouridylation in archaeon Sulfolobus islandicus

Pseudouridine, one of the most abundant RNA modifications, is synthesized by stand-alone or RNA-guided pseudouridine synthases. Here, we comprehensively mapped pseudouridines in rRNAs, tRNAs and small RNAs in the archaeon Sulfolobus islandicus and identified Cbf5-associated H/ACA RNAs. Through genetic deletion and in vitro modification assays, we determined the responsible enzymes for these modifications. The pseudouridylation machinery in S. islandicus consists of the stand-alone enzymes aPus7 and aPus10, and six H/ACA RNA-guided enzymes that account for all identified pseudouridines. These H/ACA RNAs guide the modification of all eleven sites in rRNAs, two sites in tRNAs, and two sites in CRISPR RNAs. One H/ACA RNA shows exceptional versatility by targeting eight different sites. aPus7 and aPus10 are responsible for modifying positions 13, 54 and 55 in tRNAs. We identified four atypical H/ACA RNAs that lack the lower stem and the ACA motif and confirmed their function both in vivo and in vitro. Intriguingly, atypical H/ACA RNAs can be modified by Cbf5 in a guide-independent manner. Our data provide the first global view of pseudouridylation in archaea and reveal unexpected structures, substrates, and activities of archaeal H/ACA RNPs.

H/ACA Small Ribonucleoproteins: Structural and Functional Comparison Between Archaea and Eukaryotes

During ribosome synthesis, ribosomal RNA is modified through the formation of many pseudouridines and methylations which contribute to ribosome function across all domains of life. In archaea and eukaryotes, pseudouridylation of rRNA is catalyzed by H/ACA small ribonucleoproteins (sRNPs) utilizing different H/ACA guide RNAs to identify target uridines for modification. H/ACA sRNPs are conserved in archaea and eukaryotes, as they share a common general architecture and function, but there are also several notable differences between archaeal and eukaryotic H/ACA sRNPs. Due to the higher protein stability in archaea, we have more information on the structure of archaeal H/ACA sRNPs compared to eukaryotic counterparts. However, based on the long history of yeast genetic and other cellular studies, the biological role of H/ACA sRNPs during ribosome biogenesis is better understood in eukaryotes than archaea. Therefore, this review provides an overview of the current knowledge on H/ACA sRNPs from archaea, in particular their structure and function, and relates it to our understanding of the roles of eukaryotic H/ACA sRNP during eukaryotic ribosome synthesis and beyond. Based on this comparison of our current insights into archaeal and eukaryotic H/ACA sRNPs, we discuss what role archaeal H/ACA sRNPs may play in the formation of ribosomes.

Distinct Modified Nucleosides in tRNATrp from the Hyperthermophilic Archaeon Thermococcus kodakarensis and Requirement of tRNA m2G10/m2 2G10 Methyltransferase (Archaeal Trm11) for Survival at High Temperatures

tRNA m²G10/m² ₂G10 methyltransferase (archaeal Trm11) methylates the 2-amino group in guanosine at position 10 in tRNA and forms N ²,N ²-dimethylguanosine (m² ₂G10) via N ²-methylguanosine (m²G10). We determined the complete sequence of tRNA^Trp, one of the substrate tRNAs for archaeal Trm11 from Thermococcus kodakarensis, a hyperthermophilic archaeon. Liquid chromatography/mass spectrometry following enzymatic digestion of tRNA^Trp identified 15 types of modified nucleoside at 21 positions. Several modifications were found at novel positions in tRNA, including 2'-O-methylcytidine at position 6, 2-thiocytidine at position 17, 2'-O-methyluridine at position 20, 5,2'-O-dimethylcytidine at position 32, and 2'-O-methylguanosine at position 42. Furthermore, methylwyosine was found at position 37 in this tRNA^Trp, although 1-methylguanosine is generally found at this location in tRNA^Trp from other archaea. We constructed trm11 (Δtrm11) and some gene disruptant strains and compared their tRNA^Trp with that of the wild-type strain, which confirmed the absence of m² ₂G10 and other corresponding modifications, respectively. The lack of 2-methylguanosine (m²G) at position 67 in the trm11 trm14 double disruptant strain suggested that this methylation is mediated by Trm14, which was previously identified as an m²G6 methyltransferase. The Δtrm11 strain grew poorly at 95°C, indicating that archaeal Trm11 is required for T. kodakarensis survival at high temperatures. The m² ₂G10 modification might have effects on stabilization of tRNA and/or correct folding of tRNA at the high temperatures. Collectively, these results provide new clues to the function of modifications and the substrate specificities of modification enzymes in archaeal tRNA, enabling us to propose a strategy for tRNA stabilization of this archaeon at high temperatures.IMPORTANCE Thermococcus kodakarensis is a hyperthermophilic archaeon that can grow at 60 to 100°C. The sequence of tRNA^Trp from this archaeon was determined by liquid chromatography/mass spectrometry. Fifteen types of modified nucleoside were observed at 21 positions, including 5 modifications at novel positions; in addition, methylwyosine at position 37 was newly observed in an archaeal tRNA^Trp The construction of trm11 (Δtrm11) and other gene disruptant strains confirmed the enzymes responsible for modifications in this tRNA. The lack of 2-methylguanosine (m²G) at position 67 in the trm11 trm14 double disruptant strain suggested that this position is methylated by Trm14, which was previously identified as an m²G6 methyltransferase. The Δtrm11 strain grew poorly at 95°C, indicating that archaeal Trm11 is required for T. kodakarensis survival at high temperatures.

Transfer RNA Modification Enzymes from Thermophiles and Their Modified Nucleosides in tRNA

To date, numerous modified nucleosides in tRNA as well as tRNA modification enzymes have been identified not only in thermophiles but also in mesophiles. Because most modified nucleosides in tRNA from thermophiles are common to those in tRNA from mesophiles, they are considered to work essentially in steps of protein synthesis at high temperatures. At high temperatures, the structure of unmodified tRNA will be disrupted. Therefore, thermophiles must possess strategies to stabilize tRNA structures. To this end, several thermophile-specific modified nucleosides in tRNA have been identified. Other factors such as RNA-binding proteins and polyamines contribute to the stability of tRNA at high temperatures. Thermus thermophilus, which is an extreme-thermophilic eubacterium, can adapt its protein synthesis system in response to temperature changes via the network of modified nucleosides in tRNA and tRNA modification enzymes. Notably, tRNA modification enzymes from thermophiles are very stable. Therefore, they have been utilized for biochemical and structural studies. In the future, thermostable tRNA modification enzymes may be useful as biotechnology tools and may be utilized for medical science.

Contribution of protein Gar1 to the RNA-guided and RNA-independent rRNA:Ψ-synthase activities of the archaeal Cbf5 protein

Archaeal RNA:pseudouridine-synthase (PUS) Cbf5 in complex with proteins L7Ae, Nop10 and Gar1, and guide box H/ACA sRNAs forms ribonucleoprotein (RNP) catalysts that insure the conversion of uridines into pseudouridines (Ψs) in ribosomal RNAs (rRNAs). Nonetheless, in the absence of guide RNA, Cbf5 catalyzes the in vitro formation of Ψ2603 in Pyrococcus abyssi 23S rRNA and of Ψ55 in tRNAs. Using gene-disrupted strains of the hyperthermophilic archaeon Thermococcus kodakarensis, we studied the in vivo contribution of proteins Nop10 and Gar1 to the dual RNA guide-dependent and RNA-independent activities of Cbf5 on 23S rRNA. The single-null mutants of the cbf5, nop10, and gar1 genes are viable, but display a thermosensitive slow growth phenotype. We also generated a single-null mutant of the gene encoding Pus10, which has redundant activity with Cbf5 for in vitro formation of Ψ55 in tRNA. Analysis of the presence of Ψs within the rRNA peptidyl transferase center (PTC) of the mutants demonstrated that Cbf5 but not Pus10 is required for rRNA modification. Our data reveal that, in contrast to Nop10, Gar1 is crucial for in vivo and in vitro RNA guide-independent formation of Ψ2607 (Ψ2603 in P. abyssi) by Cbf5. Furthermore, our data indicate that pseudouridylation at orphan position 2589 (2585 in P. abyssi), for which no PUS or guide sRNA has been identified so far, relies on RNA- and Gar1-dependent activity of Cbf5.

Two-subunit enzymes involved in eukaryotic post-transcriptional tRNA modification

tRNA modifications are crucial for efficient and accurate protein translation, with defects often linked to disease. There are 7 cytoplasmic tRNA modifications in the yeast Saccharomyces cerevisiae that are formed by an enzyme consisting of a catalytic subunit and an auxiliary protein, 5 of which require only a single subunit in bacteria, and 2 of which are not found in bacteria. These enzymes include the deaminase Tad2-Tad3, and the methyltransferases Trm6-Trm61, Trm8-Trm82, Trm7-Trm732, and Trm7-Trm734, Trm9-Trm112, and Trm11-Trm112. We describe the occurrence and biological role of each modification, evidence for a required partner protein in S. cerevisiae and other eukaryotes, evidence for a single subunit in bacteria, and evidence for the role of the non-catalytic binding partner. Although it is unclear why these eukaryotic enzymes require partner proteins, studies of some 2-subunit modification enzymes suggest that the partner proteins help expand substrate range or allow integration of cellular activities.

Revisiting the structure/function relationships of H/ACA(-like) RNAs: a unified model for Euryarchaea and Crenarchaea

A structural and functional classification of H/ACA and H/ACA-like motifs is obtained from the analysis of the H/ACA guide RNAs which have been identified previously in the genomes of Euryarchaea (Pyrococcus) and Crenarchaea (Pyrobaculum). A unified structure/function model is proposed based on the common structural determinants shared by H/ACA and H/ACA-like motifs in both Euryarchaea and Crenarchaea. Using a computational approach, structural and energetic rules for the guide:target RNA-RNA interactions are derived from structural and functional data on the H/ACA RNP particles. H/ACA(-like) motifs found in Pyrococcus are evaluated through the classification and their biological relevance is discussed. Extra-ribosomal targets found in both Pyrococcus and Pyrobaculum might support the hypothesis of a gene regulation mediated by H/ACA(-like) guide RNAs in archaea.

Pseudouridine: still mysterious, but never a fake (uridine)!

Pseudouridine (Ψ) is the most abundant of >150 nucleoside modifications in RNA. Although Ψ was discovered as the first modified nucleoside more than half a century ago, neither the enzymatic mechanism of its formation, nor the function of this modification are fully elucidated. We present the consistent picture of Ψ synthases, their substrates and their substrate positions in model organisms of all domains of life as it has emerged to date and point out the challenges that remain concerning higher eukaryotes and the elucidation of the enzymatic mechanism.

RNA-guided isomerization of uridine to pseudouridine--pseudouridylation

Box H/ACA ribonucleoproteins (RNPs), each consisting of one unique guide RNA and 4 common core proteins, constitute a family of complex enzymes that catalyze, in an RNA-guided manner, the isomerization of uridines to pseudouridines (Ψs) in RNAs, a reaction known as pseudouridylation. Over the years, box H/ACA RNPs have been extensively studied revealing many important aspects of these RNA modifying machines. In this review, we focus on the composition, structure, and biogenesis of H/ACA RNPs. We explain the mechanism of how this enzyme family recognizes and specifies its target uridine in a substrate RNA. We discuss the substrates of box H/ACA RNPs, focusing on rRNA (rRNA) and spliceosomal small nuclear RNA (snRNA). We describe the modification product Ψ and its contribution to RNA function. Finally, we consider possible mechanisms of the bone marrow failure syndrome dyskeratosis congenita and of prostate and other cancers linked to mutations in H/ACA RNPs.

RNA at 92 °C: the non-coding transcriptome of the hyperthermophilic archaeon Pyrococcus abyssi

The non-coding transcriptome of the hyperthermophilic archaeon Pyrococcus abyssi is investigated using the RNA-seq technology. A dedicated computational pipeline analyzes RNA-seq reads and prior genome annotation to identify small RNAs, untranslated regions of mRNAs, and cis-encoded antisense transcripts. Unlike other archaea, such as Sulfolobus and Halobacteriales, P. abyssi produces few leaderless mRNA transcripts. Antisense transcription is widespread (215 transcripts) and targets protein-coding genes that appear to evolve more rapidly than average genes. We identify at least three novel H/ACA-like guide RNAs among the newly characterized non-coding RNAs. Long 5′ UTRs in mRNAs of ribosomal proteins and amino-acid biosynthesis genes strongly suggest the presence of cis-regulatory leaders in these mRNAs. We selected a high-interest subset of non-coding RNAs based on their strong promoters, high GC-content, phylogenetic conservation, or abundance. Some of the novel small RNAs and long 5′ UTRs display high GC contents, suggesting unknown structural RNA functions. However, we were surprised to observe that most of the high-interest RNAs are AU-rich, which suggests an absence of stable secondary structure in the high-temperature environment of P. abyssi. Yet, these transcripts display other hallmarks of functionality, such as high expression or high conservation, which leads us to consider possible RNA functions that do not require extensive secondary structure.

Discovery of Pyrobaculum small RNA families with atypical pseudouridine guide RNA features

In the Eukarya and Archaea, small RNA-guided pseudouridine modification is believed to be an essential step in ribosomal RNA maturation. While readily modeled and identified by computational methods in eukaryotic species, these guide RNAs have not been found in most archaeal genomes. Using high-throughput transcriptome sequencing and comparative genomics, we have identified ten novel small RNA families that appear to function as H/ACA pseudouridylation guide sRNAs, yet surprisingly lack several expected canonical features. The new RNA genes are transcribed and highly conserved across at least six species in the archaeal hyperthermophilic genus Pyrobaculum. The sRNAs exhibit a single hairpin structure interrupted by a conserved kink-turn motif, yet only two of ten families contain the complete canonical structure found in all other H/ACA sRNAs. Half of the sRNAs lack the conserved 3'-terminal ACA sequence, and many contain only a single 3' guide region rather than the canonical 5' and 3' bipartite guides. The predicted sRNA structures contain guide sequences that exhibit strong complementarity to ribosomal RNA or transfer RNA. Most of the predicted targets of pseudouridine modification are structurally equivalent to those known in other species. One sRNA appears capable of guiding pseudouridine modification at positions U54 and U55 in most or all Pyrobaculum tRNAs. We experimentally tested seven predicted pseudouridine modifications in ribosomal RNA, and all but one was confirmed. The structural insights provided by this new set of Pyrobaculum sRNAs will augment existing models and may facilitate the identification and characterization of new guide sRNAs in other archaeal species.

Pseudouridine formation in archaeal RNAs: The case of Haloferax volcanii

Pseudouridine (Ψ), the isomer of uridine, is commonly found at various positions of noncoding RNAs of all organisms. Ψ residues are formed by a number of single- or multisite specific Ψ synthases, which generally act as stand-alone proteins. In addition, in Eukarya and Archaea, specific ribonucleoprotein complexes, each containing a distinct box H/ACA guide RNA and four core proteins, can produce Ψ at many sites of different cellular RNAs. Cbf5 is the core Ψ synthase in these complexes. Using Haloferax volcanii as an archaeal model organism, we show that, contrary to eukaryotes, the Cbf5 homolog (HVO_2493) is not essential in this archaeon. The Cbf5-deleted strain of H. volcanii completely lacks Ψ at positions 1940, 1942, 2605, and 2591 (Escherichia coli positions 1915, 1917, 2572, and 2586) of its 23S rRNA, and contains reduced steady-state levels of some box H/ACA RNAs. Archaeal Cbf5 is known to have tRNA Ψ55 synthase activity in vitro but we could not confirm this activity in vivo in H. volcanii. Conversely, the Pus10 (previously PsuX) homolog (HVO_1979), which can produce tRNA Ψ55, as well as Ψ54 in vitro, is shown here to be essential in H. volcanii, whereas the corresponding tRNA Ψ55 synthases, Pus4 and TruB, are not essential in yeast and E. coli, respectively. Finally, we demonstrate that HVO_1852, the TruA/Pus3 homolog, is responsible for the pseudouridylation of position 39 in H. volcanii tRNAs and that the corresponding gene is not essential.

Differentiating analogous tRNA methyltransferases by fragments of the methyl donor

Bacterial TrmD and eukaryotic-archaeal Trm5 form a pair of analogous tRNA methyltransferase that catalyze methyl transfer from S-adenosyl methionine (AdoMet) to N(1) of G37, using catalytic motifs that share no sequence or structural homology. Here we show that natural and synthetic analogs of AdoMet are unable to distinguish TrmD from Trm5. Instead, fragments of AdoMet, adenosine and methionine, are selectively inhibitory of TrmD rather than Trm5. Detailed structural information of the two enzymes in complex with adenosine reveals how Trm5 escapes targeting by adopting an altered structure, whereas TrmD is trapped by targeting due to its rigid structure that stably accommodates the fragment. Free energy analysis exposes energetic disparities between the two enzymes in how they approach the binding of AdoMet versus fragments and provides insights into the design of inhibitors selective for TrmD.

Genomic analysis of Acidianus hospitalis W1 a host for studying crenarchaeal virus and plasmid life cycles

The Acidianus hospitalis W1 genome consists of a minimally sized chromosome of about 2.13 Mb and a conjugative plasmid pAH1 and it is a host for the model filamentous lipothrixvirus AFV1. The chromosome carries three putative replication origins in conserved genomic regions and two large regions where non-essential genes are clustered. Within these variable regions, a few orphan orfB and other elements of the IS200/607/605 family are concentrated with a novel class of MITE-like repeat elements. There are also 26 highly diverse vapBC antitoxin–toxin gene pairs proposed to facilitate maintenance of local chromosomal regions and to minimise the impact of environmental stress. Complex and partially defective CRISPR/Cas/Cmr immune systems are present and interspersed with five vapBC gene pairs. Remnants of integrated viral genomes and plasmids are located at five intron-less tRNA genes and several non-coding RNA genes are predicted that are conserved in other Sulfolobus genomes. The putative metabolic pathways for sulphur metabolism show some significant differences from those proposed for other Acidianus and Sulfolobus species. The small and relatively stable genome of A. hospitalis W1 renders it a promising candidate for developing the first Acidianus genetic systems.

Biosynthesis and function of tRNA modifications in Archaea

tRNA modifications are important for decoding, translation accuracy, and structural integrity of tRNAs. Archaeal tRNAs contain at least 47 different tRNA modifications, some of them, including archaeosine, agmatidine, and mimG, are specific to the archaeal domain. The biosynthetic pathways for these complex signature modifications have recently been elucidated and are extensively described in this review. Archaeal organisms still lag Escherichia coli and Saccharomyces cerevisiae in terms of genetic characterization and in vivo function of tRNA modifications. However, recent advances in the model Haloferax volcanii, described here, should allow closing this gap soon. Consequently, an update on experimental characterizations of archaeal tRNA modification genes and proteins is given to set the stage for future work in this field.

Genome analyses of Icelandic strains of Sulfolobus islandicus, model organisms for genetic and virus-host interaction studies

The genomes of two Sulfolobus islandicus strains obtained from Icelandic solfataras were sequenced and analyzed. Strain REY15A is a host for a versatile genetic toolbox. It exhibits a genome of minimal size, is stable genetically, and is easy to grow and manipulate. Strain HVE10/4 shows a broad host range for exceptional crenarchaeal viruses and conjugative plasmids and was selected for studying their life cycles and host interactions. The genomes of strains REY15A and HVE10/4 are 2.5 and 2.7 Mb, respectively, and each genome carries a variable region of 0.5 to 0.7 Mb where major differences in gene content and gene order occur. These include gene clusters involved in specific metabolic pathways, multiple copies of VapBC antitoxin-toxin gene pairs, and in strain HVE10/4, a 50-kb region rich in glycosyl transferase genes. The variable region also contains most of the insertion sequence (IS) elements and high proportions of the orphan orfB elements and SMN1 miniature inverted-repeat transposable elements (MITEs), as well as the clustered regular interspaced short palindromic repeat (CRISPR)-based immune systems, which are complex and diverse in both strains, consistent with them having been mobilized both intra- and intercellularly. In contrast, the remainder of the genomes are highly conserved in their protein and RNA gene syntenies, closely resembling those of other S. islandicus and Sulfolobus solfataricus strains, and they exhibit only minor remnants of a few genetic elements, mainly conjugative plasmids, which have integrated at a few tRNA genes lacking introns. This provides a possible rationale for the presence of the introns.

Deciphering synonymous codons in the three domains of life: co-evolution with specific tRNA modification enzymes

The strategies organisms use to decode synonymous codons in cytosolic protein synthesis are not uniform. The complete isoacceptor tRNA repertoire and the type of modified nucleoside found at the wobble position 34 of their anticodons were analyzed in all kingdoms of life. This led to the identification of four main decoding strategies that are diversely used in Bacteria, Archaea and Eukarya. Many of the modern tRNA modification enzymes acting at position 34 of tRNAs are present only in specific domains and obviously have arisen late during evolution. In an evolutionary fine-tuning process, these enzymes must have played an essential role in the progressive introduction of new amino acids, and in the refinement and standardization of the canonical nuclear genetic code observed in all extant organisms (functional convergent evolutionary hypothesis).