Acquisition of a bacterial RumA-type tRNA(uracil-54, C5)-methyltransferase by Archaea through an ancient horizontal gene transfer

Pseudouridine and N1-methylpseudouridine as potent nucleotide analogues for RNA therapy and vaccine development

Modified nucleosides are integral to modern drug development, serving as crucial building blocks for creating safer, more potent, and more precisely targeted therapeutic interventions. Nucleobase modifications often confer antiviral and anti-cancer activity as monomers. When incorporated into nucleic acid oligomers, they increase stability against degradation by enzymes, enhancing the drugs' lifespan within the body. Moreover, modification strategies can mitigate potential toxic effects and reduce immunogenicity, making drugs safer and better tolerated. Particularly, N1-methylpseudouridine modification improved the efficacy of the mRNA coding for spike protein of COVID-19. This became a crucial step for developing COVID-19 vaccine applied during the 2020 pandemic. This makes N1-methylpseudouridine, and its "parent" analogue pseudouridine, potent nucleotide analogues for future RNA therapy and vaccine development. This review focuses on the structure and properties of pseudouridine and N1-methylpseudouridine. RNA has a greater structural versatility, different conformation, and chemical reactivity than DNA. Watson-Crick pairing is not strictly followed by RNA that has more unusual base pairs and base-triplets. This requires detailed structural studies and structure-activity relationship analyses for RNA, also when modifications are incorporated. Recent successes in this direction are revised in this review. We describe recent successes with using pseudouridine and N1-methylpseudouridine in mRNA drug candidates. We also highlight remaining challenges that need to be solved to develop new mRNA vaccines and therapies.

Distinct characteristics of putative archaeal 5-methylcytosine RNA methyltransferases unveil their substrate specificities and evolutionary ancestries

5-Methylcytosine methyltransferases (m5C MTases) are known to be involved in the modification of RNA. Although these enzymes have been relatively well characterized in bacteria and eukarya, a complete understanding of the archaeal counterparts is lacking. In this study, the identification and characterization of archaeal RNA m5C MTases were performed. As a case study, a hyperthermophilic archaeon, Pyrococcus horikoshii OT3, which possesses five putative RNA m5C MTases, was chosen. Among the five putative RNA m5C MTases, two proteins (PH0851 and PH1991) have been characterized as homologs of a bacterial rRNA MTase (RsmB) and eukaryal tRNA MTase (NSUN6), respectively. The in-depth characterization of the remaining three putative RNA m5C MTases (PH1078, PH1374, and PH1537) in this study suggests the presence of the signature architecture and catalytic residues plausibly involved in the binding of their cognate RNA substrates. Additionally, the results also suggest the existence of two RsmB-like proteins (PH0851 and PH1078) belonging to the same subfamily IV of m5C RNA MTase. However, the proteins PH1374 and PH1537 belong to the same subfamily V but bind to different substrates, rRNA and tRNA, respectively. The findings further indicate that archaeal RNA m5C MTases link those from bacteria and eukarya.Communicated by Ramaswamy H. Sarma.

An efficient, non-phylogenetic method for detecting genes sharing evolutionary signals in phylogenomic datasets

Assessing the compatibility between gene family phylogenies is a crucial and often computationally demanding step in many phylogenomic analyses. Here, we describe the Evolutionary Similarity Index (IES), a means to assess shared evolution between gene families using a weighted orthogonal distance regression model applied to sequence distances. The utilization of pairwise distance matrices circumvents comparisons between gene tree topologies, which are inherently uncertain and sensitive to evolutionary model choice, phylogenetic reconstruction artifacts, and other sources of error. Furthermore, IES enables the many-to-many pairing of multiple copies between similarly evolving gene families. This is done by selecting non-overlapping pairs of copies, one from each assessed family, and yielding the least sum of squared residuals. Analyses of simulated gene family data sets show that IES’s accuracy is on par with popular tree-based methods while also less susceptible to noise introduced by sequence alignment and evolutionary model fitting. Applying IES to an empirical data set of 1,322 genes from 42 archaeal genomes identified eight major clusters of gene families with compatible evolutionary trends. The most cohesive cluster consisted of 62 genes with compatible evolutionary signal, which occur as both single-copy and multiple homologs per genome; phylogenetic analysis of concatenated alignments from this cluster produced a tree closely matching previously published species trees for Archaea. Four other clusters are mainly composed of accessory genes with limited distribution among Archaea and enriched toward specific metabolic functions. Pairwise evolutionary distances obtained from these accessory gene clusters suggest patterns of interphyla horizontal gene transfer. An IES implementation is available at https://github.com/lthiberiol/evolSimIndex.

The Evolutionary Origins of Extreme Halophilic Archaeal Lineages

Interest and controversy surrounding the evolutionary origins of extremely halophilic Archaea has increased in recent years, due to the discovery and characterization of the Nanohaloarchaea and the Methanonatronarchaeia. Initial attempts in explaining the evolutionary placement of the two new lineages in relation to the classical Halobacteria (also referred to as Haloarchaea) resulted in hypotheses that imply the new groups share a common ancestor with the Haloarchaea. However, more recent analyses have led to a shift: the Nanohaloarchaea have been largely accepted as being a member of the DPANN superphylum, outside of the euryarchaeota; whereas the Methanonatronarchaeia have been placed near the base of the Methanotecta (composed of the class II methanogens, the Halobacteriales, and Archaeoglobales). These opposing hypotheses have far-reaching implications on the concepts of convergent evolution (distantly related groups evolve similar strategies for survival), genome reduction, and gene transfer. In this work, we attempt to resolve these conflicts with phylogenetic and phylogenomic data. We provide a robust taxonomic sampling of Archaeal genomes that spans the Asgardarchaea, TACK Group, euryarchaeota, and the DPANN superphylum. In addition, we assembled draft genomes from seven new representatives of the Nanohaloarchaea from distinct geographic locations. Phylogenies derived from these data imply that the highly conserved ATP synthase catalytic/noncatalytic subunits of Nanohaloarchaea share a sisterhood relationship with the Haloarchaea. We also employ a novel gene family distance clustering strategy which shows this sisterhood relationship is not likely the result of a recent gene transfer. In addition, we present and evaluate data that argue for and against the monophyly of the DPANN superphylum, in particular, the inclusion of the Nanohaloarchaea in DPANN.

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

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

Mammalian nuclear TRUB1, mitochondrial TRUB2, and cytoplasmic PUS10 produce conserved pseudouridine 55 in different sets of tRNA

Most mammalian cytoplasmic tRNAs contain ribothymidine (T) and pseudouridine (Ψ) at positions 54 and 55, respectively. However, some tRNAs contain Ψ at both positions. Several Ψ54-containing tRNAs function as primers in retroviral DNA synthesis. The Ψ54 of these tRNAs is produced by PUS10, which can also synthesize Ψ55. Two other enzymes, TRUB1 and TRUB2, can also produce Ψ55. By nearest-neighbor analyses of tRNAs treated with recombinant proteins and subcellular extracts of wild-type and specific Ψ55 synthase knockdown cells, we determined that while TRUB1, PUS10, and TRUB2 all have tRNA Ψ55 synthase activities, they have different tRNA structural requirements. Moreover, these activities are primarily present in the nucleus, cytoplasm, and mitochondria, respectively, suggesting a compartmentalization of Ψ55 synthase activity. TRUB1 produces the Ψ55 of most elongator tRNAs, but cytoplasmic PUS10 produces both Ψs of the tRNAs with Ψ54Ψ55. The nuclear isoform of PUS10 is catalytically inactive and specifically binds the unmodified U54U55 versions of Ψ54Ψ55-containing tRNAs, as well as the A54U55-containing tRNA^iMet This binding inhibits TRUB1-mediated U55 to Ψ55 conversion in the nucleus. Consequently, the U54U55 of Ψ54Ψ55-containing tRNAs are modified by the cytoplasmic PUS10. Nuclear PUS10 does not bind the U55 versions of T54Ψ55- and A54Ψ55-containing elongator tRNAs. Therefore, TRUB1 is able to produce Ψ55 in these tRNAs. In summary, the tRNA Ψ55 synthase activities of TRUB1 and PUS10 are not redundant but rather are compartmentalized and act on different sets of tRNAs. The significance of this compartmentalization needs further study.

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

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

The hyperthermophilic partners Nanoarchaeum and Ignicoccus stabilize their tRNA T-loops via different but structurally equivalent modifications

The universal L-shaped tertiary structure of tRNAs is maintained with the help of nucleotide modifications within the D- and T-loops, and these modifications are most extensive within hyperthermophilic species. The obligate-commensal Nanoarchaeum equitans and its phylogenetically-distinct host Ignicoccus hospitalis grow physically coupled under identical hyperthermic conditions. We report here two fundamentally different routes by which these archaea modify the key conserved nucleotide U54 within their tRNA T-loops. In N. equitans, this nucleotide is methylated by the S-adenosylmethionine-dependent enzyme NEQ053 to form m⁵U54, and a recombinant version of this enzyme maintains specificity for U54 in Escherichia coli. In N. equitans, m⁵U54 is subsequently thiolated to form m⁵s²U54. In contrast, I. hospitalis isomerizes U54 to pseudouridine prior to methylating its N1-position and thiolating the O4-position of the nucleobase to form the previously uncharacterized nucleotide m¹s⁴Ψ. The methyl and thiol groups in m¹s⁴Ψ and m⁵s²U are presented within the T-loop in a spatially identical manner that stabilizes the 3′-endo-anti conformation of nucleotide-54, facilitating stacking onto adjacent nucleotides and reverse-Hoogsteen pairing with nucleotide m¹A58. Thus, two distinct structurally-equivalent solutions have evolved independently and convergently to maintain the tertiary fold of tRNAs under extreme hyperthermic conditions.

Identification of the 3-amino-3-carboxypropyl (acp) transferase enzyme responsible for acp3U formation at position 47 in Escherichia coli tRNAs

tRNAs from all domains of life contain modified nucleotides. However, even for the experimentally most thoroughly characterized model organism Escherichia coli not all tRNA modification enzymes are known. In particular, no enzyme has been found yet for introducing the acp³U modification at position 47 in the variable loop of eight E. coli tRNAs. Here we identify the so far functionally uncharacterized YfiP protein as the SAM-dependent 3-amino-3-carboxypropyl transferase catalyzing this modification and thereby extend the list of known tRNA modification enzymes in E. coli. Similar to the Tsr3 enzymes that introduce acp modifications at U or m¹Ψ nucleotides in rRNAs this protein contains a DTW domain suggesting that acp transfer reactions to RNA nucleotides are a general function of DTW domain containing proteins. The introduction of the acp³U-47 modification in E. coli tRNAs is promoted by the presence of the m⁷G-46 modification as well as by growth in rich medium. However, a deletion of the enzymes responsible for the modifications at position 46 and 47 in the variable loop of E. coli tRNAs did not lead to a clearly discernible phenotype suggesting that these two modifications play only a minor role in ensuring the proper function of tRNAs in E. coli.

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.

Regulatory Factors for tRNA Modifications in Extreme- Thermophilic Bacterium Thermus thermophilus

Thermus thermophilus is an extreme-thermophilic bacterium that can grow at a wide range of temperatures (50-83°C). To enable T. thermophilus to grow at high temperatures, several biomolecules including tRNA and tRNA modification enzymes show extreme heat-resistance. Therefore, the modified nucleosides in tRNA from T. thermophilus have been studied mainly from the view point of tRNA stabilization at high temperatures. Such studies have shown that several modifications stabilize the structure of tRNA and are essential for survival of the organism at high temperatures. Together with tRNA modification enzymes, the modified nucleosides form a network that regulates the extent of different tRNA modifications at various temperatures. In this review, I describe this network, as well as the tRNA recognition mechanism of individual tRNA modification enzymes. Furthermore, I summarize the roles of other tRNA stabilization factors such as polyamines and metal ions.

The human ortholog of archaeal Pus10 produces pseudouridine 54 in select tRNAs where its recognition sequence contains a modified residue

The nearly conserved U54 of tRNA is mostly converted to a version of ribothymidine (T) in Bacteria and eukaryotes and to a version of pseudouridine (Ψ) in Archaea. Conserved U55 is nearly always modified to Ψ55 in all organisms. Orthologs of TrmA and TruB that produce T54 and Ψ55, respectively, in Bacteria and eukaryotes are absent in Archaea. Pus10 produces both Ψ54 and Ψ55 in Archaea. Pus10 orthologs are found in nearly all sequenced archaeal and most eukaryal genomes, but not in yeast and bacteria. This coincides with the presence of Ψ54 in most archaeal tRNAs and some animal tRNAs, but its absence from yeast and bacteria. Moreover, Ψ54 is found in several tRNAs that function as primers for retroviral DNA synthesis. Previously, no eukaryotic tRNA Ψ54 synthase had been identified. We show here that human Pus10 can produce Ψ54 in select tRNAs, including tRNA^Lys3, the primer for HIV reverse transcriptase. This synthase activity of Pus10 is restricted to the cytoplasm and is distinct from nuclear Pus10, which is known to be involved in apoptosis. The sequence GUUCAm¹AAUC (m¹A is 1-methyladenosine) at position 53-61 of tRNA along with a stable acceptor stem results in maximum Ψ54 synthase activity. This recognition sequence is unique for a Ψ synthase in that it contains another modification. In addition to Ψ54, SF9 cells-derived recombinant human Pus10 can also generate Ψ55, even in tRNAs that do not contain the Ψ54 synthase recognition sequence. This activity may be redundant with that of TruB.

Unveiling the structural features that determine the dual methyltransferase activities of Streptococcus pneumoniae RlmCD

Methyltransferase RlmCD was previously shown to be responsible for the introduction of C5 methylation at both U747 and U1939 of the 23S ribosomal RNA in Streptococcus pneumoniae. Intriguingly, its structural homologue, RumA, can only catalyze the methylation of U1939, while RlmC is the dedicated enzyme for m5U747 in Escherichia coli. In this study, we describe the structure of RlmCD in complex with its cofactor and the RNA substrate containing U747 at 2.00 Å or U1939 at 3.10 Å. We demonstrate that multiple structural features collaborate to establish the dual enzymatic activities of RlmCD. Of them, the side-chain rearrangement of F145 was observed to be an unusual mechanism through which RlmCD can discriminate between U747- and U1939-containing RNA substrate by switching the intermolecular aromatic stacking between protein and RNA on/off. An in-vitro methyltransferase assay and electrophoretic mobility shift assay were performed to validate these findings. Overall, our complex structures allow for a better understanding of the dual-functional mechanism of RlmCD, suggesting useful implications for the evolution of the RumA-type enzyme and the potential development of antibiotic drugs against S. pneumoniae.

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.

Unique Features and Anti-microbial Targeting of Folate- and Flavin-Dependent Methyltransferases Required for Accurate Maintenance of Genetic Information

Comparative genome analyses have led to the discovery and characterization of novel flavin- and folate-dependent methyltransferases that mainly function in DNA precursor synthesis and post-transcriptional RNA modification by forming (ribo) thymidylate and its derivatives. Here we discuss the recent literature on the novel mechanistic features of these enzymes sometimes referred to as “uracil methyltransferases,” albeit we prefer to refer to them as (ribo) thymidylate synthases. These enzyme families attest to the convergent evolution of nucleic acid methylation. Special focus is given to describing the unique characteristics of these flavin- and folate-dependent enzymes that have emerged as new models for studying the non-canonical roles of reduced flavin co-factors (FADH₂) in relaying carbon atoms between enzyme substrates. This ancient enzymatic methylation mechanism with a very wide phylogenetic distribution may be more commonly used for biological methylation reactions than previously anticipated. This notion is exemplified by the recent discovery of additional substrates for these enzymes. Moreover, similar reaction mechanisms can be reversed by demethylases, which remove methyl groups e.g., from human histones. Future work is now required to address whether the use of different methyl donors facilitates the regulation of distinct methylation reactions in the cell. It will also be of great interest to address whether the low activity flavin-dependent thymidylate synthases ThyX represent ancestral enzymes that were eventually replaced by the more active thymidylate synthases of the ThyA family to facilitate the maintenance of larger genomes in fast-growing microbes. Moreover, we discuss the recent efforts from several laboratories to identify selective anti-microbial compounds that target flavin-dependent thymidylate synthase ThyX. Altogether we underline how the discovery of the alternative flavoproteins required for methylation of DNA and/or RNA nucleotides, in addition to providing novel targets for antibiotics, has provided new insight into microbial physiology and virulence.

Evolution of Eukaryal and Archaeal Pseudouridine Synthase Pus10

In archaea, pseudouridine (Ψ) synthase Pus10 modifies uridine (U) to Ψ at positions 54 and 55 of tRNA. In contrast, Pus10 is not found in bacteria, where modifications at those two positions are carried out by TrmA (U54 to m⁵U54) and TruB (U55 to Ψ55). Many eukaryotes have an apparent redundancy; their genomes contain orthologs of archaeal Pus10 and bacterial TrmA and TruB. Although eukaryal Pus10 genes share a conserved catalytic domain with archaeal Pus10 genes, their biological roles are not clear for the two reasons. First, experimental evidence suggests that human Pus10 participates in apoptosis induced by the tumor necrosis factor-related apoptosis-inducing ligand. Whether the function of human Pus10 is in place or in addition to of Ψ synthesis in tRNA is unknown. Second, Pus10 is found in earlier evolutionary branches of fungi (such as chytrid Batrachochytrium) but is absent in all dikaryon fungi surveyed (Ascomycetes and Basidiomycetes). We did a comprehensive analysis of sequenced genomes and found that orthologs of Pus10, TrmA, and TruB were present in all the animals, plants, and protozoa surveyed. This indicates that the common eukaryotic ancestor possesses all the three genes. Next, we examined 116 archaeal and eukaryotic Pus10 protein sequences to find that Pus10 existed as a single copy gene in all the surveyed genomes despite ancestral whole genome duplications had occurred. This indicates a possible deleterious gene dosage effect. Our results suggest that functional redundancy result in gene loss or neofunctionalization in different evolutionary lineages.

Folate-/FAD-dependent tRNA methyltransferase from Thermus thermophilus regulates other modifications in tRNA at low temperatures

TrmFO is a N(5) , N(10) -methylenetetrahydrofolate (CH2 THF)-/FAD-dependent tRNA methyltransferase, which synthesizes 5-methyluridine at position 54 (m(5) U54) in tRNA. Thermus thermophilus is an extreme-thermophilic eubacterium, which grows in a wide range of temperatures (50-83 °C). In T. thermophilus, modified nucleosides in tRNA and modification enzymes form a network, in which one modification regulates the degrees of other modifications and controls the flexibility of tRNA. To clarify the role of m(5) U54 and TrmFO in the network, we constructed the trmFO gene disruptant (∆trmFO) strain of T. thermophilus. Although this strain did not show any growth retardation at 70 °C, it showed a slow-growth phenotype at 50 °C. Nucleoside analysis showed increase in 2'-O-methylguanosine at position 18 and decrease in N(1) -methyladenosine at position 58 in the tRNA mixture from the ∆trmFO strain at 50 °C. These in vivo results were reproduced by in vitro experiments with purified enzymes. Thus, we concluded that the m(5) U54 modification have effects on the other modifications in tRNA through the network at 50 °C. (35) S incorporations into proteins showed that the protein synthesis activity of ∆trmFO strain was inferior to the wild-type strain at 50 °C, suggesting that the growth delay at 50 °C was caused by the inferior protein synthesis activity.

Diversity in mechanism and function of tRNA methyltransferases

tRNA molecules undergo extensive post-transcriptional processing to generate the mature functional tRNA species that are essential for translation in all organisms. These processing steps include the introduction of numerous specific chemical modifications to nucleotide bases and sugars; among these modifications, methylation reactions are by far the most abundant. The tRNA methyltransferases comprise a diverse enzyme superfamily, including members of multiple structural classes that appear to have arisen independently during evolution. Even among closely related family members, examples of unusual substrate specificity and chemistry have been observed. Here we review recent advances in tRNA methyltransferase mechanism and function with a particular emphasis on discoveries of alternative substrate specificities and chemistry associated with some methyltransferases. Although the molecular function for a specific tRNA methylation may not always be clear, mutations in tRNA methyltransferases have been increasingly associated with human disease. The impact of tRNA methylation on human biology is also discussed.

The universal tree of life: an update

Biologists used to draw schematic “universal” trees of life as metaphors illustrating the history of life. It is indeed a priori possible to construct an organismal tree connecting the three major domains of ribosome encoding organisms: Archaea, Bacteria and Eukarya, since they originated by cell division from LUCA. Several universal trees based on ribosomal RNA sequence comparisons proposed at the end of the last century are still widely used, although some of their main features have been challenged by subsequent analyses. Several authors have proposed to replace the traditional universal tree with a ring of life, whereas others have proposed more recently to include viruses as new domains. These proposals are misleading, suggesting that endosymbiosis can modify the shape of a tree or that viruses originated from the last universal common ancestor (LUCA). I propose here an updated version of Woese’s universal tree that includes several rootings for each domain and internal branching within domains that are supported by recent phylogenomic analyses of domain specific proteins. The tree is rooted between Bacteria and Arkarya, a new name proposed for the clade grouping Archaea and Eukarya. A consensus version, in which each of the three domains is unrooted, and a version in which eukaryotes emerged within archaea are also presented. This last scenario assumes the transformation of a modern domain into another, a controversial evolutionary pathway. Viruses are not indicated in these trees but are intrinsically present because they infect the tree from its roots to its leaves. Finally, I present a detailed tree of the domain Archaea, proposing the sub-phylum neo-Euryarchaeota for the monophyletic group of euryarchaeota containing DNA gyrase. These trees, that will be easily updated as new data become available, could be useful to discuss controversial scenarios regarding early life evolution.

Insights into archaeal evolution and symbiosis from the genomes of a nanoarchaeon and its inferred crenarchaeal host from Obsidian Pool, Yellowstone National Park

Background

A single cultured marine organism, Nanoarchaeum equitans, represents the Nanoarchaeota branch of symbiotic Archaea, with a highly reduced genome and unusual features such as multiple split genes.

Results

The first terrestrial hyperthermophilic member of the Nanoarchaeota was collected from Obsidian Pool, a thermal feature in Yellowstone National Park, separated by single cell isolation, and sequenced together with its putative host, a Sulfolobales archaeon. Both the new Nanoarchaeota (Nst1) and N. equitans lack most biosynthetic capabilities, and phylogenetic analysis of ribosomal RNA and protein sequences indicates that the two form a deep-branching archaeal lineage. However, the Nst1 genome is more than 20% larger, and encodes a complete gluconeogenesis pathway as well as the full complement of archaeal flagellum proteins. With a larger genome, a smaller repertoire of split protein encoding genes and no split non-contiguous tRNAs, Nst1 appears to have experienced less severe genome reduction than N. equitans. These findings imply that, rather than representing ancestral characters, the extremely compact genomes and multiple split genes of Nanoarchaeota are derived characters associated with their symbiotic or parasitic lifestyle. The inferred host of Nst1 is potentially autotrophic, with a streamlined genome and simplified central and energetic metabolism as compared to other Sulfolobales.

Conclusions

Comparison of the N. equitans and Nst1 genomes suggests that the marine and terrestrial lineages of Nanoarchaeota share a common ancestor that was already a symbiont of another archaeon. The two distinct Nanoarchaeota-host genomic data sets offer novel insights into the evolution of archaeal symbiosis and parasitism, enabling further studies of the cellular and molecular mechanisms of these relationships.

Reviewers

This article was reviewed by Patrick Forterre, Bettina Siebers (nominated by Michael Galperin) and Purification Lopez-Garcia

Updated clusters of orthologous genes for Archaea: a complex ancestor of the Archaea and the byways of horizontal gene transfer

Background

Collections of Clusters of Orthologous Genes (COGs) provide indispensable tools for comparative genomic analysis, evolutionary reconstruction and functional annotation of new genomes. Initially, COGs were made for all complete genomes of cellular life forms that were available at the time. However, with the accumulation of thousands of complete genomes, construction of a comprehensive COG set has become extremely computationally demanding and prone to error propagation, necessitating the switch to taxon-specific COG collections. Previously, we reported the collection of COGs for 41 genomes of Archaea (arCOGs). Here we present a major update of the arCOGs and describe evolutionary reconstructions to reveal general trends in the evolution of Archaea.

Results

The updated version of the arCOG database incorporates 91% of the pangenome of 120 archaea (251,032 protein-coding genes altogether) into 10,335 arCOGs. Using this new set of arCOGs, we performed maximum likelihood reconstruction of the genome content of archaeal ancestral forms and gene gain and loss events in archaeal evolution. This reconstruction shows that the last Common Ancestor of the extant Archaea was an organism of greater complexity than most of the extant archaea, probably with over 2,500 protein-coding genes. The subsequent evolution of almost all archaeal lineages was apparently dominated by gene loss resulting in genome streamlining. Overall, in the evolution of Archaea as well as a representative set of bacteria that was similarly analyzed for comparison, gene losses are estimated to outnumber gene gains at least 4 to 1. Analysis of specific patterns of gene gain in Archaea shows that, although some groups, in particular Halobacteria, acquire substantially more genes than others, on the whole, gene exchange between major groups of Archaea appears to be largely random, with no major ‘highways’ of horizontal gene transfer.

Conclusions

The updated collection of arCOGs is expected to become a key resource for comparative genomics, evolutionary reconstruction and functional annotation of new archaeal genomes. Given that, in spite of the major increase in the number of genomes, the conserved core of archaeal genes appears to be stabilizing, the major evolutionary trends revealed here have a chance to stand the test of time.

Reviewers

This article was reviewed by (for complete reviews see the Reviewers’ Reports section): Dr. PLG, Prof. PF, Dr. PL (nominated by Prof. JPG).

The tRNA recognition mechanism of folate/FAD-dependent tRNA methyltransferase (TrmFO)

The conserved U54 in tRNA is often modified to 5-methyluridine (m(5)U) and forms a reverse Hoogsteen base pair with A58 that stabilizes the L-shaped tRNA structure. In Gram-positive and some Gram-negative eubacteria, m(5)U54 is produced by folate/FAD-dependent tRNA (m(5)U54) methyltransferase (TrmFO). TrmFO utilizes N(5),N(10)-methylenetetrahydrofolate (CH(2)THF) as a methyl donor. We previously reported an in vitro TrmFO assay system, in which unstable [(14)C]CH(2)THF was supplied from [(14)C]serine and tetrahydrofolate by serine hydroxymethyltransferase. In the current study, we have improved the TrmFO assay system by optimization of enzyme and substrate concentrations and introduction of a filter assay system. Using this assay, we have focused on the tRNA recognition mechanism of TrmFO. 42 tRNA mutant variants were prepared, and experiments with truncated tRNA and microhelix RNAs revealed that the minimum requirement of TrmFO exists in the T-arm structure. The positive determinants for TrmFO were found to be the U54U55C56 sequence and G53-C61 base pair. The gel mobility shift assay and fluorescence quenching showed that the affinity of TrmFO for tRNA in the initial binding process is weak. The inhibition experiments showed that the methylated tRNA is released before the structural change process. Furthermore, we found that A38 prevents incorrect methylation of U32 in the anticodon loop. Moreover, the m(1)A58 modification clearly accelerates the TrmFO reaction, suggesting a synergistic effect of the m(5)U54, m(1)A58, and s(2)U54 modifications on m(5)s(2)U54 formation in Thermus thermophilus cells. The docking model of TrmFO and the T-arm showed that the G53-C61 base pair is not able to directly contact the enzyme.

A single methyltransferase YefA (RlmCD) catalyses both m5U747 and m5U1939 modifications in Bacillus subtilis 23S rRNA

Methyltransferases that use S-adenosylmethionine (AdoMet) as a cofactor to catalyse 5-methyl uridine (m(5)U) formation in tRNAs and rRNAs are widespread in Bacteria and Eukaryota, and are also found in certain Archaea. These enzymes belong to the COG2265 cluster, and the Gram-negative bacterium Escherichia coli possesses three paralogues. These comprise the methyltransferases TrmA that targets U54 in tRNAs, RlmC that modifies U747 in 23S rRNA and RlmD that is specific for U1939 in 23S rRNA. The tRNAs and rRNAs of the Gram-positive bacterium Bacillus subtilis have the same three m(5)U modifications. However, as previously shown, the m(5)U54 modification in B. subtilis tRNAs is catalysed in a fundamentally different manner by the folate-dependent enzyme TrmFO, which is unrelated to the E. coli TrmA. Here, we show that methylation of U747 and U1939 in B. subtilis rRNA is catalysed by a single enzyme, YefA that is a COG2265 member. A recombinant version of YefA functions in an E. coli m(5)U-null mutant adding the same two rRNA methylations. The findings suggest that during evolution, COG2265 enzymes have undergone a series of changes in target specificity and that YefA is closer to an archetypical m(5)U methyltransferase. To reflect its dual specificity, YefA is renamed RlmCD.

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.

Expression and purification of untagged and histidine-tagged folate-dependent tRNA:m5U54 methyltransferase from Bacillus subtilis

Folate-dependent tRNA m(5)U methyltransferase TrmFO is a flavoprotein that catalyzes the C(5)-methylation of uridine at position 54 in the TPsiC loop of tRNA in several bacteria. Here we report the cloning and optimization of expression in Escherichia coli BL21 (DE3) of untagged, N-terminus, C-terminus (His)(6)-tagged TrmFO from Bacillus subtilis. Tagged and untagged TrmFO were purified to homogeneity by metal affinity or ion exchange and heparin affinity, respectively, followed by size-exclusion chromatography. The tag did not significantly alter the expression level, flavin content, activity and secondary structure of the protein.

The closest relatives of icosahedral viruses of thermophilic bacteria are among viruses and plasmids of the halophilic archaea

We have sequenced the genome and identified the structural proteins and lipids of the novel membrane-containing, icosahedral virus P23-77 of Thermus thermophilus. P23-77 has an approximately 17-kb circular double-stranded DNA genome, which was annotated to contain 37 putative genes. Virions were subjected to dissociation analysis, and five protein species were shown to associate with the internal viral membrane, while three were constituents of the protein capsid. Analysis of the bacteriophage genome revealed it to be evolutionarily related to another Thermus phage (IN93), archaeal Halobacterium plasmid (pHH205), a genetic element integrated into Haloarcula genome (designated here as IHP for integrated Haloarcula provirus), and the Haloarcula virus SH1. These genetic elements share two major capsid proteins and a putative packaging ATPase. The ATPase is similar with the ATPases found in the PRD1-type viruses, thus providing an evolutionary link to these viruses and furthering our knowledge on the origin of viruses.

Atomic structure of a folate/FAD-dependent tRNA T54 methyltransferase

tRNAs from all 3 phylogenetic domains have a 5-methyluridine at position 54 (T54) in the T-loop. The methyl group is transferred from S-adenosylmethionine by TrmA methyltransferase in most Gram-negative bacteria and some archaea and eukaryotes, whereas it is transferred from 5,10-methylenetetrahydrofolate (MTHF) by TrmFO, a folate/FAD-dependent methyltransferase, in most Gram-positive bacteria and some Gram-negative bacteria. However, the catalytic mechanism remains unclear, because the crystal structure of TrmFO has not been solved. Here, we report the crystal structures of Thermus thermophilus TrmFO in its free form, tetrahydrofolate (THF)-bound form, and glutathione-bound form at 2.1-, 1.6-, and 1.05-A resolutions, respectively. TrmFO consists of an FAD-binding domain and an insertion domain, which both share structural similarity with those of GidA, an enzyme involved in the 5-carboxymethylaminomethylation of U34 of some tRNAs. However, the overall structures of TrmFO and GidA are basically different because of their distinct domain orientations, which are consistent with their respective functional specificities. In the THF complex, the pteridin ring of THF is sandwiched between the flavin ring of FAD and the imidazole ring of a His residue. This structure provides a snapshot of the folate/FAD-dependent methyl transfer, suggesting that the transferring methylene group of MTHF is located close to the redox-active N5 atom of FAD. Furthermore, we established an in vitro system to measure the methylation activity. Our TrmFO-tRNA docking model, in combination with mutational analyses, suggests a catalytic mechanism, in which the methylene of MTHF is directly transferred onto U54, and then the exocyclic methylene of U54 is reduced by FADH(2).

Happy together: genomic insights into the unique Nanoarchaeum/Ignicoccus association

The complete genome sequence of the crenarchaeon Ignicoccus hospitalis published recently in Genome Biology provides a great leap forward in the dissection of its unique association with another hyperthermophilic archaeon, Nanoarchaeum equitans.

Archaeal Pus10 proteins can produce both pseudouridine 54 and 55 in tRNA

Pus10, a recently identified pseudouridine (Psi) synthase, does not belong to any of the five commonly identified families of Psi synthases. Pyrococcus furiosus Pus10 has been shown to produce Psi55 in tRNAs. However, in vitro studies have identified another mechanism for tRNA Psi55 production in Archaea, which uses Cbf5 and other core proteins of the H/ACA ribonucleoprotein complex, in a guide RNA-independent manner. Pus10 homologs have been observed in nearly all sequenced archaeal genomes and in some higher eukaryotes, but not in yeast and bacteria. This coincides with the presence of Psi54 in the tRNAs of Archaea and higher eukaryotes and its absence in yeast and bacteria. No tRNA Psi54 synthase has been reported so far. Here, using recombinant Methanocaldococcus jannaschii and P. furiosus Pus10, we show that these proteins can function as synthase for both tRNA Psi54 and Psi55. The two modifications seem to occur independently. Salt concentration dependent variations in these activities of both proteins are observed. The Psi54 synthase activity of M. jannaschii protein is robust, while the same activity of P. furiosus protein is weak. Probable reasons for these differences are discussed. Furthermore, unlike bacterial TruB and yeast Pus4, archaeal Pus10 does not require a U54 x A58 reverse Hoogstein base pair and pyrimidine at position 56 to convert tRNA U55 to Psi55. The homology of eukaryal Pus10 with archaeal Pus10 suggests that the former may also have a tRNA Psi54 synthase activity.

RNomics and Modomics in the halophilic archaea Haloferax volcanii: identification of RNA modification genes

Background

Naturally occurring RNAs contain numerous enzymatically altered nucleosides. Differences in RNA populations (RNomics) and pattern of RNA modifications (Modomics) depends on the organism analyzed and are two of the criteria that distinguish the three kingdoms of life. If the genomic sequences of the RNA molecules can be derived from whole genome sequence information, the modification profile cannot and requires or direct sequencing of the RNAs or predictive methods base on the presence or absence of the modifications genes.

Results

By employing a comparative genomics approach, we predicted almost all of the genes coding for the t+rRNA modification enzymes in the mesophilic moderate halophile Haloferax volcanii. These encode both guide RNAs and enzymes. Some are orthologous to previously identified genes in Archaea, Bacteria or in Saccharomyces cerevisiae, but several are original predictions.

Conclusion

The number of modifications in t+rRNAs in the halophilic archaeon is surprisingly low when compared with other Archaea or Bacteria, particularly the hyperthermophilic organisms. This may result from the specific lifestyle of halophiles that require high intracellular salt concentration for survival. This salt content could allow RNA to maintain its functional structural integrity with fewer modifications. We predict that the few modifications present must be particularly important for decoding, accuracy of translation or are modifications that cannot be functionally replaced by the electrostatic interactions provided by the surrounding salt-ions. This analysis also guides future experimental validation work aiming to complete the understanding of the function of RNA modifications in Archaeal translation.

The crystal structure of Pyrococcus abyssi tRNA (uracil-54, C5)-methyltransferase provides insights into its tRNA specificity

The 5-methyluridine is invariably found at position 54 in the TΨC loop of tRNAs of most organisms. In Pyrococcus abyssi, its formation is catalyzed by the S-adenosyl-l-methionine-dependent tRNA (uracil-54, C5)-methyltransferase (_PabTrmU54), an enzyme that emerged through an ancient horizontal transfer of an RNA (uracil, C5)-methyltransferase-like gene from bacteria to archaea. The crystal structure of _PabTrmU54 in complex with S-adenosyl-l-homocysteine at 1.9 Å resolution shows the protein organized into three domains like Escherichia coli RumA, which catalyzes the same reaction at position 1939 of 23S rRNA. A positively charged groove at the interface between the three domains probably locates part of the tRNA-binding site of _PabTrmU54. We show that a mini-tRNA lacking both the D and anticodon stem-loops is recognized by _PabTrmU54. These results were used to model yeast tRNA^Asp in the _PabTrmU54 structure to get further insights into the different RNA specificities of RumA and _PabTrmU54. Interestingly, the presence of two flexible loops in the central domain, unique to _PabTrmU54, may explain the different substrate selectivities of both enzymes. We also predict that a large TΨC loop conformational change has to occur for the flipping of the target uridine into the _PabTrmU54 active site during catalysis.