The putative tRNA 2-thiouridine synthetase Ncs6 is an essential sulfur carrier in Methanococcus maripaludis

[4Fe-4S]-dependent enzymes in non-redox tRNA thiolation

Post-transcriptional modification of nucleosides in transfer RNAs (tRNAs) is an important process for accurate and efficient translation of the genetic information during protein synthesis in all domains of life. In particular, specific enzymes catalyze the biosynthesis of sulfur-containing nucleosides, such as the derivatives of 2-thiouridine (s2U), 4-thiouridine (s4U), 2-thiocytidine (s2C), and 2-methylthioadenosine (ms2A), within tRNAs. Whereas the mechanism that has prevailed for decades involved persulfide chemistry, more and more tRNA thiolation enzymes have now been shown to contain a [4Fe-4S] cluster. This review summarizes the information over the last ten years concerning the biochemical, spectroscopic and structural characterization of [4Fe-4S]-dependent non-redox tRNA thiolation enzymes.

Model Organisms To Study Methanogenesis, a Uniquely Archaeal Metabolism

Methanogenic archaea are the only organisms that produce CH4 as part of their energy-generating metabolism. They are ubiquitous in oxidant-depleted, anoxic environments such as aquatic sediments, anaerobic digesters, inundated agricultural fields, the rumen of cattle, and the hindgut of termites, where they catalyze the terminal reactions in the degradation of organic matter. Methanogenesis is the only metabolism that is restricted to members of the domain Archaea. Here, we discuss the importance of model organisms in the history of methanogen research, including their role in the discovery of the archaea and in the biochemical and genetic characterization of methanogenesis. We also discuss outstanding questions in the field and newly emerging model systems that will expand our understanding of this uniquely archaeal metabolism.

The thiolation of uridine 34 in tRNA, which controls protein translation, depends on a [4Fe-4S] cluster in the archaeum Methanococcus maripaludis

Thiolation of uridine 34 in the anticodon loop of several tRNAs is conserved in the three domains of life and guarantees fidelity of protein translation. U34-tRNA thiolation is catalyzed by a complex of two proteins in the eukaryotic cytosol (named Ctu1/Ctu2 in humans), but by a single NcsA enzyme in archaea. We report here spectroscopic and biochemical experiments showing that NcsA from Methanococcus maripaludis (MmNcsA) is a dimer that binds a [4Fe-4S] cluster, which is required for catalysis. Moreover, the crystal structure of MmNcsA at 2.8 Å resolution shows that the [4Fe-4S] cluster is coordinated by three conserved cysteines only, in each monomer. Extra electron density on the fourth nonprotein-bonded iron most likely locates the binding site for a hydrogenosulfide ligand, in agreement with the [4Fe-4S] cluster being used to bind and activate the sulfur atom of the sulfur donor. Comparison of the crystal structure of MmNcsA with the AlphaFold model of the human Ctu1/Ctu2 complex shows a very close superposition of the catalytic site residues, including the cysteines that coordinate the [4Fe-4S] cluster in MmNcsA. We thus propose that the same mechanism for U34-tRNA thiolation, mediated by a [4Fe-4S]-dependent enzyme, operates in archaea and eukaryotes.

Naturally occurring modified ribonucleosides

The chemical identity of RNA molecules beyond the four standard ribonucleosides has fascinated scientists since pseudouridine was characterized as the "fifth" ribonucleotide in 1951. Since then, the ever-increasing number and complexity of modified ribonucleosides have been found in viruses and throughout all three domains of life. Such modifications can be as simple as methylations, hydroxylations, or thiolations, complex as ring closures, glycosylations, acylations, or aminoacylations, or unusual as the incorporation of selenium. While initially found in transfer and ribosomal RNAs, modifications also exist in messenger RNAs and noncoding RNAs. Modifications have profound cellular outcomes at various levels, such as altering RNA structure or being essential for cell survival or organism viability. The aberrant presence or absence of RNA modifications can lead to human disease, ranging from cancer to various metabolic and developmental illnesses such as Hoyeraal-Hreidarsson syndrome, Bowen-Conradi syndrome, or Williams-Beuren syndrome. In this review article, we summarize the characterization of all 143 currently known modified ribonucleosides by describing their taxonomic distributions, the enzymes that generate the modifications, and any implications in cellular processes, RNA structure, and disease. We also highlight areas of active research, such as specific RNAs that contain a particular type of modification as well as methodologies used to identify novel RNA modifications. This article is categorized under: RNA Processing > RNA Editing and Modification.

Structure-based mechanistic insights into catalysis by tRNA thiolation enzymes

In all domains of life, ribonucleic acid (RNA) maturation includes post-transcriptional chemical modifications of nucleosides. Many sulfur-containing nucleosides have been identified in transfer RNAs (tRNAs), such as the derivatives of 2-thiouridine (s²U), 4-thiouridine (s⁴U), 2-thiocytidine (s²C), 2-methylthioadenosine (ms²A). These modifications are essential for accurate and efficient translation of the genetic code from messenger RNA (mRNA) for protein synthesis. This review summarizes the recent discoveries concerning the mechanistic and structural characterization of tRNA thiolation enzymes that catalyze the non-redox substitution of oxygen for sulfur in nucleosides. Two mechanisms have been described. One involves persulfide formation on catalytic cysteines, while the other uses a [4Fe-4S] cluster, chelated by three conserved cysteines only, as a sulfur carrier.

Nonredox thiolation in tRNA occurring via sulfur activation by a [4Fe-4S] cluster

Sulfur is present in several nucleosides within tRNAs. In particular, thiolation of the universally conserved methyl-uridine at position 54 stabilizes tRNAs from thermophilic bacteria and hyperthermophilic archaea and is required for growth at high temperature. The simple nonredox substitution of the C2-uridine carbonyl oxygen by sulfur is catalyzed by tRNA thiouridine synthetases called TtuA. Spectroscopic, enzymatic, and structural studies indicate that TtuA carries a catalytically essential [4Fe-4S] cluster and requires ATP for activity. A series of crystal structures shows that (i) the cluster is ligated by only three cysteines that are fully conserved, allowing the fourth unique iron to bind a small ligand, such as exogenous sulfide, and (ii) the ATP binding site, localized thanks to a protein-bound AMP molecule, a reaction product, is adjacent to the cluster. A mechanism for tRNA sulfuration is suggested, in which the unique iron of the catalytic cluster serves to bind exogenous sulfide, thus acting as a sulfur carrier.

Biochemical and structural characterization of oxygen-sensitive 2-thiouridine synthesis catalyzed by an iron-sulfur protein TtuA

Two-thiouridine (s²U) at position 54 of transfer RNA (tRNA) is a posttranscriptional modification that enables thermophilic bacteria to survive in high-temperature environments. s²U is produced by the combined action of two proteins, 2-thiouridine synthetase TtuA and 2-thiouridine synthesis sulfur carrier protein TtuB, which act as a sulfur (S) transfer enzyme and a ubiquitin-like S donor, respectively. Despite the accumulation of biochemical data in vivo, the enzymatic activity by TtuA/TtuB has rarely been observed in vitro, which has hindered examination of the molecular mechanism of S transfer. Here we demonstrate by spectroscopic, biochemical, and crystal structure analyses that TtuA requires oxygen-labile [4Fe-4S]-type iron (Fe)-S clusters for its enzymatic activity, which explains the previously observed inactivation of this enzyme in vitro. The [4Fe-4S] cluster was coordinated by three highly conserved cysteine residues, and one of the Fe atoms was exposed to the active site. Furthermore, the crystal structure of the TtuA-TtuB complex was determined at a resolution of 2.5 Å, which clearly shows the S transfer of TtuB to tRNA using its C-terminal thiocarboxylate group. The active site of TtuA is connected to the outside by two channels, one occupied by TtuB and the other used for tRNA binding. Based on these observations, we propose a molecular mechanism of S transfer by TtuA using the ubiquitin-like S donor and the [4Fe-4S] cluster.

Biosynthesis of Sulfur-Containing tRNA Modifications: A Comparison of Bacterial, Archaeal, and Eukaryotic Pathways

Post-translational tRNA modifications have very broad diversity and are present in all domains of life. They are important for proper tRNA functions. In this review, we emphasize the recent advances on the biosynthesis of sulfur-containing tRNA nucleosides including the 2-thiouridine (s²U) derivatives, 4-thiouridine (s⁴U), 2-thiocytidine (s²C), and 2-methylthioadenosine (ms²A). Their biosynthetic pathways have two major types depending on the requirement of iron–sulfur (Fe–S) clusters. In all cases, the first step in bacteria and eukaryotes is to activate the sulfur atom of free l-cysteine by cysteine desulfurases, generating a persulfide (R-S-SH) group. In some archaea, a cysteine desulfurase is missing. The following steps of the bacterial s²U and s⁴U formation are Fe–S cluster independent, and the activated sulfur is transferred by persulfide-carrier proteins. By contrast, the biosynthesis of bacterial s²C and ms²A require Fe–S cluster dependent enzymes. A recent study shows that the archaeal s⁴U synthetase (ThiI) and the eukaryotic cytosolic 2-thiouridine synthetase (Ncs6) are Fe–S enzymes; this expands the role of Fe–S enzymes in tRNA thiolation to the Archaea and Eukarya domains. The detailed reaction mechanisms of Fe–S cluster depend s²U and s⁴U formation await further investigations.

Sulfur Modifications of the Wobble U34 in tRNAs and their Intracellular Localization in Eukaryotic Cells

The wobble uridine (U₃₄) of transfer RNAs (tRNAs) for two-box codon recognition, i.e., tRNA^Lys_UUU, tRNA^Glu_UUC, and tRNA^Gln_UUG, harbor a sulfur- (thio-) and a methyl-derivative structure at the second and fifth positions of U₃₄, respectively. Both modifications are necessary to construct the proper anticodon loop structure and to enable them to exert their functions in translation. Thio-modification of U₃₄ (s²U₃₄) is found in both cytosolic tRNAs (cy-tRNAs) and mitochondrial tRNAs (mt-tRNAs). Although l-cysteine desulfurase is required in both cases, subsequent sulfur transfer pathways to cy-tRNAs and mt-tRNAs are different due to their distinct intracellular locations. The s²U₃₄ formation in cy-tRNAs involves a sulfur delivery system required for the biosynthesis of iron-sulfur (Fe/S) clusters and certain resultant Fe/S proteins. This review addresses presumed sulfur delivery pathways for the s²U₃₄ formation in distinct intracellular locations, especially that for cy-tRNAs in comparison with that for mt-tRNAs.

Persulfide Formation Mediates Cysteine and Homocysteine Biosynthesis in Methanosarcina acetivorans

The mechanisms of sulfur uptake and trafficking in methanogens inhabiting sulfidic environments are highly distinctive. In aerobes, sulfur transfers between proteins occur via persulfide relay, but direct evidence for persulfides in methanogens has been lacking. Here, we use mass spectrometry to analyze tryptic peptides of the Methanosarcina acetivorans SepCysS and MA1821 proteins purified anaerobically from methanogen cells. These enzymes insert sulfide into phosphoseryl(Sep)-tRNA^Cys and aspartate semialdehyde, respectively, to form Cys-tRNA^Cys and homocysteine. A high frequency of persulfidation at conserved cysteines of each protein was identified, while the substantial presence of persulfides in peptides from other cellular proteins suggests that this modification plays a general physiological role in the organism. Purified native SepCysS containing persulfide at conserved Cys260 generates Cys-tRNA^Cys in anaerobic single-turnover reactions without exogenously added sulfur, directly linking active-site persulfide formation in vivo with catalytic activity.

Evolution of the archaeal and mammalian information processing systems: towards an archaeal model for human disease

Current evolutionary models suggest that Eukaryotes originated from within Archaea instead of being a sister lineage. To test this model of ancient evolution, we review recent studies and compare the three major information processing subsystems of replication, transcription and translation in the Archaea and Eukaryotes. Our hypothesis is that if the Eukaryotes arose within the archaeal radiation, their information processing systems will appear to be one of kind and not wholly original. Within the Eukaryotes, the mammalian or human systems are emphasized because of their importance in understanding health. Biochemical as well as genetic studies provide strong evidence for the functional similarity of archaeal homologs to the mammalian information processing system and their dissimilarity to the bacterial systems. In many independent instances, a simple archaeal system is functionally equivalent to more elaborate eukaryotic homologs, suggesting that evolution of complexity is likely an central feature of the eukaryotic information processing system. Because fewer components are often involved, biochemical characterizations of the archaeal systems are often easier to interpret. Similarly, the archaeal cell provides a genetically and metabolically simpler background, enabling convenient studies on the complex information processing system. Therefore, Archaea could serve as a parsimonious and tractable host for studying human diseases that arise in the information processing systems.

A [3Fe-4S] cluster is required for tRNA thiolation in archaea and eukaryotes

The sulfur-containing nucleosides in transfer RNA (tRNAs) are present in all three domains of life; they have critical functions for accurate and efficient translation, such as tRNA structure stabilization and proper codon recognition. The tRNA modification enzymes ThiI (in bacteria and archaea) and Ncs6 (in archaea and eukaryotic cytosols) catalyze the formation of 4-thiouridine (s⁴U) and 2-thiouridine (s²U), respectively. The ThiI homologs were proposed to transfer sulfur via cysteine persulfide enzyme adducts, whereas the reaction mechanism of Ncs6 remains unknown. Here we show that ThiI from the archaeon Methanococcus maripaludis contains a [3Fe-4S] cluster that is essential for its tRNA thiolation activity. Furthermore, the archaeal and eukaryotic Ncs6 homologs as well as phosphoseryl-tRNA (Sep-tRNA):Cys-tRNA synthase (SepCysS), which catalyzes the Sep-tRNA to Cys-tRNA conversion in methanogens, also possess a [3Fe-4S] cluster similar to the methanogenic archaeal ThiI. These results suggest that the diverse tRNA thiolation processes in archaea and eukaryotic cytosols share a common mechanism dependent on a [3Fe-4S] cluster for sulfur transfer.

Anticodon Modifications in the tRNA Set of LUCA and the Fundamental Regularity in the Standard Genetic Code

Based on (i) an analysis of the regularities in the standard genetic code and (ii) comparative genomics of the anticodon modification machinery in the three branches of life, we derive the tRNA set and its anticodon modifications as it was present in LUCA. Previously we proposed that an early ancestor of LUCA contained a set of 23 tRNAs with unmodified anticodons that was capable of translating all 20 amino acids while reading 55 of the 61 sense codons of the standard genetic code (SGC). Here we use biochemical and genomic evidence to derive that LUCA contained a set of 44 or 45 tRNAs containing 2 or 3 modifications while reading 59 or 60 of the 61 sense codons. Subsequent tRNA modifications occurred independently in the Bacteria and Eucarya, while the Archaea have remained quite close to the tRNA set as it was present in LUCA.

Abbreviated Pathway for Biosynthesis of 2-Thiouridine in Bacillus subtilis

The 2-thiouridine (s(2)U) modification of the wobble position in glutamate, glutamine, and lysine tRNA molecules serves to stabilize the anticodon structure, improving ribosomal binding and overall efficiency of the translational process. Biosynthesis of s(2)U in Escherichia coli requires a cysteine desulfurase (IscS), a thiouridylase (MnmA), and five intermediate sulfur-relay enzymes (TusABCDE). The E. coli MnmA adenylates and subsequently thiolates tRNA to form the s(2)U modification. Bacillus subtilis lacks IscS and the intermediate sulfur relay proteins, yet its genome contains a cysteine desulfurase gene, yrvO, directly adjacent to mnmA. The genomic synteny of yrvO and mnmA combined with the absence of the Tus proteins indicated a potential functionality of these proteins in s(2)U formation. Here, we provide evidence that the B. subtilis YrvO and MnmA are sufficient for s(2)U biosynthesis. A conditional B. subtilis knockout strain showed that s(2)U abundance correlates with MnmA expression, and in vivo complementation studies in E. coli IscS- or MnmA-deficient strains revealed the competency of these proteins in s(2)U biosynthesis. In vitro experiments demonstrated s(2)U formation by YrvO and MnmA, and kinetic analysis established a partnership between the B. subtilis proteins that is contingent upon the presence of ATP. Furthermore, we observed that the slow-growth phenotype of E. coli ΔiscS and ΔmnmA strains associated with s(2)U depletion is recovered by B. subtilis yrvO and mnmA. These results support the proposal that the involvement of a devoted cysteine desulfurase, YrvO, in s(2)U synthesis bypasses the need for a complex biosynthetic pathway by direct sulfur transfer to MnmA.

IMPORTANCE

The 2-thiouridine (s(2)U) modification of the wobble position in glutamate, glutamine, and lysine tRNA is conserved in all three domains of life and stabilizes the anticodon structure, thus guaranteeing fidelity in translation. The biosynthesis of s(2)U in Escherichia coli requires seven proteins: the cysteine desulfurase IscS, the thiouridylase MnmA, and five intermediate sulfur-relay enzymes (TusABCDE). Bacillus subtilis and most Gram-positive bacteria lack a complete set of biosynthetic components. Interestingly, the mnmA coding sequence is located adjacent to yrvO, encoding a cysteine desulfurase. In this work, we provide evidence that the B. subtilis YrvO is able to transfer sulfur directly to MnmA. Both proteins are sufficient for s(2)U biosynthesis in a pathway independent of the one used in E. coli.