Kinetic analysis of 3'-5' nucleotide addition catalyzed by eukaryotic tRNA(His) guanylyltransferase

The life and times of a tRNA

The study of eukaryotic tRNA processing has given rise to an explosion of new information and insights in the last several years. We now have unprecedented knowledge of each step in the tRNA processing pathway, revealing unexpected twists in biochemical pathways, multiple new connections with regulatory pathways, and numerous biological effects of defects in processing steps that have profound consequences throughout eukaryotes, leading to growth phenotypes in the yeast Saccharomyces cerevisiae and to neurological and other disorders in humans. This review highlights seminal new results within the pathways that comprise the life of a tRNA, from its birth after transcription until its death by decay. We focus on new findings and revelations in each step of the pathway including the end-processing and splicing steps, many of the numerous modifications throughout the main body and anticodon loop of tRNA that are so crucial for tRNA function, the intricate tRNA trafficking pathways, and the quality control decay pathways, as well as the biogenesis and biology of tRNA-derived fragments. We also describe the many interactions of these pathways with signaling and other pathways in the cell.

Analysis of GTP addition in the reverse (3'-5') direction by human tRNAHis guanylyltransferase

Human tRNA^His guanylyltransferase (HsThg1) catalyzes the 3'-5' addition of guanosine triphosphate (GTP) to the 5'-end (-1 position) of tRNA^His, producing mature tRNA^His In human cells, cytoplasmic and mitochondrial tRNA^His have adenine (A) or cytidine (C), respectively, opposite to G_-1 Little attention has been paid to the structural requirements of incoming GTP in 3'-5' nucleotidyl addition by HsThg1. In this study, we evaluated the incorporation efficiencies of various GTP analogs by HsThg1 and compared the reaction mechanism with that of Candida albicans Thg1 (CaThg1). HsThg1 incorporated GTP opposite A or C in the template most efficiently. In contrast to CaThg1, HsThg1 could incorporate UTP opposite A, and guanosine diphosphate (GDP) opposite C. These results suggest that HsThg1 could transfer not only GTP, but also other NTPs, by forming Watson-Crick (WC) hydrogen bonds between the incoming NTP and the template base. On the basis of the molecular mechanism, HsThg1 succeeded in labeling the 5'-end of tRNA^His with biotinylated GTP. Structural analysis of HsThg1 was also performed in the presence of the mitochondrial tRNA^His Structural comparison of HsThg1 with other Thg1 family enzymes suggested that the structural diversity of the carboxy-terminal domain of the Thg1 enzymes might be involved in the formation of WC base-pairing between the incoming GTP and template base. These findings provide new insights into an unidentified biological function of HsThg1 and also into the applicability of HsThg1 to the 5'-terminal modification of RNAs.

Fidelity of base-pair recognition by a 3'-5' polymerase: mechanism of the Saccharomyces cerevisiae tRNAHis guanylyltransferase

The tRNA^His guanylyltransferase (Thg1) was originally discovered in Saccharomyces cerevisiae where it catalyzes 3'-5' addition of a single nontemplated guanosine (G_-1) to the 5' end of tRNA^His In addition to this activity, S. cerevisiae Thg1 (SceThg1) also catalyzes 3'-5' polymerization of Watson-Crick (WC) base pairs, utilizing nucleotides in the 3'-end of a tRNA as the template for addition. Subsequent investigation revealed an entire class of enzymes related to Thg1, called Thg1-like proteins (TLPs). TLPs are found in all three domains of life and preferentially catalyze 3'-5' polymerase activity, utilizing this unusual activity to repair tRNA, among other functions. Although both Thg1 and TLPs utilize the same chemical mechanism, the molecular basis for differences between WC-dependent (catalyzed by Thg1 and TLPs) and non-WC-dependent (catalyzed exclusively by Thg1) reactions has not been fully elucidated. Here we investigate the mechanism of base-pair recognition by 3'-5' polymerases using transient kinetic assays, and identify Thg1-specific residues that play a role in base-pair discrimination. We reveal that, regardless of the identity of the opposing nucleotide in the RNA "template," addition of a non-WC G_-1 residue is driven by a unique kinetic preference for GTP. However, a secondary preference for forming WC base pairs is evident for all possible templating residues. Similar to canonical 5'-3' polymerases, nucleotide addition by SceThg1 is driven by the maximal rate rather than by NTP substrate affinity. Together, these data provide new insights into the mechanism of base-pair recognition by 3'-5' polymerases.

Chemical footprinting and kinetic assays reveal dual functions for highly conserved eukaryotic tRNAHis guanylyltransferase residues

tRNA^His guanylyltransferase (Thg1) adds a single guanine to the -1 position of tRNA^His as part of its maturation. This seemingly modest addition of one nucleotide to tRNA^His ensures translational fidelity by providing a critical identity element for the histidyl aminoacyl tRNA synthetase (HisRS). Like HisRS, Thg1 utilizes the GUG anticodon for selective tRNA^His recognition, and Thg1-tRNA complex structures have revealed conserved residues that interact with anticodon nucleotides. Separately, kinetic analysis of alanine variants has demonstrated that many of these same residues are required for catalytic activity. A model in which loss of activity with the variants was attributed directly to loss of the critical anticodon interaction has been proposed to explain the combined biochemical and structural results. Here we used RNA chemical footprinting and binding assays to test this model and further probe the molecular basis for the requirement for two critical tRNA-interacting residues, His-152 and Lys-187, in the context of human Thg1 (hThg1). Surprisingly, we found that His-152 and Lys-187 alanine-substituted variants maintain a similar overall interaction with the anticodon region, arguing against the sufficiency of this interaction for driving catalysis. Instead, conservative mutagenesis revealed a new direct function for these residues in recognition of a non-Watson-Crick G_-1:A₇₃ bp, which had not been described previously. These results have important implications for the evolution of eukaryotic Thg1 from a family of ancestral promiscuous RNA repair enzymes to the highly selective enzymes needed for their essential function in tRNA^His maturation.

The Role of 3' to 5' Reverse RNA Polymerization in tRNA Fidelity and Repair

The tRNA^His guanylyltransferase (Thg1) superfamily includes enzymes that are found in all three domains of life that all share the common ability to catalyze the 3′ to 5′ synthesis of nucleic acids. This catalytic activity, which is the reverse of all other known DNA and RNA polymerases, makes this enzyme family a subject of biological and mechanistic interest. Previous biochemical, structural, and genetic investigations of multiple members of this family have revealed that Thg1 enzymes use the 3′ to 5′ chemistry for multiple reactions in biology. Here, we describe the current state of knowledge regarding the catalytic features and biological functions that have been so far associated with Thg1 and its homologs. Progress toward the exciting possibility of utilizing this unusual protein activity for applications in biotechnology is also discussed.

A Temporal Order in 5'- and 3'- Processing of Eukaryotic tRNAHis

For flawless translation of mRNA sequence into protein, tRNAs must undergo a series of essential maturation steps to be properly recognized and aminoacylated by aminoacyl-tRNA synthetase, and subsequently utilized by the ribosome. While all tRNAs carry a 3'-terminal CCA sequence that includes the site of aminoacylation, the additional 5'-G-1 position is a unique feature of most histidine tRNA species, serving as an identity element for the corresponding synthetase. In eukaryotes including yeast, both 3'-CCA and 5'-G-1 are added post-transcriptionally by tRNA nucleotidyltransferase and tRNA^His guanylyltransferase, respectively. Hence, it is possible that these two cytosolic enzymes compete for the same tRNA. Here, we investigate substrate preferences associated with CCA and G-1-addition to yeast cytosolic tRNA^His, which might result in a temporal order to these important processing events. We show that tRNA nucleotidyltransferase accepts tRNA^His transcripts independent of the presence of G-1; however, tRNA^His guanylyltransferase clearly prefers a substrate carrying a CCA terminus. Although many tRNA maturation steps can occur in a rather random order, our data demonstrate a likely pathway where CCA-addition precedes G-1 incorporation in S. cerevisiae. Evidently, the 3'-CCA triplet and a discriminator position A73 act as positive elements for G-1 incorporation, ensuring the fidelity of G-1 addition.

Molecular mechanism of substrate recognition and specificity of tRNAHis guanylyltransferase during nucleotide addition in the 3'-5' direction

The tRNA^His guanylyltransferase (Thg1) transfers a guanosine triphosphate (GTP) in the 3'-5' direction onto the 5'-terminal of tRNA^His, opposite adenosine at position 73 (A₇₃). The guanosine at the -1 position (G_-1) serves as an identity element for histidyl-tRNA synthetase. To investigate the mechanism of recognition for the insertion of GTP opposite A₇₃, first we constructed a two-stranded tRNA^His molecule composed of a primer and a template strand through division at the D-loop. Next, we evaluated the structural requirements of the incoming GTP from the incorporation efficiencies of GTP analogs into the two-piece tRNA^His Nitrogen at position 7 and the 6-keto oxygen of the guanine base were important for G_-1 addition; however, interestingly, the 2-amino group was found not to be essential from the highest incorporation efficiency of inosine triphosphate. Furthermore, substitution of the conserved A₇₃ in tRNA^His revealed that the G_-1 addition reaction was more efficient onto the template containing the opposite A₇₃ than onto the template with cytidine (C₇₃) or other bases forming canonical Watson-Crick base-pairing. Some interaction might occur between incoming GTP and A₇₃, which plays a role in the prevention of continuous templated 3'-5' polymerization. This study provides important insights into the mechanism of accurate tRNA^His maturation.

Biochemical analysis of human tRNAHis guanylyltransferase in mitochondrial tRNAHis maturation

Mitochondria contain their own protein synthesis machinery, which includes mitochondrial tRNA maturation. It has been suggested that mammalian mitochondrial tRNA^His (mtRNA^His) is matured by post-transcriptional addition of guanosine at the -1 position (G_-1), which serves as an identity element for mitochondrial histidyl-tRNA synthetase. However, the exact maturation process of mammalian mtRNA^His remains unclear. In cytoplasmic tRNA^His (ctRNA^His) maturation, tRNA^His guanylyltransferase (Thg1) adds a GTP onto the 5'-terminal of ctRNA^His and then removes the 5'-pyrophosphate to yield the mature 5'-monophospholylated G_-1-ctRNA^His (pG_-1-ctRNA^His). Although mammalian Thg1 is localized to both the cytoplasm and mitochondria, it remains unclear whether mammalian Thg1 plays a role in mtRNA^His maturation in mitochondria. Here, we demonstrated that human Thg1 (hThg1) catalyzes the G_-1 addition reaction for both human ctRNA^His and mtRNA^His through recognition of the anticodon. While hThg1 catalyzed consecutive GTP additions to mtRNA^Hisin vitro, it did not exhibit any activity toward mature pG_-1-mtRNA^His. We further found that hThg1 could add a GMP directly to the 5'-terminal of mtRNA^His in a template-dependent manner, but fungal Thg1 could not. Therefore, we hypothesized that acceleration of the pyrophosphate removal activity before or after the G_-1 addition reaction is a key feature of hThg1 for maintaining a normal 5'-terminal of mtRNA^His in human mitochondria. This study provided a new insight into the differences between tRNA^His maturation in the cytoplasm and mitochondria of humans.

Minimal requirements for reverse polymerization and tRNA repair by tRNAHis guanylyltransferase

tRNA^His guanylyltransferase (Thg1) has unique reverse (3'-5') polymerase activity occurring in all three domains of life. Most eukaryotic Thg1 homologs are essential genes involved in tRNA^His maturation. These enzymes normally catalyze a single 5' guanylation of tRNA^His lacking the essential G^-1 identity element required for aminoacylation. Recent studies suggest that archaeal type Thg1, which includes most archaeal and bacterial Thg1 enzymes is phylogenetically distant from eukaryotic Thg1. Thg1 is evolutionarily related to canonical 5'-3' forward polymerases but catalyzes reverse 3'-5'polymerization. Similar to its forward polymerase counterparts, Thg1 encodes the conserved catalytic palm domain and fingers domain. Here we investigate the minimal requirements for reverse polymerization. We show that the naturally occurring minimal Thg1 enzyme from Ignicoccus hospitalis (IhThg1), which lacks parts of the conserved fingers domain, is catalytically active. And adds all four natural nucleotides to RNA substrates, we further show that the entire fingers domain of Methanosarcina acetivorans Thg1 and Pyrobaculum aerophilum Thg1 (PaThg1) is dispensable for enzymatic activity. In addition, we identified residues in yeast Thg1 that play a part in preventing extended polymerization. Mutation of these residues with alanine resulted in extended reverse polymerization. PaThg1 was found to catalyze extended, template dependent tRNA repair, adding up to 13 nucleotides to a truncated tRNA^His substrate. Sequencing results suggest that PaThg1 fully restored the near correct sequence of the D- and acceptor stem, but also produced incompletely and incorrectly repaired tRNA products. This research forms the basis for future engineering efforts towards a high fidelity, template dependent reverse polymerase.

Identification of distinct biological functions for four 3'-5' RNA polymerases

The superfamily of 3'-5' polymerases synthesize RNA in the opposite direction to all other DNA/RNA polymerases, and its members include eukaryotic tRNA(His) guanylyltransferase (Thg1), as well as Thg1-like proteins (TLPs) of unknown function that are broadly distributed, with family members in all three domains of life. Dictyostelium discoideum encodes one Thg1 and three TLPs (DdiTLP2, DdiTLP3 and DdiTLP4). Here, we demonstrate that depletion of each of the genes results in a significant growth defect, and that each protein catalyzes a unique biological reaction, taking advantage of specialized biochemical properties. DdiTLP2 catalyzes a mitochondria-specific tRNA(His) maturation reaction, which is distinct from the tRNA(His) maturation reaction typically catalyzed by Thg1 enzymes on cytosolic tRNA. DdiTLP3 catalyzes tRNA repair during mitochondrial tRNA 5'-editing in vivo and in vitro, establishing template-dependent 3'-5' polymerase activity of TLPs as a bona fide biological activity for the first time since its unexpected discovery more than a decade ago. DdiTLP4 is cytosolic and, surprisingly, catalyzes robust 3'-5' polymerase activity on non-tRNA substrates, strongly implying further roles for TLP 3'-5' polymerases in eukaryotes.

A mutation in the THG1L gene in a family with cerebellar ataxia and developmental delay

Autosomal-recessive cerebellar atrophy is usually associated with inactivating mutations and early-onset presentation. The underlying molecular diagnosis suggests the involvement of neuronal survival pathways, but many mechanisms are still lacking and most patients elude genetic diagnosis. Using whole exome sequencing, we identified homozygous p.Val55Ala in the THG1L (tRNA-histidine guanylyltransferase 1 like) gene in three siblings who presented with cerebellar signs, developmental delay, dysarthria, and pyramidal signs and had cerebellar atrophy on brain MRI. THG1L protein was previously reported to participate in mitochondrial fusion via its interaction with MFN2. Abnormal mitochondrial fragmentation, including mitochondria accumulation around the nuclei and confinement of the mitochondrial network to the nuclear vicinity, was observed when patient fibroblasts were cultured in galactose containing medium. Culturing cells in galactose containing media promotes cellular respiration by oxidative phosphorylation and the action of the electron transport chain thus stimulating mitochondrial activity. The growth defect of the yeast thg1Δ strain was rescued by the expression of either yeast Thg1 or human THG1L; however, clear growth defect was observed following the expression of the human p.Val55Ala THG1L or the corresponding yeast mutant. A defect in the protein tRNA^His guanylyltransferase activity was excluded by the normal in vitro G_-1 addition to either yeast tRNA^His or human mitochondrial tRNA^His in the presence of the THG1L mutation. We propose that homozygosity for the p.Val55Ala mutation in THG1L is the cause of the abnormal mitochondrial network in the patient fibroblasts, likely by interfering with THG1L activity towards MFN2. This may result in lack of mitochondria in the cerebellar Purkinje dendrites, with degeneration of Purkinje cell bodies and apoptosis of granule cells, as reported for MFN2 deficient mice.

In vitro substrate specificities of 3'-5' polymerases correlate with biological outcomes of tRNA 5'-editing reactions

Protozoan mitochondrial tRNAs (mt-tRNAs) are repaired by a process known as 5'-editing. Mt-tRNA sequencing revealed organism-specific patterns of editing G-U base pairs, wherein some species remove G-U base pairs during 5'-editing, while others retain G-U pairs in the edited tRNA. We tested whether 3'-5' polymerases that catalyze the repair step of 5'-editing exhibit organism-specific preferences that explain the treatment of G-U base pairs. Biochemical and kinetic approaches revealed that a 3'-5' polymerase from Acanthamoeba castellanii tolerates G-U wobble pairs in editing substrates much more readily than several other enzymes, consistent with its biological pattern of editing.

From end to end: tRNA editing at 5'- and 3'-terminal positions

During maturation, tRNA molecules undergo a series of individual processing steps, ranging from exo- and endonucleolytic trimming reactions at their 5'- and 3'-ends, specific base modifications and intron removal to the addition of the conserved 3'-terminal CCA sequence. Especially in mitochondria, this plethora of processing steps is completed by various editing events, where base identities at internal positions are changed and/or nucleotides at 5'- and 3'-ends are replaced or incorporated. In this review, we will focus predominantly on the latter reactions, where a growing number of cases indicate that these editing events represent a rather frequent and widespread phenomenon. While the mechanistic basis for 5'- and 3'-end editing differs dramatically, both reactions represent an absolute requirement for generating a functional tRNA. Current in vivo and in vitro model systems support a scenario in which these highly specific maturation reactions might have evolved out of ancient promiscuous RNA polymerization or quality control systems.

Mitochondrial tRNA 5'-editing in Dictyostelium discoideum and Polysphondylium pallidum

Mitochondrial tRNA (mt-tRNA) 5'-editing was first described more than 20 years ago; however, the first candidates for 5'-editing enzymes were only recently identified in a eukaryotic microbe (protist), the slime mold Dictyostelium discoideum. In this organism, eight of 18 mt-tRNAs are predicted to be edited based on the presence of genomically encoded mismatched nucleotides in their aminoacyl-acceptor stem sequences. Here, we demonstrate that mt-tRNA 5'-editing occurs at all predicted sites in D. discoideum as evidenced by changes in the sequences of isolated mt-tRNAs compared with the expected sequences encoded by the mitochondrial genome. We also identify two previously unpredicted editing events in which G-U base pairs are edited in the absence of any other genomically encoded mismatches. A comparison of 5'-editing in D. discoideum with 5'-editing in another slime mold, Polysphondylium pallidum, suggests organism-specific idiosyncrasies in the treatment of U-G/G-U pairs. In vitro activities of putative D. discoideum editing enzymes are consistent with the observed editing reactions and suggest an overall lack of tRNA substrate specificity exhibited by the repair component of the editing enzyme. Although the presence of terminal mismatches in mt-tRNA sequences is highly predictive of the occurrence of mt-tRNA 5'-editing, the variability in treatment of U-G/G-U base pairs observed here indicates that direct experimental evidence of 5'-editing must be obtained to understand the complete spectrum of mt-tRNA editing events in any species.

Saccharomyces cerevisiae Thg1 uses 5'-pyrophosphate removal to control addition of nucleotides to tRNA(His.)

In eukaryotes, the tRNA^His guanylyltransferase (Thg1) catalyzes 3′–5′ addition of a single guanosine residue to the −1 position (G_–1) of tRNA^His, across from a highly conserved adenosine at position 73 (A₇₃). After addition of G_–1, Thg1 removes pyrophosphate from the tRNA 5′-end, generating 5′-monophosphorylated G_–1-containing tRNA. The presence of the 5′-monophosphorylated G_–1 residue is important for recognition of tRNA^His by its cognate histidyl-tRNA synthetase. In addition to the single-G_–1 addition reaction, Thg1 polymerizes multiple G residues to the 5′-end of tRNA^His variants. For 3′–5′ polymerization, Thg1 uses the 3′-end of the tRNA^His acceptor stem as a template. The mechanism of reverse polymerization is presumed to involve nucleophilic attack of the 3′-OH from each incoming NTP on the intact 5′-triphosphate created by the preceding nucleotide addition. The potential exists for competition between 5′-pyrophosphate removal and 3′–5′ polymerase reactions that could define the outcome of Thg1-catalyzed addition, yet the interplay between these competing reactions has not been investigated for any Thg1 enzyme. Here we establish transient kinetic assays to characterize the pyrophosphate removal versus nucleotide addition activities of yeast Thg1 with a set of tRNA^His substrates in which the identity of the N_–1:N₇₃ base pair was varied to mimic various products of the N_–1 addition reaction catalyzed by Thg1. We demonstrate that retention of the 5′-triphosphate is correlated with efficient 3′–5′ reverse polymerization. A kinetic partitioning mechanism that acts to prevent addition of nucleotides beyond the −1 position with wild-type tRNA^His is proposed.

Structural basis of reverse nucleotide polymerization

Nucleotide polymerization proceeds in the forward (5'-3') direction. This tenet of the central dogma of molecular biology is found in diverse processes including transcription, reverse transcription, DNA replication, and even in lagging strand synthesis where reverse polymerization (3'-5') would present a "simpler" solution. Interestingly, reverse (3'-5') nucleotide addition is catalyzed by the tRNA maturation enzyme tRNA(His) guanylyltransferase, a structural homolog of canonical forward polymerases. We present a Candida albicans tRNA(His) guanylyltransferase-tRNA(His) complex structure that reveals the structural basis of reverse polymerization. The directionality of nucleotide polymerization is determined by the orientation of approach of the nucleotide substrate. The tRNA substrate enters the enzyme's active site from the opposite direction (180° flip) compared with similar nucleotide substrates of canonical 5'-3' polymerases, and the finger domains are on opposing sides of the core palm domain. Structural, biochemical, and phylogenetic data indicate that reverse polymerization appeared early in evolution and resembles a mirror image of the forward process.

Structural studies of a bacterial tRNA(HIS) guanylyltransferase (Thg1)-like protein, with nucleotide in the activation and nucleotidyl transfer sites

All nucleotide polymerases and transferases catalyze nucleotide addition in a 5' to 3' direction. In contrast, tRNA(His) guanylyltransferase (Thg1) enzymes catalyze the unusual reverse addition (3' to 5') of nucleotides to polynucleotide substrates. In eukaryotes, Thg1 enzymes use the 3'-5' addition activity to add G-1 to the 5'-end of tRNA(His), a modification required for efficient aminoacylation of the tRNA by the histidyl-tRNA synthetase. Thg1-like proteins (TLPs) are found in Archaea, Bacteria, and mitochondria and are biochemically distinct from their eukaryotic Thg1 counterparts TLPs catalyze 5'-end repair of truncated tRNAs and act on a broad range of tRNA substrates instead of exhibiting strict specificity for tRNA(His). Taken together, these data suggest that TLPs function in distinct biological pathways from the tRNA(His) maturation pathway, perhaps in tRNA quality control. Here we present the first crystal structure of a TLP, from the gram-positive soil bacterium Bacillus thuringiensis (BtTLP). The enzyme is a tetramer like human THG1, with which it shares substantial structural similarity. Catalysis of the 3'-5' reaction with 5'-monophosphorylated tRNA necessitates first an activation step, generating a 5'-adenylylated intermediate prior to a second nucleotidyl transfer step, in which a nucleotide is transferred to the tRNA 5'-end. Consistent with earlier characterization of human THG1, we observed distinct binding sites for the nucleotides involved in these two steps of activation and nucleotidyl transfer. A BtTLP complex with GTP reveals new interactions with the GTP nucleotide in the activation site that were not evident from the previously solved structure. Moreover, the BtTLP-ATP structure allows direct observation of ATP in the activation site for the first time. The BtTLP structural data, combined with kinetic analysis of selected variants, provide new insight into the role of key residues in the activation step.

Absence of a universal element for tRNAHis identity in Acanthamoeba castellanii

The additional G(-1) nucleotide on tRNA(His) is a nearly universal feature that specifies tRNA(His) identity in all three domains of life. In eukaryotes, the G(-1) identity element is obtained by a post-transcriptional pathway, through the unusual 3'-5' polymerase activity of the highly conserved tRNA(His) guanylyltransferase (Thg1) enzyme, and no examples of eukaryotic histidyl-tRNAs that lack this essential element have been identified. Here we report that the eukaryote Acanthamoeba castellanii lacks the G(-1) identity element on its tRNA(His), consistent with the lack of a gene encoding a bona fide Thg1 ortholog in the A. castellanii genome. Moreover, the cytosolic histidyl-tRNA synthetase in A. castellanii exhibits an unusual tRNA substrate specificity, efficiently aminoacylating tRNA(His) regardless of the presence of G(-1). A. castellanii does contain two Thg1-related genes (encoding Thg1-like proteins, TLPs), but the biochemical properties we associate here with these proteins are consistent with a function for these TLPs in separate pathways unrelated to tRNA(His) metabolism, such as mitochondrial tRNA repair during 5'-editing.

Doing it in reverse: 3'-to-5' polymerization by the Thg1 superfamily

The tRNA(His) guanylyltransferase (Thg1) family of enzymes comprises members from all three domains of life (Eucarya, Bacteria, Archaea). Although the initial activity associated with Thg1 enzymes was a single 3'-to-5' nucleotide addition reaction that specifies tRNA(His) identity in eukaryotes, the discovery of a generalized base pair-dependent 3'-to-5' polymerase reaction greatly expanded the scope of Thg1 family-catalyzed reactions to include tRNA repair and editing activities in bacteria, archaea, and organelles. While the identification of the 3'-to-5' polymerase activity associated with Thg1 enzymes is relatively recent, the roots of this discovery and its likely physiological relevance were described ≈ 30 yr ago. Here we review recent advances toward understanding diverse Thg1 family enzyme functions and mechanisms. We also discuss possible evolutionary origins of Thg1 family-catalyzed 3'-to-5' addition activities and their implications for the currently observed phylogenetic distribution of Thg1-related enzymes in biology.