tRNA Recognition of tRNA-guanine Transglycosylase from a Hyperthermophilic Archaeon, Pyrococcus horikoshii

Abstract

In the biosynthesis of archaeosine, archaeal tRNAguanine transglycosylase (TGT) catalyzes the replacement of guanine at position 15 in the D loop of most tRNAs by a free precursor base. We examined the tRNA recognition of TGT from a hyperthermophilic archaeon, Pyrococcus horikoshii. Mutational studies using variant tRNA Val transcripts revealed that both guanine and its location (position 15) were strictly recognized by TGT without any other sequence-specific requirements. It appeared that neither the global L-shaped structure of a tRNA nor the local conformation of the D loop contributed to recognition by TGT. A minihelix composed of the acceptor stem and D arm of tRNA Val , designed as a potential minimal substrate, failed to serve as a substrate for TGT. Only a minihelix with mismatched nucleotides at the junction between the two domains served as a good substrate, suggesting that mismatched nucleotides in the helix provide the specific information that allows TGT to recognize the guanine in the D loop. Our findings indicate that the tRNA recognition requirements of P. horikoshii TGT are sufficiently limited and specific to allow the enzyme to recognize efficiently any tRNA species whose structure is not fully stabilized in an extremely high temperature environment.Among the large number of modified nucleosides found in tRNAs, only two hypermodified nucleotides have a 7-deazaguanosine structure, namely, queuosine and archaeosine. Queuosine (Q 1 ; 7-(((4,5-cis-dihydroxy-2-cyclopenten-1-yl)amino)methyl)-7-deazaguanosine) has been found at position 34, the wobble position of the anticodon, of four tRNAs with the anticodon sequence GUN (specific for amino acids Asp, Asn, His, and Tyr), but it has been found in most eubacteria and eukaryotes (1-3). Queuosine is involved not only in the fine tuning of translation in eubacteria, as suggested by its location at the wobble position, but also in cellular events such as development, differentiation, aging, and cancer in eukaryotes. However, the mechanisms responsible for such phenomena are not fully understood (for reviews, see Refs. 4 and 5). Archaeosine (G*; 7-formamidino-7-deazaguanosine) has been found at position 15 in the D loop of tRNAs from many archaea (6, 7). In Haloferax volcanii, the archaeon that has been examined in the greatest detail, archaeosine has been identified in more than 15 tRNA species (8, 9), but its role remains unknown.A characteristic feature of the biosynthesis of both queuosine and archaeosine is the base replacement reaction that is catalyzed by tRNA-guanine transglycosylase (TGT) (10 -13). Eubacterial TGTs catalyze the replacement of guanine at position 34 by the free precursor, 7-aminomethyl-7-deazaguanine (preQ 1 ), and archaeal TGT catalyzes that at position 15 by 7-cyano-7-deazaguanine (preQ 0 ). Subsequent modification is catalyzed by two different enzymes to yield the final products in Escherichia coli (14, 15), whereas such reactions are presumably catalyzed by at least one as yet unidentified enzyme in archaea. By contrast, eukar...

Evolutionary Divergence of the Archaeal Aspartyl-tRNA Synthetases into Discriminating and Nondiscriminating Forms

“…The P. kodakaraensis and M. thermautotrophicus tRNA transcripts were poorly charged (Ͻ10%) under a range of conditions by all AspRSs tested except at high enzyme concentrations (not shown). The poor aminoacylation levels of these transcripts may result from incorrect folding because of a lack of modifications that have been shown to be necessary for stability of the tRNA at high temperatures (28). However, the F. acidarmanus tRNA transcripts could be highly charged.…”

Substrate Specificity for 4-Thiouridine Modification in Escherichia coli

“…Other tRNA-modifying enzymes have minimal recognition elements similar to that of ThiI. These include RNase P (53) and the archael G15 guanine trans-glycosylase that initiates archaeosine biosynthesis (54). Both recognize bulged mini-helix portions of the tRNA.…”

Mg2+ binding and archaeosine modification stabilize the G15–C48 Levitt base pair in tRNAs

“…Commonly known as archaeosine from its domain specificity, this is one of the most remarkable post-transcriptional chemical modifications identified in tRNA molecules. As a result of the G15 replacement by a 7-cyano-7-deazaguanine (preQ 0 ), performed by Archaeal tRNA-guanine transglycosylae (ArcTGT) (Watanabe et al 2001;Iwata-Reuyl 2003), and of subsequent enzymatic steps that remain to be clarified, archaeosine bears a charged imidino side chain on the C7 atom of the 7-deazaguanine core.…”