tRNA 5′-end repair activities of tRNA(His) guanylyltransferase (Thg1)-like proteins from Bacteria and Archaea

The tRNA(His) guanylyltransferase (Thg1) family comprises a set of unique 3′–5′ nucleotide addition enzymes found ubiquitously in Eukaryotes, where they function in the critical G(−1) addition reaction required for tRNA(His) maturation. However, in most Bacteria and Archaea, G(−1) is genomically encoded; thus post-transcriptional addition of G(−1) to tRNA(His) is not necessarily required. The presence of highly conserved Thg1-like proteins (TLPs) in more than 40 bacteria and archaea therefore suggests unappreciated roles for TLP-catalyzed 3′–5′ nucleotide addition. Here, we report that TLPs from Bacillus thuringiensis (BtTLP) and Methanosarcina acetivorans (MaTLP) display biochemical properties consistent with a prominent role in tRNA 5′-end repair. Unlike yeast Thg1, BtTLP strongly prefers addition of missing N(+1) nucleotides to 5′-truncated tRNAs over analogous additions to full-length tRNA (k(cat)/K(M) enhanced 5–160-fold). Moreover, unlike for −1 addition, BtTLP-catalyzed additions to truncated tRNAs are not biased toward addition of G, and occur with tRNAs other than tRNA(His). Based on these distinct biochemical properties, we propose that rather than functioning solely in tRNA(His) maturation, bacterial and archaeal TLPs are well-suited to participate in tRNA quality control pathways. These data support more widespread roles for 3′–5′ nucleotide addition reactions in biology than previously expected.