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

Abstract

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.