Figure 3.

A specialized 5'-editing pathway that acts on tRNA^His. In Bacteria and Archaea, tRNA^His species are almost always encoded with an additional 5'-G nucleotide that is retained during 5'-end maturation by RNase P (indicated by the blue lightning bolt), ensuring the presence of the 5'-G-1 residue that is required to recognize the tRNA^His by its canonical Histidyl-tRNA synthetase (HisRS). However, in most eukaryotes, the pre-tRNA^His does not encode the additional G-1 and eukaryotic Thg1 enzymes catalyze a specialized 5'-end editing reaction to post-transcriptionally add G-1 to the tRNA and thus ensure proper recognition. Recently, some eukaryotes have been identified that lack a bona fide Thg1 ortholog, and in these cases, the lack of the 5'-edited tRNA^His is accommodated by the presence of a non-canonical HisRS that is capable of efficiently aminoacylating the tRNA even in the absence of G-1. The G-1 nucleotide is indicated in red, and the N+1 nucleotide that represents the normal 5'-end for all other tRNA species is highlighted in blue.