Life without post-transcriptional addition of G-1: two alternatives for tRNAHis identity in Eukarya

The identity of tRNA(His) is strongly associated with the presence of an additional 5'-guanosine residue (G-1) in all three domains of life. The critical nature of the G-1 residue is underscored by the fact that two entirely distinct mechanisms for its acquisition are observed, with cotranscriptional incorporation observed in Bacteria, while post-transcriptional addition of G-1 occurs in Eukarya. Here, through our investigation of eukaryotes that lack obvious homologs of the post-transcriptional G-1-addition enzyme Thg1, we identify alternative pathways to tRNA(His) identity that controvert these well-established rules. We demonstrate that Trypanosoma brucei, like Acanthamoeba castellanii, lacks the G-1 identity element on tRNA(His) and utilizes a noncanonical G-1-independent histidyl-tRNA synthetase (HisRS). Purified HisRS enzymes from A. castellanii and T. brucei exhibit a mechanism of tRNA(His) recognition that is distinct from canonical G-1-dependent synthetases. Moreover, noncanonical HisRS enzymes genetically complement the loss of THG1 in Saccharomyces cerevisiae, demonstrating the biological relevance of the G-1-independent aminoacylation activity. In contrast, in Caenorhabditis elegans, which is another Thg1-independent eukaryote, the G-1 residue is maintained, but here its acquisition is noncanonical. In this case, the G-1 is encoded and apparently retained after 5' end processing, which has so far only been observed in Bacteria and organelles. Collectively, these observations unearth a widespread and previously unappreciated diversity in eukaryotic tRNA(His) identity mechanisms.