Absence of a universal element for tRNA(His) identity in Acanthamoeba castellanii

The additional G(−1) nucleotide on tRNA(His) is a nearly universal feature that specifies tRNA(His) identity in all three domains of life. In eukaryotes, the G(−1) identity element is obtained by a post-transcriptional pathway, through the unusual 3′–5′ polymerase activity of the highly conserved tRNA(His) guanylyltransferase (Thg1) enzyme, and no examples of eukaryotic histidyl-tRNAs that lack this essential element have been identified. Here we report that the eukaryote Acanthamoeba castellanii lacks the G(−1) identity element on its tRNA(His), consistent with the lack of a gene encoding a bona fide Thg1 ortholog in the A. castellanii genome. Moreover, the cytosolic histidyl-tRNA synthetase in A. castellanii exhibits an unusual tRNA substrate specificity, efficiently aminoacylating tRNA(His) regardless of the presence of G(−1). A. castellanii does contain two Thg1-related genes (encoding Thg1-like proteins, TLPs), but the biochemical properties we associate here with these proteins are consistent with a function for these TLPs in separate pathways unrelated to tRNA(His) metabolism, such as mitochondrial tRNA repair during 5′-editing.