Absence of a universal element for tRNAHis identity in Acanthamoeba castellanii

Abstract

The additional G-1 nucleotide on tRNAHis is a nearly universal feature that specifies tRNAHis 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 tRNAHis 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 tRNAHis, 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 tRNAHis 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 tRNAHis metabolism, such as mitochondrial tRNA repair during 5′-editing. © 2012 The Author(s) 2012. Published by Oxford University Press.