dc.description.abstract | Previous studies show that many yeast aminoacyl-tRNA synthetases (aaRSs) contain a lysine-rich polypeptide extension or appended domain, which is absent from their bacterial relatives. Yeast valyl-tRNA synthetase (ValRS) possesses an N-terminal appended domain. This domain acts as an auxiliary tRNA-binding domain. Deletion of this domain from ValRS impaired its tRNA-binding, aminoacylation, and complementation activities. This finding underscores the necessity of an appended domain for functioning of a yeast aaRS. So far, only a few examples of non-AUG initiation have been identified in yeast. Our results show that, except for AAG and AGG, all triplets that differ from AUG by a single nucleotide were able to serve as an initiator in yeast. Moreover, each non-AUG initiator codon appeared to have its own favorite sequence context. In yeast, the ALA1 gene encodes the cytoplasmic and mitochondrial isoforms of AlaRS through alternative initiation from an AUG and an upstream non-AUG codon. Interestingly, except for Vanderwaltozyma polyspora, all yeast species studied contained a single dual-functional AlaRS gene. In contrast, V. polyspora contained two distinct nuclear AlaRS genes, one specifying the cytoplasmic form and the other its mitochondrial counterpart. A phylogenetic analysis revealed that all yeast AlaRS genes, including those in V. polyspora, are of mitochondrial origin. These and other results imply that the AlaRS genes in V. polyspora underwent a major genetic recombination at least twice. First, the mitochondrion-type gene gained cytoplasmic function and became a dual-functional gene, while its orthologue was lost. Later, the dual-functional gene of mitochondrial origin was duplicated into two copies during the whole-genome duplication event, each retaining a single function (cytoplasmic or mitochondrial).
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