Rescuing a dysfunctional homologue of a yeast glycyl-tRNA synthetase gene

The yeast Saccharomyces cerevisiae contains two distinct nuclear glycyl-tRNA synthetase (GlyRS) genes, GRS1 and GRS2. GRS1 is dual functional in that possesses both cytoplasmic and mitochondrial activities, whereas GRS2 is pseudogene-like. GlyRS1 and GlyRS2 are highly similar on the whole but are distinguished by a lysine-rich insertion domain of 44 amino acid residues, present only in GlyRS1. We herein present evidence that whereas the insertion domain is dispensable for the complementary activity of GRS1in vivo, deletion of this domain from GlyRS1 reduced its aminoacylation activity by up to 9-fold. On the other hand, fusion of a constitutive ADH promoter to GRS2 failed to confer a functional phenotype to the gene, but further fusion of ARC1 (a yeast gene encoding a tRNA-binding protein, Arc1p) to this hybrid gene successfully rescued its activity. Most intriguingly, purified GlyRS2 retained a substantial level of aminoacylation activity. Fusion of Arc1p to this enzyme further enhanced its activity and stability. These findings highlight not only the structural integrity of the pseudogene-encoded enzyme but also the necessity of obtaining an auxiliary tRNA-binding domain for functioning of a yeast tRNA synthetase.