dc.description.abstract | Previous studies showed that ALA1 and GRS1 of Saccharomyces cerevisiae can initiate translation of their respective mitochondrial forms from a non-AUG codon. Our results showed that the translation efficiency of non-AUG initiation is about 30% (or less) relative to that of AUG initiation. In addition, it appeared that a non-AUG initiator codon is much more sensitive to its sequence context than is an AUG initiator codon, and AAA (the nucleotides at position -3 to -1 relative to the initiator) is the most favorable sequence context. Moreover, redundancy of non-AUG initiators, for instance ACGACG, significantly increased the translational efficiency. However, some redundant non-AUG initiators such as UUGUUG that have a poor sequence context (especially at position -3 relative to the second UUG codon), reduced the efficiency of translation. Another interesting discovery reported here was that the majority of yeast species possess a single glycyl-tRNA synthetase (GlyRS) gene (named GRS1). In contrast, S. cerevisiae and Vanderwaltozyma polyspora possessed two GlyRS genes (named GRS1 and GRS2). In all cases, GRS1 was dual-functional, because it encodes both cytoplasmic and mitochondrial forms of GlyRS. In contrast, GRS2 was pseudogene-like and dispensable for growth. Surprisingly, while GRS2 was silent under normal growth conditions (30°C), its expression was induced by certain stresses such as high temperature (37°C) and high external pH (pH 8.0). In addition, purified recombinant GlyRS2 retained a substantial level of aminoacylation activity. Both GlyRS1 and GlyRS2 were appreciably stable in vivo. When overexpressed, the GRS2 gene could rescue the growth defect of a GRS1 knockout strain. Altogether, these data suggest that GRS2 may function to substitute for GRS1 under certain circumstances. Alternatively, it may be involved in other as-yet-unidentified metabolic pathways.
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