Abstract
Although previous studies have already shown that both cytoplasmic and mitochondrial activities of glycyl-tRNA synthetase are provided by a single gene, GRS1,in the yeast Saccharomyces cerevisiae, the mechanism by which this occurs remains unclear. Evidence presented here indicates that this bifunctional property is actually a result of two distinct translational products alternatively generated from a single transcript of this gene. Except for an amino-terminal 23-amino acid extension, these two isoforms have the same polypeptide sequence and function exclusively in their respective compartments under normal conditions. Reporter gene assays further suggest that this leader peptide can function independently as a mitochondrial targeting signal and plays the major role in the subcellular localization of the isoforms. Additionally, whereas the short protein is translationally initiated from a traditional AUG triplet, the longer isoform is generated from an upstream inframe UUG codon. To our knowledge, GRS1 appears to be the first example in the yeast wherein a functional protein isoform is initiated from a naturally occurring non-AUG codon. The results suggest that non-AUG initiation might be a mechanism existing throughout all kingdoms.
Highlights
S ribosomal subunit [6]
Previous studies have already shown that both cytoplasmic and mitochondrial activities of glycyltRNA synthetase are provided by a single gene, GRS1, in the yeast Saccharomyces cerevisiae, the mechanism by which this occurs remains unclear
Compartmentalization of the protein synthesis machinery within the cytoplasm and organelles of eukaryotes leads to isoaccepting tRNA species that are distinguished by nucleotide sequence, subcellular location, and enzyme specificity
Summary
GlyRS, glycyl-tRNA synthetase; RACE, rapid amplification of cDNA ends; ADH, alcohol dehydrogenase; 5-FOA, 5-fluoroorotic acid; YPG, yeast extract peptone glycerol; RT, reverse transcriptase. Even more remarkable is the fact that one of the two protein isoforms is initiated from a UUG triplet, a codon that was believed inappropriate for initiation in yeast. The implications of this novel mechanism of translation initiation, in yeast, are further discussed
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