Translation Initiation from a Naturally Occurring Non-AUG Codon in Saccharomyces cerevisiae

Abstract

Although 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. 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.

Alternative translation start sites and hidden coding potential of eukaryotic mRNAs

“…(10)(11)(12)(13) The initiation of translation may also sometimes occur at non-AUG codons. (14)(15)(16) Commonly, in these cases certain additional signals are essential (e.g. perfect context, specifically positioned downstream secondary structure, reiterated initiator codon.…”

“…(23) Cytoplasmic and mitochondrial isoforms of Saccharomyces cerevisiae alanyl-tRNA synthetase and glycyl-tRNA synthetase are also translated from alternative TISs by leaky scanning mechanism. (14) DNA ligase 1 (AtLIG1) is the only essential DNA ligase activity in Arabidopsis thaliana. The mitochondrial and nuclear forms of DNA ligase 1 are translated from a single mRNA species through translation initiation from either the first or second in-frame AUG codons, respectively.…”

Plant organellar protein targeting: a traffic plan still under construction

“…More surprising, however, is that this relaxation can result in the establishment of non-AUG translation initiation sites. This is the case in yeast glycine [26] and alanine [27] tRNA synthetases, to permit cytosolic and mitochondrial dual targeting, and in plant DNA polymerase [13] for mitochondrial-plastid dual targeting. In the organellar DNA polymerase, the upstream initiator CTG appears to be as effective as an ATG in the same position, based on site-directed mutagenesis, and relative mitochondrial versus plastid targeting appears to be modulated developmentally [13].…”

Mechanism and Regulation of Protein Synthesis in Saccharomyces cerevisiae

“…Likewise, all possible single nucleotide substitutions at the AUG start codon of a HIS4-lacZ reporter lowered expression to #2% of the AUG control . However, the mRNAs encoding glycyl (Grs1) and alanyl (Ala1) tRNA synthetases initiate at both AUG and non-AUG codons (Chang and Wang 2004;Tang et al 2004;Chen et al 2008). Whereas the cytoplasmic synthetases initiate at an AUG start codon, the extended, mitochondrial enzymes initiate at upstream codons: UUG for Grs1 (Chen et al 2008) and ACG for Ala1 (Tang et al 2004).…”