AUG codons in the RNA leader sequences of the yeast PET genes CBS1 and SCO1 have no influence on translation efficiency.

We report that the major transcription start sites of the yeast PET gene SCO1 are located at positions -149 and -125 relative to the AUG initiation codon of the SCO1 reading frame. The leader sequences of the resulting mRNAs possess a single AUG codon at position -49, which initiates a short open reading frame of three amino acids. The recent finding of a similar situation in the case of the PET gene CBS1 prompted us to address the question as to whether these AUG codons might play some role in the expression of these PET genes. After removal of the upstream AUG codons by site-directed mutagenesis, expression was monitored by use of lacZ fusions and compared to the respective wild-type constructs. Our data show that under all growth conditions tested the leader-contained AUG initiation codons have no significant influence on the expression of both PET genes.