Characterization of mRNAs and coding potential of the PET54 gene from Saccharomyces cerevisiae

The nuclear PET54 gene in yeast controls expression of two mitochondrial genes: COX1 at the level of pre-mRNA splicing and COX3 at the level of mRNA translation. Two size classes (1.6 and 1.1 kb) of transcripts that contain the PET54 coding region are produced in vivo. Relative to the majority of yeast mRNAs analyzed so far, the 5' untranslated leader region of the 1.6 kb transcript is unusually long (254 bases), while that for the major 1.1 kb transcript is unusually short (1 base). The majority of each class of PET54 mRNA was associated with polysomes in vivo. The possibility that two polypeptides are produced in vivo from the 1.1 kb PET54 mRNA was raised by the work of Sedman et al. [J. Virol. 64: 453-457, 1990], which showed that translation initiation at a downstream AUG occurs with increased efficiency when the upstream AUG is located very close to the 5' end of the mRNA. However, two sensitive assays for production of a second polypeptide, which is predicted to be 22 kD, were employed and no second polypeptide was detected. Furthermore, a nonsense mutation introduced near the beginning of the PET54 open reading frame abolished both COX1 and COX3 gene expression. These results indicate that the PET54 gene encodes predominantly a single functional polypeptide that is employed for expression of both the COX1 and COX3 genes of mitochondrial DNA.