Reduced but accurate translation from a mutant AUA initiation codon in the mitochondrial COX2 mRNA of Saccharomyces cerevisiae.

We have changed the translation initiation codon of the COX2 mRNA of Saccharomyces cerevisiae from AUG to AUA, generating a mutation termed cox2-10. This mutation reduced translation of the COX2 mRNA at least five-fold without affecting the steady-state level of the mRNA, and produced a leaky nonrespiratory growth phenotype. To address the question of whether residual translation of the cox2-10 mRNA was initiating at the altered initiation codon or at the next AUG codon downstream (at position 14), we took advantage of the fact that the mature coxII protein is generated from the electrophoretically distinguishable coxII precursor by removal of the amino-terminal 15 residues, and that this processing can be blocked by a mutation in the nuclear gene PET2858. We constructed a pet2858, cox2-10 double mutant strain using a pet2858 allele from our mutant collection. The double mutant accumulated low levels of a polypeptide which comigrated with the coxII precursor protein, not the mature species, providing strong evidence that residual initiation was occurring at the mutant AUA codon. Residual translation of the mutant mRNA required the COX2 mRNA-specific activator PET111. Furthermore, growth of cox2-10 mutant strains was sensitive to alterations in PET111 gene dosage: the respiratory-defective growth phenotype was partially suppressed in haploid strains containing PET111 on a high-copy-number vector, but became more severe in diploid strains containing only one functional copy of PET111.