PET111 acts in the 5'-leader of the Saccharomyces cerevisiae mitochondrial COX2 mRNA to promote its translation.

PET111 is a yeast nuclear gene specifically required for the expression of the mitochondrial gene COX2, encoding cytochrome c oxidase subunit II (coxII). Previous studies have shown that PET111 activates translation of the COX2 mRNA. To map the site of PET111 action we have constructed, in vitro, genes coding for chimeric mRNAs, introduced them into mitochondria by transformation and studied their expression. Translation of a chimeric mRNA with the 612-base 5'-untranslated leader of the COX3 mRNA fused precisely to the structural gene for the coxII-precursor protein is independent of PET111, but does require a COX3 mRNA-specific translational activator known to work on the COX3 5'-leader. This result demonstrates that PET111 is not required for any post-translational step. Translation of a chimeric mRNA with the 54-base 5'-leader of the COX2 mRNA fused precisely to the structural gene for cytochrome c oxidase subunit III was dependent on PET111 activity. These results demonstrate that PET111 acts specifically at a site in the short COX2 5'-leader to activate translation of downstream coding sequences.