A point mutation in the 5'-untranslated leader that affects translational activation of the mitochondrial COX3 mRNA.

The 613-base 5'-untranslated leader (5'-UTL) of the Saccharomyces cerevisiae mitochondrial COX3 mRNA contains the target of an mRNA-specific translational activator complex composed of at least three nuclearly encoded proteins. We have genetically mapped a collection of cox3 point mutations, using a set of defined COX3 deletions, and found one to be located in the region coding the 5'-UTL. The strain carrying this allele was specifically defective in translation of the COX3 mRNA. Nucleotide-sequence analysis showed that the allele was in fact a double mutation comprised of a single-base insertion in the 5'-UTL (T inserted between bases -428 and -427 with respect to the start of translation) and a G to A substitution at +3 that changed the ATG initiation codon to ATA. Both mutations were required to block translation completely. The effects of the ATG to ATA mutation alone (cox3-1) had previously been analyzed in this laboratory: it reduces, but does not eliminate, translation, causing a slow respiratory growth phenotype. The T insertion in the 5'-UTL had no detectable respiratory growth phenotype as a single mutation. However, the 5'-UTL insertion mutation enhanced the respiratory defective phenotype of missense mutations in pet54, one of the COX3-specific translational-activator genes. This phenotypic enhancement suggests that the -400 region of the 5'-UTL, where the mutation is located, is important for Pet54p-COX3 mRNA interaction.