ACCUMULATION OF MITOCHONDRIALLY SYNTHESIZED SACCHAROMYCES-CEREVISIAE COX2P AND COX3P DEPENDS ON TARGETING INFORMATION IN UNTRANSLATED PORTIONS OF THEIR MESSENGER-RNAS

The essential products of the yeast mitochondrial translation system a re seven hydrophobic membrane proteins and Var1p, a hydrophilic protei n in the small ribosomal subunit. Translation of the membrane proteins depends on nuclearly encoded, mRNA-specific translational activators that recognize the 5'-untranslated leaders of their target mRNAs. Thes e translational activators are themselves membrane associated and coul d therefore tether translation to the inner membrane. In this study, w e tested whether chimeric mRNAs with the untranslated sequences normal ly present on the mRNA encoding soluble Var1p, can direct functional e xpression of coding sequences specifying the integral membrane protein s Cox2p and Cox3p, DNA sequences specifying these chimeric mRNAs were inserted into mtDNA at the VAR1 locus and expressed in strains contain ing a nuclearly localized plasmid that supplies a functional form of V ar1p, imported from the cytoplasm. Although cells expressing these chi meric mRNAs actively synthesized both membrane proteins, they were sev erely deficient in cytochrome c oxidase activity and in the accumulati on of Cox2p and Cox3p, respectively. These data strongly support the p hysiological importance of interactions between membrane-bound mRNA-sp ecific translational activators and the native 5'-untranslated leaders of the COX2 and COX3 mRNAs for localizing productive synthesis of Cox 2p and Cox3p to the inner membrane.