Suppressor mutations define two regions in the Cbp1 protein important for mitochondrial cytochrome b mRNA stability in Saccharomyces cerevisiae

Nuclear-encoded Cbp1 stabilizes and promotes translation of mitochondrial cytochrome b (COB) mRNA. A CCG triplet within the 5′UTR of COB mRNA is essential for Cbp1-dependent stability. Like cbp1 mutations, mutation of any nucleotide in CCG results in degradation of COB transcripts. In this study, CBP1-linked pseudorevertants of the temperature-sensitive CCU strain were isolated. The suppressors are missense mutations within a central cluster or a carboxyl cluster in the linear sequence of Cbp1. Strains with mutations in the carboxyl half of the central cluster or the carboxyl cluster respire better than those with mutations in the amino half of the central cluster. COB mRNA levels in the suppressor strains were increased compared with that in the CCU strain and were positively correlated with respiratory capability. This correlation supports a model in which the primary role of Cbp1 is to protect COB mRNAs and deliver them to the mitochondrial translational apparatus.