GENETIC-EVIDENCE FOR INTERACTION BETWEEN CBP1 AND SPECIFIC NUCLEOTIDES IN THE 5'-UNTRANSLATED REGION OF MITOCHONDRIAL CYTOCHROME-B MESSENGER-RNA IN SACCHAROMYCES-CEREVISIAE

The cytochrome b (COB) gene is encoded by the mitochondrial genome; ho wever, its expression requires the participation of several nuclearly encoded protein factors. The yeast Cbp1 protein, which is encoded by t he nuclear CBP1 gene, is required for the stabilization of COB mRNA. A previous deletion analysis identified an Il-nucleotide-long sequence within the 5' untranslated region of COB mRNA that is important for Cb p1-dependent COB mRNA stability, In the present study, site-directed m utagenesis experiments were carried out to define further the features of this cis element. The CCG sequence within this region was shown to be necessary for stability. A change in residue 533 of Cbp1 from aspa rtate to tyrosine suppresses the effects of a single-base change in th e CCG element. This is strong genetic evidence that the nuclearly enco ded Cbp1 protein recognizes and binds directly to the sequence contain ing CCG and thus protects COB mRNA from degradation.