Assembly of the mitochondrial membrane system. CBP6, a yeast nuclear gene necessary for synthesis of cytochrome b.

A new gene function involved in the expression of mitochondrial cytochrome b is described. E158 is a respiratory deficient strain of Saccharomyces cerevisiae with a recessive mutation in nuclear DNA. The block in mitochondrial respiration is a consequence of the mutant's inability to synthesize cytochrome b. This has been confirmed by analysis of the mitochondrial translation products in E158. Since the mutant has wild-type concentrations of mature cytochrome b mRNA, the absence of the cytochrome b protein cannot be due to a transcriptional or RNA-processing defect. The wild-type gene (CBP6) responsible for the observed phenotype has been cloned by transformation of E158 with a genomic library of yeast nuclear DNA. The cloned gene has been sequenced and shown to code for a basic protein with a molecular weight of 18,657. Both deletion and disruption of the CBP6 coding sequence in chromosomal DNA of wild-type yeast lead to respiratory deficiency with a concomitant loss of cytochrome b. Part of the CBP6 gene has been fused to the trpE gene on a high expression Escherichia coli vector. The hybrid protein encoded by the trpE/CBP6 fusion has been purified from E. coli and used as an antigen for antibody production. Antibodies to the hybrid protein cross-react with an 18-kDa protein present in yeast mitochondria.