A point mutation in the mitochondrial cytochrome b gene obviates the requirement for the nuclear encoded core protein 2 subunit in the cytochrome bc(1) complex in Saccharomyces cerevisiae

A yeast mutant (cor2-45) in which approximately half of the C terminus of core protein 2 of the cytochrome bc(1) complex is lacking due to a frameshift mutation that introduces a stop at codon 197 in the COR2 gene fails to assemble the cytochrome bc(1) complex and does not grow on non-fermentable carbon sources that require respiration. The loss of respiration is more severe with this frameshift mutation than with the complete deletion of the COR2 gene, suggesting deleterious effects of the truncated core 2 protein. A search for extragenic suppressors of the nuclear cor2-45 mutation resulted (in addition to the expected nuclear suppressors) in the isolation of a suppressor mutation in the mitochondrial DNA that replaces serine 223 by proline in cytochrome b. Assembly of the cytochrome bc(1) complex and the respiratory deficient phenotype of the cor2-45 mutant are restored by the proline for serine replacement in cytochrome b. Surprisingly, this amino acid replacement in cytochrome b corrects not only the phenotype resulting from the cor2-45 frameshift mutation, but it also obviates the need for core protein 2 in the cytochrome bc(1) complex since it alleviates the respiratory deficiency resulting from the complete deletion of the COR2 gene. This is the first report of a homoplasmic missense point mutation of the mitochondrial DNA acting as a functional suppressor of a mutation located in a nuclear gene and the first demonstration that the supernumerary core protein 2 subunit is not essential for the electron transfer and energy transducing functions of the mitochondrial cytochrome bc(1) complex.