Electron‐transfer restoration by vitamin K 3 in a complex III‐deficient mutant of S. cerevisiae and sequence of the corresponding cytochrome b mutation

The yeast box‐mutant W7 exhibits deficiencies in cytochrome b and in nuclear coded complex III subunits, a phenotype observed previously in a patient with mitochondrial myopathy. DNA sequence analysis of mutant W7 revealed a single base transition in the cytochrome b gene; the mutated residue Gly 131 is perfectly conserved in all known cytochromes b and belongs to the Q 0 domain. Mutant W7 provides a model system for evaluating the action of therapeutic agents, such as vitamin K 3 which restored NADH‐oxidase activity in the mutant as well as in the antimycin‐inhibited wild type. However, with the mutant, a greater quantity of menadione was necessary due to a decrease in other complex activities, and a much lower electron‐flow fraction passed through cytochrome oxidase.