The single nuclear gene encoding the 14-kDa subunit VII of yeast ubiquinol:cytochrome c oxidoreductase has been inactivated by one-step gene disruption, as verified by Southern blot analysis and immunoblotting. The resulting mutant has no ubiquinol:cytochrome c oxidoreductase activity and is respiratory-deficient. Immunoblotting shows that cells lacking the 14-kDa protein, also have lowered steady-state levels of other subunits of complex III, the nuclear-encoded 11-kDa subunit VIII, the Rieske Fe-S protein and the mitochondrially encoded cytochrome b. No cytochrome b can be detected spectrally. The steady-state levels of the transcripts from genes encoding these proteins are not reduced, implying that the mutation exerts its pleiotropic effects at a post-transcriptional level. The residual amounts of subunits of complex III are recovered in the mutant mitochondria, suggesting that import is unaffected. The results strongly suggest that the 14-kDa protein plays an essential role in the biosynthesis of the complex, most probably at the level of assembly. Field-inversion gel electrophoresis was used to separate chromosomes of HR2 wild type and the (14-kDa-protein) degrees mutant, after which the gene encoding the 14-kDa protein was located on chromosome IV by Southern blot analysis.