CHARACTERIZATION OF MUTATIONS IN THE MITOCHONDRIAL CYTOCHROME-B GENE OF SACCHAROMYCES-CEREVISIAE AFFECTING THE QUINONE REDUCTASE SITE (Q(N))

The revertant [G33A]cytochrome b recently isolated from the [G33D]cyto chrome b mutant [Coppee, J. Y., Tokutake, N., Marc, D., di Rage, J.-P. , Miyoshi, H. and Colson, A.-M. (1994) FEES Lett. 339, 1-6] exhibits c ross resistance to center-N inhibitors 2-heptyl-4-hydroxyquinoline N-o xide (HQNO) and funiculosin and a spectral shift in the cytochrome b(5 62) heme. This indicates that the conserved G33 residue is in the vici nity of this heme, and thus agrees with the previous suggestion that g lycine may play a role in the helix packing around the hemes. The [S20 6L]cytochrome b and [M221K]cytochrome b respiratory-growth-deficient m utants [Lemesle-Meunier, D., Brivet-Chevillotte, P., di Rage, J. P., S lonimski, P. P., Bruel, C., Tron, T. and Forget, N. (1993) J. Biol. Ch em. 268, 15626-15632], which synthesize cytochrome b and retain little or no bc(1) complex activity, show no change in the reduction kinetic s of cytochrome b via center P, which suggests that the oxidizing site is functional. Impairment of both the reduction and oxidation of heme b(562) at the ubiquinone reduction center of the mitochondrial ubiqui none-cytochrome-c oxidoreductase site is, therefore, responsible for t he deficient catalytic activity and respiratory growth in these strain s.