It was previously reported that disruption or deletion of QCR6, the nuclear gene encoding subunit 6 of the cytochrome bc1 complex, does not impair growth of yeast on non-fermentable carbon sources (Schoppink, P. J., Hemrika, W., Reyne, J. M., Grivell, L.A., and Berden, J. A. (1988) Eur. J. Biochem. 113, 115-122; Crivellone, M. D., Wu, M. M., and Tzagoloff, A. (1988) J. Biol. Chem. 262, 14323-14333; Schmitt, M. E., and Trumpower, B. L. (1990) J. Biol. Chem. 265, 17005-17011). We have discovered that deletion of QCR6 results in a temperature-sensitive petite phenotype, manifested at 37 degrees C, and that this phenotype can be masked by spontaneously arising suppressor mutations. Mitochondrial membranes from the deletion strain grown at 37 degrees C lack ubiquinol-cytochrome c reductase activity, and optical spectra reveal an extensive decrease in cytochrome b absorption, but little or no decrease in cytochrome c1 absorption. Immunoblots of mitochondrial membrane proteins from the deletion strain indicate that processing of cytochrome c1 from intermediate to mature size is blocked coincident with the loss of subunit 6. This is the first example where mutation of a subunit within the bc1 complex blocks maturation of cytochrome c1.