ISOLATION AND CHARACTERIZATION OF QCR10, THE NUCLEAR GENE ENCODING THE 8.5-KDA SUBUNIT-10 OF THE SACCHAROMYCES-CEREVISIAE CYTOCHROME BC(1),COMPLEX

We have cloned and sequenced QCR10, the nuclear gene encoding an 8.5-k Da protein proposed to be a subunit of the cytochrome bc(1) complex of Saccharomyces cerevisiae. QCR10 includes a 231-base pair open reading frame capable of encoding a protein of 77 amino acids with a predicte d molecular mass of 8492 Da. The codons for the amino-terminal methion ine and alanine are separated from the remainder of the open reading f rame by a 63-base pair intron that contains 5'-donor, 3'-acceptor, and TACTAAC sequences known to be necessary for splicing. The deduced ami no acid sequence of the 8.5-kDa yeast protein is 28% identical to that of the 6.4-kDa subunit 11 from the bovine heart cytochrome be, comple x, and the predicted secondary structures of the two proteins are very similar. Deletion of the chromosomal copy of QCR10 did not affect the growth of yeast on nonfermentable carbon sources. However, deletion o f QCR10 synergistically contributes to the temperature-dependent pheno type resulting from deletion of QCR6, the gene for subunit 6 of the cy tochrome bc(1) complex. In addition, ubiquinol-cytochrome c oxidoreduc tase activity was reduced 40% in mitochondrial membranes from the QCR1 0 deletion strain, and the Rieske iron-sulfur protein was lost when th e cytochrome bc(1) complex lacking the 8.5-kDa protein was purified. W e conclude that the 8.5-kDa protein encoded by QCR10 is a subunit of t he yeast cytochrome bc(1) complex and that the presence of this subuni t during assembly is required for stable association of the iron-sulfu r protein with the complex.