THE COQ5 GENE ENCODES A YEAST MITOCHONDRIAL PROTEIN NECESSARY FOR UBIQUINONE BIOSYNTHESIS AND THE ASSEMBLY OF THE RESPIRATORY-CHAIN

Saccharomyces cerevisiae is a facultative anaerobe capable of meeting its energy requirements by fermentation and is thus an ideal system fo r studying the biogenesis of respiring mitochondria. We have isolated a respiration-deficient mutant exhibiting a pleiotropic loss of the mi tochondrial electron transport chain. The corresponding wild-type gene , COQ5, was cloned, sequenced, and able to restore respiratory growth. Deletion of the chromosomal COQ5 gene results in a respiration defici ency and reduced levels of respiratory protein components. Exogenously added decylubiquinone can partially restore electron transport chain function to mitochondrial membranes from the deletion mutant. The COQ5 nucleotide sequence predicts a polypeptide of 307 amino acids contain ing a mitochondrial targeting signal. COQ5p is 43% identical to the po lypeptide predicted by the Escherichia coli open reading frame, o251 ( 1). The COQ5 gene, when introduced into E. coli, complements the respi ratory deficiency of an ubiE mutant that maps near o251, suggesting th at it is the yeast homolog of the ubiE gene product. We conclude that the COQ5 gene encodes the mitochondria-localized 2-hexaprenyl-6-methox y-1,4-benzoquinone methyltransferase of the yeast ubiquinone biosynthe tic pathway.