CHARACTERIZATION OF THE COQ5 GENE FROM SACCHAROMYCES-CEREVISIAE - EVIDENCE FOR A C-METHYLTRANSFERASE IN UBIQUINONE BIOSYNTHESIS

Ubiquinone (coenzyme Q or Q) is a lipophilic metabolite that functions in the electron transport chain in the plasma membrane of prokaryotes and in the inner mitochondrial membrane of eukaryotes. Q-deficient mu tants of Saccharomyces cerevisiae fall into eight complementation grou ps (coq1-coq8), Yeast mutants from the coq5 complementation group lack Q and as a result are respiration-defective and fail to grow on nonfe rmentable carbon sources, A nuclear gene, designated COQ5 was isolated from a yeast genomic library based on its ability to restore growth o f a representative coq5 mutant on media containing glycerol as the sol e carbon source, The DNA segment responsible for the complementation c ontained an open reading frame (GenBank(TM) accession number Z49210) w ith 44% Sequence identity over 262 amino acids to UbiE, which is requi red for a C-methyltransferase step in the Q and menaquinone biosynthet ic pathways in Escherichia coil, Both the ubiE and COQ5 coding sequenc es contain sequence motifs common to a wide variety of S-adenosyl-L-me thionine-dependent methyltransferases. A gene fusion expressing a biot inylated form of Coq5p retains function, as assayed by the complementa tion of the coq5 mutant, This Coq5-biotinylated fusion protein is loca ted in mitochondria. The synthesis of two farnesylated analogs of inte rmediates in the ubiquinone biosynthetic pathway is reported, These re agents have been used to develop in vitro C-methylation assays with is olated yeast mitochondria, These studies show that Coq5p is required f or the C-methyltransferase step that converts 2-methoxy-5-polyprenyl-1 ,4-benzoquinone to 2-methoxy-5-methyl-6-polyprenyl-1,4-benioquinone.