Abstract
Ubiquinone (coenzyme Q) is a lipid that transports electrons in the respiratory chains of both prokaryotes and eukaryotes. Mutants of Saccharomyces cerevisiae deficient in ubiquinone biosynthesis fail to grow on nonfermentable carbon sources and have been classified into eight complementation groups (coq1 coq8; Tzagoloff, A., and Dieckmann, C. L.(1990) Microbiol. Rev. 54, 211-225). In this study we show that although yeast coq7 mutants lack detectable ubiquinone, the coq7 1 mutant does synthesize demethoxyubiquinone (2-hexaprenyl-3-methyl-6-methoxy-1,4-benzoquinone), a ubiquinone biosynthetic intermediate. The corresponding wild-type COQ7 gene was isolated, sequenced, and found to restore growth on nonfermentable carbon sources and the synthesis of ubiquinone. The sequence predicts a polypeptide of 272 amino acids which is 40% identical to a previously reported Caenorhabditis elegans open reading frame. Deletion of the chromosomal COQ7 gene generates respiration defective yeast mutants deficient in ubiquinone. Analysis of several coq7 deletion strains indicates that, unlike the coq7 1 mutant, demethoxyubiquinone is not produced. Both coq7 1 and coq7 deletion mutants, like other coq mutants, accumulate an early intermediate in the ubiquinone biosynthetic pathway, 3-hexaprenyl-4-hydroxybenzoate. The data suggest that the yeast COQ7 gene may encode a protein involved in one or more monoxygenase or hydroxylase steps of ubiquinone biosynthesis.
Highlights
Ubiquinone1 is a lipid component of the electron transfer chain and functions in the transport of electrons from Complex I or II to the cytochrome bc1 complex found in the inner mitochondrial membrane of eukaryotes, and in the plasma membrane of prokaryotes [1, 2]
We demonstrate here that the COQ7 gene encodes a protein of 272 amino acids, which is necessary for growth on nonfermentable carbon sources and which restores Q biosynthesis in the coq7–1 mutant
A coq7–1 Mutant (C97) Lacks Q and Accumulates a “Quinone-like” Intermediate—To study the biochemical defects responsible for Q deficiency in the uncharacterized coq complementation groups, lipid extracts were prepared from 100-ml cultures of one representative strain from each coq complementation group grown in the presence of either [U-14C]p-hydroxybenzoic acid or [carboxyl-14C]p-hydroxybenzoic acid
Summary
Ubiquinone (coenzyme Q, or Q) is a lipid component of the electron transfer chain and functions in the transport of electrons from Complex I or II to the cytochrome bc complex found in the inner mitochondrial membrane of eukaryotes, and in the plasma membrane of prokaryotes [1, 2]. Recent evidence suggests that the Q biosynthetic pathway in higher eukaryotes mirrors that of S. cerevisiae, since a rat cDNA homologue to the yeast COQ3 gene was isolated based on its ability to restore synthesis of Q in a coq mutant [23, 24]. Deletion of the COQ7 gene generates mutant strains that do not accumulate DMQ, but accumulate large amounts of 3-hexaprenyl-4-hydroxybenzoic acid (compound 1). Botstein 12, 13 12, 13 55 55 55 13 Winston and Dollard (unpublished results) This study This study This study This study This study This study This study
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