coq-3(
qm188) homozygous mutant C. elegans derived from heterozygous mothers develop more slowly and are smaller in size than wild-type animals. The majority of these homozygous mutants are completely sterile with only a few laying 5-10 eggs. These second generation homozygous mutants arrest as L1 larvae. Based on amino acid sequence identity with yeast and human Coq3 polypeptides, the C. elegans
coq-3 gene has been identified as a candidate O-methyltransferase in the biosynthesis of coenzyme Q (ubiquinone or Q). Q functions as an essential factor of the respiratory chain and as a lipid soluble antioxidant that prevents peroxidation of membrane lipids. The Q biosynthetic pathway consists of at least nine polypeptides encoded by coq genes designated
coq-1 through
coq-9. The Q biosynthetic pathway in eukaryotes includes two O-methylation steps catalyzed by Coq3, one C-methylation by Coq5, and at least two hydroxylases, encoded by the
coq-6 and
clk-1/coq7 genes. It is intriguing that the C. elegans
coq-3 mutants exhibit a much more drastic phenotype than the C. elegans
clk-1/coq7 mutants, which exhibit slow development, sluggish adult behaviors, and an extended life span. C. elegans
clk-1/coq-7 mutants fail to produce Q, accumulate demethoxyubiquinone (DMQ), and are dependent on a dietary supply of Q for growth and fertility. However, dietary Q fails to rescue the fertility of the C. elegans
coq-3 mutant. We have recovered a C. elegans
coq-3 cDNA and have expressed it in the Saccharoymyces cerevisiae
coq3 null strain. The rescued yeast strain is able to grow on glycerol, a non-fermentable carbon source. Q<sub>6</sub> biosynthesis is restored in this rescued strain, although not to wild-type levels. Expression of the C. elegans
coq-3 cDNA in the Escherichia coli mutant GD1, which lacks the
coq-3 ortholog ubiG, restores growth on succinate and enables the bacteria to produce Q<sub>8</sub>. These data indicate that the COQ-3 polypeptide acts as an O-methyltransferase in the C. elegans Q biosynthetic pathway. Using the yeast and bacterial expression systems we hope to further characterize the functional roles of the C. elegans COQ-3 polypeptide. This work was supported by NIA Grant AG19777 and NIH Grant GM45952, and by NIH Cell and Molecular Training Grant GM07185 to AL and FG.