C. elegans worms with mutations in the gene CLK-1 develop slowly and have an extended lifespan. CLK-1 encodes a mitochondrial protein that is responsible for the hydroxylation of 5-demethoxyubiquinone (DMQ), the penultimate step of ubiquinone (Coenzyme-Q or UQ) biosynthesis. Structural homologues of CLK-1 are found in mammals, fruit flies, yeast and some types of bacteria. Interestingly, however, there is no structural homologue of CLK-1 in the Arabidopsis genome and no plant homologue can be found in other sequence databases. Yeast with the CLK-1 homologue COQ7 deleted fail to grow on non-fermentable carbon sources. To identify a plant functional homologue of COQ7/CLK-1, an Arabidopsis cDNA expression library was screened for complementation of a yeast coq7 deletion mutant. A clone was identified that rescued the coq7 respiratory deficiency. Although the sequence of the encoded protein has no structural similarity to proteins in the COQ7/CLK-1 family, it contains a monooxygenase/hydroxylase domain that has sequence similarity with the E. coli DMQ hydroxylase encoded by the UBIF gene. Like the structural homologues of COQ7/CLK-1 found in other eukaryotes, the gene (AQX for 'Alternate Quinone monooXygenase') contains a likely mitochondrial targeting presequence at its N-terminus. HPLC analysis of quinone extracts from rescued cog7 strains does not detect ubiquinone, but instead shows another peak that may be DMQ. It is likely that AQX does not hydroxylate yeast DMQ effectively enough to generate detectable levels of UQ. A unique pathway for UQ biosynthesis in plants is proposed that is defined by AQX and Arabidopsis genes identified on the basis of homology to known E. coli and yeast UQ biosynthesis genes.

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