CLONING AND CHARACTERIZATION OF ERG25, THE SACCHAROMYCES-CEREVISIAE GENE ENCODING C-4 STEROL METHYL OXIDASE

We have cloned the Saccharomyces cerevisiae C-4 sterol methyl oxidase ERG25 gene, The sterol methyl oxidase performs the first of three enzy mic steps required to remove the two C-4 methyl groups leading to chol esterol (animal), ergosterol (fungal), and stigmasterol (plant) biosyn thesis, An ergosterol auxotroph, erg25, which fails to demethylate and concomitantly accumulates 4,4-dimethylzymosterol, was isolated after mutagenesis, A complementing clone consisting of a 1.35-kb Dra I fragm ent encoded a 309-amino acid polypeptide (calculated molecular mass, 3 6.48 kDa). The amino acid sequence shows a C-terminal endoplasmic reti culum retrieval signal KKXX and three histidine-rich clusters found in eukaryotic membrane desaturases and in a bacterial alkane hydroxylase and xylene monooxygenase, The sterol profile of an ERG25 disruptant w as consistent with the erg25 allele obtained by mutagenesis.