Molecular cloning, characterization, and overexpression of ERG7, the Saccharomyces cerevisiae gene encoding lanosterol synthase.

We report the cloning, characterization, and overexpression of Saccharomyces cerevisiae ERG7, which encodes lanosterol synthase [(S)-2,3-epoxysqualene mutase (cyclizing, lanosterol forming), EC 5.4.99.7], the enzyme responsible for the complex cyclization/rearrangement step in sterol biosynthesis. Oligonucleotide primers were designed corresponding to protein sequences conserved between Candida albicans ERG7 and the related Arabidopsis thaliana cycloartenol synthase [(S)-2,3-epoxysqualene mutase (cyclizing, cycloartenol forming), EC 5.4.99.8]. A PCR product was amplified from yeast genomic DNA using these primers and was used to probe yeast libraries by hybridization. Partial-length clones homologous to the two known epoxysqualene mutases were isolated, but a full-length sequence was found neither in cDNA nor genomic libraries, whether in phage or plasmids. Two overlapping clones were assembled to make a functional reconstruction of the gene, which contains a 2196-bp open reading frame capable of encoding an 83-kDa protein. The reconstruction complemented the erg7 mutation when driven from either its native promoter or the strong ADH1 promoter.