Cloning, sequencing, expression and allelic sequence diversity of ERG3 (C-5 sterol desaturase gene) in Candida albicans.

The C-5 sterol desaturase gene (ERG3), essential for yeast ergosterol biosynthesis, was cloned and sequenced from Candida albicans by homology with the Saccharomyces cerevisiae ERG3. The ERG3 ORF contained 1158bp and encoded 386 deduced amino acids. The clone was used to transform a gal1 mutant derived from the Darlington strain of C. albicans, using galactose selection. The Darlington strain is known to lack Delta(5,6) sterols, i.e. to have an erg3 phenotype (Howell, S.A., et al., 1990. J. Appl. Bacteriol. 69, 692-696). The transformant (CDTR1) contained six tandem integrated ERG3GAL1 repeats, had double the abundance of ERG3 transcript found in the host strain, and synthesized ergosterol, a Delta(5,6) sterol.The Darlington strain was noted to have an abundance of ERG3 transcript. Both ERG3 alleles in Darlington were cloned and sequenced in order to look for changes that might explain the erg3 phenotype. One allele, called Dar-2, contained a stop codon in place of tryptophan-292. The other ERG3 allele, called Dar-1, had changes in three amino acids, two of which were conserved in three fungal and one plant species. EcoRI genomic fragments containing ERG3 from the Dar-1 allele and from B311, the wild-type strain, were inserted into the plasmid pRS316 and used to transform a Saccharomyces cerevisiae erg3,ura3 mutant using uracil selection. The 4.1kb ERG3 fragments from the B311 and Dar-1 both contained 1. 4kb 5' and 1.5kb 3' flanking sequences around the coding region. Transformants with ERG3 from B311 but not from Dar-1 showed restored ergosterol synthesis. One or more of these three deduced amino acids in the Dar-1 allele of ERG3 appeared critical for function.