Formation of azole-resistant Candida albicans by mutation of sterol 14-demethylase P450

The sterol 14-demethylase P450 (CYP51) of a fluconazole-resistant isolate o f Candida albicans, DUMC136, showed reduced susceptibility to this azole bu t with little change in its catalytic activity. Twelve nucleotide substitut ions, resulting in four amino acid changes, were identified in the DUMC136 CYP51 gene in comparison with a reported CYP51 sequence from a wild-type, f luconazole-susceptible C. albicans strain. Seven of these substitutions, in cluding all of those causing amino acid changes, were located within a regi on covering one of the putative substrate recognition sites of the enzyme ( SRS-1). Polymorphisms within this region were observed in several C. albica ns isolates, and some were found to be CYP51 heterozygotes. Among the amino acid changes occurring in this region, only an alteration of Y132 was comm on among these fluconazole-resistant isolates, which suggests the importanc e of this residue to the fluconazole resistance of the target enzyme. DUMC1 36 and another fluconazole-resistant isolate were homozygotes with respect to CYP51, although the typical wild-type, fluconazole-susceptible C. albica ns was a CYP51 heterozygote. These findings suggest that part of the flucon azole-resistant phenotype of C. albicans DUMC136 was acquired through a mut ation-prone area of CYP51, an area which might promote the formation of flu conazole-resistant CYP51, along with a mechanism(s) which allows the format ion of a homozygote of this altered CYP51 in this diploid pathogenic yeast.