Clade-specific flucytosine resistance is due to a single nucleotide change in the FUR1 gene of Candida albicans.

Population studies have indicated that natural resistance to flucytosine (5FC) in Candida albicans is limited to one of the five major clades, clade I. In addition, while 73% of clade I isolates are less susceptible to 5FC (MIC >/= 0.5 microg/ml), only 2% of non-clade I isolates are less susceptible. In order to determine the genetic basis for this clade-specific resistance, we sequenced two genes involved in the metabolism of 5FC that had previously been linked to resistance (cytosine deaminase and uracil phosphoribosyltransferase), in 48 isolates representative of all clades. Our results demonstrate that a single nucleotide change from cytosine to thymine at position 301 in the uracil phosphoribosyltransferase gene (FUR1) of C. albicans is responsible for 5FC resistance. The mutant allele was found only in group I isolates. The 5FC MICs for strains without copies of the mutant allele were almost exclusively </=0.25 microg/ml, those for strains with one copy of the mutant allele were >/=0.5 microg/ml, and those for strains with two copies of the mutant allele were >/=16 microg/ml. Thus, the two alleles were codominant. The presence of this allele is responsible for clade I-specific resistance to 5FC within the C. albicans population and thus by inference is likely to be the major underlying 5FC resistance mechanism in C. albicans. This represents the first description of the genetic mutation responsible for 5FC resistance.