title: Molecular mechanisms of primary resistance to flucytosine in Candida albicans.
abstract: Primary resistance in Candida albicans to flucytosine (5-FC) was investigated in 25 strains by identifying and sequencing the genes FCA1, FUR1, FCY21, and FCY22, which code for cytosine deaminase, uracil phosphoribosyltransferase (UPRT), and two purine-cytosine permeases, respectively. These proteins are involved in pyrimidine salvage and 5-FC metabolism. An association between a polymorphic nucleotide and resistance to 5-FC was found within FUR1 where the substitution of cytidylate for thymidylate at nucleotide position 301 results in the replacement of arginine with cysteine at amino acid position 101 in UPRT. Isolates that are homozygous for this mutation display increased levels of resistance to 5-FC, whereas heterozygous isolates have reduced susceptibility. Three-dimensional protein modeling of UPRT suggests that the Arg101Cys mutation disturbs the quaternary structure of the enzyme, which is postulated to compromise optimal enzyme activity. A single resistant isolate, lacking the above polymorphism in FUR1, has a homozygous polymorphism in FCA1 that results in a glycine-to-aspartate substitution at position 28 in cytosine deaminase.
authors.affiliation: Immunocompromised Host Section POB, NCI, NIH CRC, Room 1-5700, 10 Center Dr., MSC 1100, Bethesda, MD 20892-1100, USA. hopew@mail.nih.gov
authors.forename: William W
authors.initials: WW
authors.lastname: Hope
authors.forename: Lydia
authors.initials: L
authors.lastname: Tabernero
authors.forename: David W
authors.initials: DW
authors.lastname: Denning
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Anderson
mesh_terms: D000595:Amino Acid Sequence; D000935:Antifungal Agents / Q000494:pharmacology*; D000963:Antimetabolites / Q000494:pharmacology; D002176:Candida albicans / Q000187:drug effects*; D043525:Cytosine Deaminase / Q000378:metabolism; D017931:DNA Primers; D004271:DNA, Fungal / Q000096:biosynthesis / Q000235:genetics; D025141:Drug Resistance, Fungal; D005437:Flucytosine / Q000494:pharmacology*; D005472:Fluorouracil / Q000494:pharmacology; D005800:Genes, Fungal / Q000235:genetics; D026901:Membrane Transport Proteins / Q000378:metabolism; D008826:Microbial Sensitivity Tests; D008958:Models, Molecular; D008969:Molecular Sequence Data; D033702:Nucleobase Transport Proteins; D010430:Pentosyltransferases / Q000378:metabolism; D011110:Polymorphism, Genetic / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism
chemical_list: D000935:Antifungal Agents; D000963:Antimetabolites; D017931:DNA Primers; D004271:DNA, Fungal; C043181:FCY2 protein, S cerevisiae; D026901:Membrane Transport Proteins; D033702:Nucleobase Transport Proteins; D029701:Saccharomyces cerevisiae Proteins; D005437:Flucytosine; D010430:Pentosyltransferases; C023269:uracil phosphoribosyltransferase; D043525:Cytosine Deaminase; D005472:Fluorouracil