title: Genetic prerequisites for additive or synergistic actions of 5-fluorocytosine and fluconazole in baker's yeast.
abstract: During applications of 5-fluorocytosine (5FC) and fluconazole (FLC), additive or synergistic action may even occur when primary resistance to 5FC is established. Here, we analysed conjoint drug action in Saccharomyces cerevisiae strains deficient in genes known to be essential for 5FC or FLC function. Despite clear primary resistance, residual 5FC activity and additive 5FC+FLC action in cells lacking cytosine permease (Fcy2p) or uracil phosphoribosyl transferase (Fur1p) were detected. In contrast, Deltafcy1 mutants, lacking cytosine deaminase, became entirely resistant to 5FC, concomitantly losing 5FC+FLC additivity. Disruption of the orotate phosphoribosyltransferase gene (URA5) in the wild-type led to low-level 5FC tolerance, while an alternative orotate phosphoribosyltransferase, encoded by URA10, contributed to 5FC toxicity only in the Deltaura5 background. Remarkably, combination of Deltaura5 and Deltafur1 resulted in complete 5FC resistance. Thus, yeast orotate phosphoribosyltransferases are involved in 5FC metabolism. Similarly, disruption of the ergosterol Delta(5,6)-desaturase-encoding gene ERG3 resulted only in partial resistance to FLC, and concomitantly a synergistic effect with 5FC became evident. Full resistance to FLC occurred in Deltaerg3 Deltaerg11 double mutants and, simultaneously, synergism or even an additive effect with FLC and 5FC was no longer discernible. Since the majority of spontaneously occurring resistant yeast clones displayed residual sensitivity to either 5FC or FLC and those strains responded to combined drug treatment in a predictable manner, careful resistance profiling based on the findings reported here may help to address yeast infections by combined application of antimycotic compounds.
authors.affiliation: Institut für Molekulare Mikrobiologie und Biotechnologie, Westfälische Wilhelms-Universität Münster, Corrensstr. 3, D-48149 Münster, Germany.
authors.forename: John P
authors.initials: JP
authors.lastname: Paluszynski
authors.affiliation: Institut für Molekulare Mikrobiologie und Biotechnologie, Westfälische Wilhelms-Universität Münster, Corrensstr. 3, D-48149 Münster, Germany.
authors.forename: Roland
authors.initials: R
authors.lastname: Klassen
authors.affiliation: Institut für Molekulare Mikrobiologie und Biotechnologie, Westfälische Wilhelms-Universität Münster, Corrensstr. 3, D-48149 Münster, Germany.
authors.forename: Friedhelm
authors.initials: F
authors.lastname: Meinhardt
mesh_terms: D000595:Amino Acid Sequence; D000935:Antifungal Agents / Q000494:pharmacology*; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics; D043525:Cytosine Deaminase / Q000235:genetics; D004271:DNA, Fungal / Q000235:genetics; D025141:Drug Resistance, Fungal / Q000235:genetics; D004357:Drug Synergism; D015725:Fluconazole / Q000494:pharmacology*; D005437:Flucytosine / Q000494:pharmacology*; D005800:Genes, Fungal; D026901:Membrane Transport Proteins / Q000235:genetics; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D009154:Mutation; D033702:Nucleobase Transport Proteins; D009962:Orotate Phosphoribosyltransferase / Q000235:genetics; D010088:Oxidoreductases / Q000235:genetics; D010430:Pentosyltransferases / Q000235:genetics; D012441:Saccharomyces cerevisiae / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics; D014170:Transformation, Genetic
chemical_list: D000935:Antifungal Agents; D004271:DNA, Fungal; C043181:FCY2 protein, S cerevisiae; D026901:Membrane Transport Proteins; D033702:Nucleobase Transport Proteins; D029701:Saccharomyces cerevisiae Proteins; D015725:Fluconazole; D003577:Cytochrome P-450 Enzyme System; D005437:Flucytosine; D010088:Oxidoreductases; C526348:Erg11 protein, S cerevisiae; C039306:sterol delta-5 desaturase; D010430:Pentosyltransferases; D009962:Orotate Phosphoribosyltransferase; C023269:uracil phosphoribosyltransferase; D043525:Cytosine Deaminase