title: Various cytosine/adenine permease homologues are involved in the toxicity of 5-fluorocytosine in Saccharomyces cerevisiae.
abstract: 5-Fluorocytosine (5-FC), a medically applied antifungal agent (Ancotil), is also active against the model organism Saccharomyces cerevisiae. 5-FC uptake in S. cerevisiae was considered to be mediated by the FCY2-encoded cytosine/adenine permease. By applying a highly sensitive assay, a low-level but dose-dependent toxicity of 5-FC in fcy2 mutants was detected, whereas cells deficient in the cytosine deaminase (encoded by FCY1), which is essential for intracellular conversion of 5-FC to 5-fluorouracil, display strong dose-independent resistance. Thus, an alternative, Fcy2-independent access pathway for 5-FC exists in S. cerevisiae. A genome-wide search for cytosine permease homologues identified two uncharacterized candidate genes, designated FCY21 and FCY22, both of which exhibit highest similarity to FCY2. Disruption of either FCY21 or FCY22 resulted in strains displaying low-level resistance, indicating the functional involvement of both gene products in 5-FC toxicity. When mutations in FCY21 or FCY22 were combined with the FCY2 disruption, both double mutants displayed stronger resistance when compared to the FCY2 mutant alone. Disruptions in all three permease genes consequently conferred the highest degree of resistance, not only towards 5-FC but also to the toxic adenine analogon 8-azaadenine. As residual 5-FC sensitivity was, however, even detectable in the fcy2 fcy21 fcy22 mutant, we analysed the relevance of other FCY2 homologues, i.e. TPN1, FUR4, DAL4, FUI1 and yOR071c, for 5-FC toxicity. Among these, Tpn1, Fur4 and the one encoded by yOR071c were found to contribute significantly to 5-FC toxicity, thus revealing alternative entry routes for 5-FC via other cytosine/adenine permease homologues.
authors.affiliation: Institut für Molekulare Mikrobiologie und Biotechnologie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 3, D-48149 Münster, Germany.
authors.forename: John P
authors.initials: JP
authors.lastname: Paluszynski
authors.forename: Roland
authors.initials: R
authors.lastname: Klassen
authors.forename: Matthias
authors.initials: M
authors.lastname: Rohe
authors.forename: Friedhelm
authors.initials: F
authors.lastname: Meinhardt
mesh_terms: D000595:Amino Acid Sequence; D000935:Antifungal Agents / Q000494:pharmacology*; D004271:DNA, Fungal / Q000737:chemistry / Q000235:genetics; D005437:Flucytosine / Q000494:pharmacology*; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D033702:Nucleobase Transport Proteins; D016133:Polymerase Chain Reaction; D012441:Saccharomyces cerevisiae / Q000187:drug effects* / Q000201:enzymology* / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D016415:Sequence Alignment; 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; D005437:Flucytosine