FCY1 (YPR062W) encodes cytosine deaminase (EC 3.5.4.1), a 158-residue zinc-dependent homodimeric enzyme of the cytidine/deoxycytidylate deaminase (CDA-fold) family, unrelated to bacterial CodA. It hydrolyses cytosine (and 5-methylcytosine) to uracil and ammonia in the cytoplasm, allowing cytosine, and cytidine after Urh1 cleavage, to be salvaged via Fur1 to UMP; it is the limiting step for cytidine utilisation as a UMP precursor. fcy1 mutants lose all cytosine deaminase activity and are highly resistant to 5-fluorocytosine, which Fcy1 converts to 5-fluorouracil; this property makes yeast cytosine deaminase a widely used prodrug-activating enzyme and counter-selectable marker. It has been extensively characterised structurally and mechanistically.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004131 cytosine deaminase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN000871231 (C. albicans FCA1 and yeast FCY1 donors). Correct. Reason: Core activity. Supporting Evidence: PMID:9000374 Disruption of FCY1 resulted in high resistance to 5-fluorocytosine (10(-2) M) and in total loss of cytosine deaminase activity |
| GO:0004131 cytosine deaminase activity | IDA PMID:15535715 Catalytic mechanism of yeast cytosine deaminase: an ONIOM co... | ACCEPT | Summary: SGD IDA citing Sklenak et al. 2004, an ONIOM computational study of the yeast cytosine deaminase reaction mechanism; the abstract describes modelling, not an assay, so the IDA evidence code is questionable. The activity itself is firmly established for Fcy1 (PMID:9000374, many crystal structures). Reason: Correct activity; evidence code for this particular reference is doubtful but the annotation content is right. Supporting Evidence: PMID:15535715 a zinc metalloenzyme of significant biomedical interest PMID:9000374 Disruption of FCY1 resulted in high resistance to 5-fluorocytosine (10(-2) M) and in total loss of cytosine deaminase activity |
| GO:0004131 cytosine deaminase activity | IDA PMID:9000374 Characterization of the Saccharomyces cerevisiae FCY1 gene e... | ACCEPT | Summary: Erbs et al. 1997: fcy1 disruption causes total loss of cytosine deaminase activity and 5-fluorocytosine resistance; FCY1 restores growth on cytosine. Reason: Direct experimental demonstration. Supporting Evidence: PMID:9000374 Disruption of FCY1 resulted in high resistance to 5-fluorocytosine (10(-2) M) and in total loss of cytosine deaminase activity PMID:9000374 restored sensitivity to 5-fluorocytosine and allowed growth on cytosine, as a source of pyrimidine, or ammonium |
| GO:0004131 cytosine deaminase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Combined IEA (ARBA, RHEA:20605, EC 3.5.4.1). Correct. Reason: Matches experimental activity. Supporting Evidence: PMID:9000374 Disruption of FCY1 resulted in high resistance to 5-fluorocytosine (10(-2) M) and in total loss of cytosine deaminase activity |
| GO:0005634 nucleus | HDA PMID:14562095 Global analysis of protein localization in budding yeast. | KEEP AS NON CORE | Summary: Huh et al. 2003 GFP screen: nucleus (and cytoplasm). No nuclear role known for this small soluble enzyme. Reason: Observed location without functional relevance. Supporting Evidence: PMID:14562095 Global analysis of protein localization in budding yeast. file:yeast/FCY1/FCY1-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:14562095}. Nucleus |
| GO:0005634 nucleus | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: UniProt SubCell 'Nucleus' mapping from the Huh et al. data. Reason: Non-core location. Supporting Evidence: file:yeast/FCY1/FCY1-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:14562095}. Nucleus |
| GO:0005737 cytoplasm | HDA PMID:14562095 Global analysis of protein localization in budding yeast. | ACCEPT | Summary: Huh et al. 2003 GFP screen: cytoplasm. Reason: Site of cytosine salvage. Supporting Evidence: PMID:14562095 Global analysis of protein localization in budding yeast. file:yeast/FCY1/FCY1-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:14562095}. Nucleus |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt SubCell 'Cytoplasm' mapping. Correct. Reason: Correct. Supporting Evidence: file:yeast/FCY1/FCY1-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:14562095}. Nucleus |
| GO:0005829 cytosol | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA default cytosol (YEAST-RNT-SALV). Consistent with cytoplasmic GFP signal. Reason: Default compartment, consistent with data. Supporting Evidence: file:yeast/FCY1/FCY1-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:14562095}. Nucleus |
| GO:0008270 zinc ion binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro IPR016192 (APOBEC/CMP deaminase zinc-binding) mapping. Zinc is an experimentally established cofactor of Fcy1 (crystal structures; zinc-coordinated hydroxide mechanism). Reason: Catalytic cofactor binding; non-core. Supporting Evidence: PMID:15535715 a zinc metalloenzyme of significant biomedical interest |
| GO:0008270 zinc ion binding | RCA PMID:30358795 The cellular economy of the Saccharomyces cerevisiae zinc pr... | KEEP AS NON CORE | Summary: SGD RCA from the Wang et al. 2018 zinc-proteome survey; consistent with structural data. Reason: Cofactor binding; non-core. Supporting Evidence: PMID:30358795 The yeast zinc proteome of 582 known or potential zinc-binding proteins was identified using a bioinformatics analysis that combined global domain searches with local motif searches |
| GO:0008655 pyrimidine-containing compound salvage | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN000871231 (FCA1, FCY1). Correct. Reason: Correct. Supporting Evidence: PMID:9000374 restored sensitivity to 5-fluorocytosine and allowed growth on cytosine, as a source of pyrimidine, or ammonium |
| GO:0008655 pyrimidine-containing compound salvage | IMP PMID:9000374 Characterization of the Saccharomyces cerevisiae FCY1 gene e... | ACCEPT | Summary: Erbs et al. 1997 IMP: FCY1 is required for use of cytosine as pyrimidine source. Reason: Genetic evidence for the salvage role. Supporting Evidence: PMID:9000374 restored sensitivity to 5-fluorocytosine and allowed growth on cytosine, as a source of pyrimidine, or ammonium |
| GO:0008835 diaminohydroxyphosphoribosylaminopyrimidine deaminase activity | IBA GO_REF:0000033 | REMOVE | Summary: IBA diaminohydroxyphosphoribosylaminopyrimidine (DRAP) deaminase activity from PTN002254009 with E. coli RibD (P25539) as the only donor: a deep node in the CMP/dCMP deaminase superfamily covering the riboflavin-pathway deaminases. Fcy1 is a 158-residue single-domain cytosine deaminase with no reported activity on riboflavin intermediates; in S. cerevisiae the DRAP deaminase step of riboflavin synthesis is carried out by the deaminase domain of Rib2, and fcy1 nulls are not reported as riboflavin auxotrophs (they grow normally and are used as a 5-FC counter-selectable marker). Reason: Over-propagation of a riboflavin-pathway deaminase function across the deaminase superfamily into the cytosine-deaminase lineage; the yeast DRAP deaminase is Rib2. Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE Sources checked: UniProtKB:P25539 · E. coli RibD SUPPORTS SOURCE BUT NOT TARGET Bifunctional riboflavin deaminase/reductase; activity valid for RibD but the cytosine-deaminase clade containing Fcy1 diverged. PANTHER:PTN002254009 · PTN002254009 UNRESOLVED Deep node placement in the CDA superfamily covers Fcy1; yeast DRAP deaminase is Rib2. Supporting Evidence: PMID:9000374 Disruption of FCY1 resulted in high resistance to 5-fluorocytosine (10(-2) M) and in total loss of cytosine deaminase activity file:yeast/RIB2/RIB2-uniprot.txt RecName: Full=Diaminohydroxyphosphoribosylaminopyrimidine deaminase; |
| GO:0010138 pyrimidine ribonucleotide salvage | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA (YEAST-RNT-SALV). Fcy1 converts cytosine to uracil, the substrate of Fur1 for UMP formation. Reason: Correct pathway membership. Supporting Evidence: PMID:9000374 restored sensitivity to 5-fluorocytosine and allowed growth on cytosine, as a source of pyrimidine, or ammonium |
| GO:0016787 hydrolase activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro IPR016192 to root hydrolase activity. True but uninformative. Reason: Specific activity known. Proposed replacements: cytosine deaminase activity Supporting Evidence: PMID:9000374 Disruption of FCY1 resulted in high resistance to 5-fluorocytosine (10(-2) M) and in total loss of cytosine deaminase activity |
| GO:0019858 cytosine metabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN000871231. Fcy1 deaminates cytosine; correct. Reason: Correct. Supporting Evidence: PMID:9000374 Disruption of FCY1 resulted in high resistance to 5-fluorocytosine (10(-2) M) and in total loss of cytosine deaminase activity |
| GO:0019858 cytosine metabolic process | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA rule for cytosine metabolic process. Correct. Reason: Correct. Supporting Evidence: PMID:9000374 Disruption of FCY1 resulted in high resistance to 5-fluorocytosine (10(-2) M) and in total loss of cytosine deaminase activity |
| GO:0019858 cytosine metabolic process | IMP PMID:9000374 Characterization of the Saccharomyces cerevisiae FCY1 gene e... | ACCEPT | Summary: Erbs et al. 1997 IMP: fcy1 cells cannot use cytosine. Reason: Correct. Supporting Evidence: PMID:9000374 restored sensitivity to 5-fluorocytosine and allowed growth on cytosine, as a source of pyrimidine, or ammonium |
| GO:0044206 UMP salvage | IEA GO_REF:0000041 | ACCEPT | Summary: UniPathway UPA00574 mapping (UMP biosynthesis via salvage; uracil from cytosine). Correct: Fcy1 feeds uracil to Fur1. Reason: Core process. Supporting Evidence: PMID:9000374 restored sensitivity to 5-fluorocytosine and allowed growth on cytosine, as a source of pyrimidine, or ammonium |
| GO:0046087 cytidine metabolic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: IBA cytidine metabolic process from PTN000871231. Fcy1 does not act on cytidine; it deaminates cytosine released from cytidine by Urh1, and is the limiting step for cytidine utilisation. Literally indirect. Reason: Downstream participation in cytidine utilisation, not a cytidine-modifying activity. Supporting Evidence: PMID:10501935 a block in cytosine deaminase (Fcy1p), but not in cytidine deaminase (Cdd1p), constitutes a limiting step in cytidine utilisation as a UMP precursor |
| GO:0046087 cytidine metabolic process | IMP PMID:10501935 New insights into the pyrimidine salvage pathway of Saccharo... | KEEP AS NON CORE | Summary: Kurtz et al. 1999 IMP: a block in Fcy1 limits cytidine utilisation as a UMP precursor. Reason: Indirect role via cytosine produced from cytidine. Supporting Evidence: PMID:10501935 a block in cytosine deaminase (Fcy1p), but not in cytidine deaminase (Cdd1p), constitutes a limiting step in cytidine utilisation as a UMP precursor |
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Cytosine deaminase; zinc metalloenzyme that catalyzes the hydrolytic deamination of cytosine to uracil; of biomedical interest because it also catalyzes the deamination of 5-fluorocytosine (5FC) to form anticancer drug 5-fluorouracil (5FU)
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