FCY1/YPR062W Summary Help

Standard Name FCY1 1
Systematic Name YPR062W
Feature Type ORF, Verified
Description 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) (1, 2 and see Summary Paragraph)
Also known as: yCD 3
Name Description FluoroCYtosine resistance 1
Chromosomal Location
ChrXVI:677165 to 677641 | ORF Map | GBrowse
Gene Ontology Annotations All FCY1 GO evidence and references
  View Computational GO annotations for FCY1
Molecular Function
Manually curated
Biological Process
Manually curated
Cellular Component
Regulators 5 genes
Classical genetics
Large-scale survey
32 total interaction(s) for 25 unique genes/features.
Physical Interactions
  • Affinity Capture-MS: 7
  • Affinity Capture-RNA: 5
  • Co-crystal Structure: 1
  • PCA: 1
  • Two-hybrid: 2

Genetic Interactions
  • Negative Genetic: 14
  • Positive Genetic: 2

Expression Summary
Length (a.a.) 158
Molecular Weight (Da) 17,507
Isoelectric Point (pI) 5.79
Phosphorylation PhosphoGRID | PhosphoPep Database
sequence information
ChrXVI:677165 to 677641 | ORF Map | GBrowse
Last Update Coordinates: 2011-02-03 | Sequence: 1996-07-31
Subfeature details
Most Recent Updates
Coordinates Sequence
CDS 1..477 677165..677641 2011-02-03 1996-07-31
Retrieve sequences
Analyze Sequence
S288C only
S288C vs. other species
S288C vs. other strains
External Links All Associated Seq | E.C. | Entrez Gene | Entrez RefSeq Protein | MIPS | Search all NCBI (Entrez) | UniProtKB
Primary SGDIDS000006266

Fcy1p is a cytosine deaminase (EC: that catalyzes the hydrolytic deamination of cytosine to uracil (1, 2, 4). Fcy1p is homodimeric, and each subunit contains an active center composed of a catalytic zinc ion coordinated with a histidine (His62), two cysteines (Cys91 and Cys94), and a water molecule in the substrate-free enzyme, with the latter serving as a nucleophile in the deamination reaction (2). The conversion of cytosine to uracil is required for the pyrimidine salvage pathway of Saccharomyces cerevisiae, and is important for providing pyrimidines for the synthesis of nucleic acids and amino acids, and as a source of energy (4, 5). A block in Fcy1p cytosine deaminase activity, but not in Cdd1p cytidine deaminase activity, constitutes a limiting step in cytidine utilization (4). Fcy1p is of great biomedical interest because it also catalyzes the deamination of the prodrug 5-fluorocytosine (5FC) to form the anticancer drug 5-fluorouracil (5FU) (2).

FCY1 is similar to Candida albicans (FCA1) and Escherichia coli cytidine deaminases (1, 2), and displays significant structural similarity to Bacillus subtilis cytidine deaminase (cdd) and chicken ATIC (6). Fcy1p also displays two regions of similarity to the dCMP/CMP deaminases from human (DCTD), S. cerevisiae (DCD1) and T4 phage (dCD), and to the human cytidine/deoxycytidine deaminase CDA (1).

Fcy1 null mutants are viable, but display a total loss of cytosine deaminase activity, high resistance to 5-fluorocytosine, and resistance to 5-fluorocytidine (4, 1, 5). Although wild-type Fcy1p shows a rapid loss of activity at 50 C, with a half-life of 4 hours in vitro, Fcy1p-[A23L/I140L/V108I] has a half-life of 117 hours at 50 C, a 30-fold increase (7). Fcy1p-[A23L/I140L/V108I] is also better than wild-type Fcy1p at allowing growth at 37 C of an E. coli strain of dependent on cytosine deaminase function for uracil synthesis (7). The defects of S. cerevisiae fcy1 mutants can be complemented by C. albicans FCA1 (1).

A novel and highly potent suicide gene, FCU1, designed for expression in tumorogenic mammalian cells, has been derived from S. cerevisiae FCY1 cytosine deaminase and FUR1 uracil phosphoribosyltransferase. The bifunctional chimeric Fcu1p combines the enzymatic activities of Fcy1p and Fur1p, and efficiently catalyzes the direct conversion of nontoxic antifungal 5FC into the toxic metabolites 5FU and 5-fluorouridine-5'monophosphate, thus bypassing the natural resistance of certain human tumor cells to 5-fluorouracil. The uracil phosphoribosyltransferase activity of Fcu1p is equivalent to that of Fur1p, but its cytosine deaminase activity is 100-fold higher than that of Fcy1p. As a consequence, tumor cells transduced with an adenovirus expressing FCU1 (Ad-FCU1) are sensitive to concentrations of 5FC 1000-fold lower than those used for cells transduced with a vector expressing only FCY1 (Ad-FCY1). Furthermore, bystander cell killing is also more effective in cells transduced with Ad-FCU1 than in cultures infected with Ad-FCY1 or Ad-FUR1, alone or in combination. In a mouse model of tumor progression, injection of Ad-FCU1 into tumors in conjunction with systemic administration of 5FC, leads to substantial delays in tumor growth (8).

Last updated: 2005-11-04 Contact SGD

References cited on this page View Complete Literature Guide for FCY1
1) Erbs P, et al.  (1997) Characterization of the Saccharomyces cerevisiae FCY1 gene encoding cytosine deaminase and its homologue FCA1 of Candida albicans. Curr Genet 31(1):1-6
2) Yao L, et al.  (2005) Product release is rate-limiting in the activation of the prodrug 5-fluorocytosine by yeast cytosine deaminase. Biochemistry 44(15):5940-7
3) Kievit E, et al.  (2000) Yeast cytosine deaminase improves radiosensitization and bystander effect by 5-fluorocytosine of human colorectal cancer xenografts. Cancer Res 60(23):6649-55
4) Kurtz JE, et al.  (1999) New insights into the pyrimidine salvage pathway of Saccharomyces cerevisiae: requirement of six genes for cytidine metabolism. Curr Genet 36(3):130-6
5) Kurtz JE, et al.  (2002) The URH1 uridine ribohydrolase of Saccharomyces cerevisiae. Curr Genet 41(3):132-41
6) Ko TP, et al.  (2003) Crystal structure of yeast cytosine deaminase. Insights into enzyme mechanism and evolution. J Biol Chem 278(21):19111-7
7) Korkegian A, et al.  (2005) Computational thermostabilization of an enzyme. Science 308(5723):857-60
8) Erbs P, et al.  (2000) In vivo cancer gene therapy by adenovirus-mediated transfer of a bifunctional yeast cytosine deaminase/uracil phosphoribosyltransferase fusion gene. Cancer Res 60(14):3813-22