CHARACTERIZATION OF THE SACCHAROMYCES-CEREVISIAE FCY1 GENE ENCODING CYTOSINE DEAMINASE AND ITS HOMOLOG FCA1 OF CANDIDA-ALBICANS

By functional complementation of a fcy1 null mutant of Saccharomyces c erevisiae, we have cloned and characterized the FCY1 gene, encoding cy tosine deaminase in Saccharomyces cerevisiae, and its homologue FCA1, encoding cytosine deaminase in Candida albicans. Disruption of FCY1 re sulted in high resistance to 5-fluorocytosine (10(-2) M) and in total loss of cytosine deaminase activity. By contrast the transformation by FCY1 or FCA1 of the haploid FCY1-disrupted host strain restored sensi tivity to 5-fluorocytosine and allowed growth on cytosine, as a source of pyrimidine, or ammonium. FCA1 as opposed to FCY1 contains an intro n. FCA1 and FCY1 encode respectively 150- and 158- residue proteins of 60% identity. Both Fcy1p and Fca1p share common motifs with cytidine and CMP deaminases, but homology with cytosine deaminase of E. coli co uld not be detected.