THE CANDIDA-ALBICANS CDR3 GENE CODES FOR AN OPAQUE-PHASE ABC TRANSPORTER

We report the cloning and functional analysis of a third member of the CDR gene family in Candida albicans, named CDR3. This gene codes for an ABC (ATP-binding cassette) transporter of 1,501 amino acids highly homologous to Cdr1p and Cdr2p (56 and 55% amino acid sequence identity , respectively), two transporters involved in fluconazole resistance i n C. albicans. The predicted structure of Cdr3p is typical of the PDR/ CDR family, with two similar halves, each comprising an N-terminal hyd rophilic domain with consensus sequences for ATP binding and a C-termi nal hydrophobic domain with six predicted transmembrane segments. Nort hern analysis showed that CDR3 expression is regulated, in a cell-type -specific manner, with low levels of CDR3 mRNA in CAI4 yeast and hypha l cells, high levels in WO-1 opaque sells, and undetectable levels in WO-1 white cells. Disruption of both alleles of CDR3 in CAI4 resulted in no obvious changes in cell morphology, growth rate, or susceptibili ty to fluconazole. Overexpression of Cdr3p in C. albicans did not resu lt in increased cellular resistance to fluconazole, cycloheximide, and 4-nitroquinoline-N-oxide, which are known substrates for different tr ansporters of the PDR/CDR family. These results indicate that despite a high degree of sequence conservation with C. albicans Cdr1p and Cdr2 p, Cdr3p does not appear to be involved in drug resistance, at least t o the compounds tested which include the clinically relevant antifunga l agent fluconazole. Rather, the high level of Cdr3p expression in WO- 1 opaque cells suggests an opaque-phase-associated biological function which remains to be identified.