Spontaneous Second-site Suppressors of the Filamentation Defect of prr1Delta Mutants Define a Critical Domain of Rim101p in Candida Albicans

In response to changes in ambient pH the opportunistic pathogen Candida albicans differentially expresses a number of genes. The response to pH affects morphological differentiation and virulence. The pathway controlling the pH response terminates in the zinc-finger containing transcription factor encoded by RIM101/PRR2. By analogy to the pH response pathway of Aspergillus nidulans, PRR1 of C. albicans encodes a protein that is presumably required to convert Rim101p from an inactive to an active form by proteolytic removal of a C-terminal peptide. A prr1Delta mutant is compromised in its ability to differentiate into the filamentous form. Spontaneous phenotypic revertants of a prr1Delta mutant were selected by their ability to form filamentous colonies. These mutants were also found to be defective in pH-dependent gene expression. Each of the eight mutants examined contained a heterozygous dominant mutation at the RIM101 locus. This was demonstrated genetically in all of the mutants, and directly by sequence determination of both alleles in two of the mutants. The mutant alleles conferred the ability to filament to a prr1Delta mutant, thus demonstrating that they were directly responsible for suppressing the filamentation defect. Seven of the mutant alleles contained a 1-bp substitution and one contained two substitutions at adjacent positions. The mutations were clustered within a 90-bp region near the 3'-end of the gene. In all cases the mutation generated a nonsense codon that resulted in premature termination of Rim101p; the mutant proteins were truncated by 75-104 amino acids. The results define a critical region in the C-terminal region of Rim101p and are consistent with the proposed proteolytic activation of Rim101p.