Effect of environmental pH on morphological development of Candida albicans is mediated via the PacC-related transcription factor encoded by PRR2

Abstract

The ability to respond to ambient pH is critical to the growth and virulence of the fungal pathogen Candida albicans. This response entails the differential expression of several genes affecting morphogenesis. To investigate the mechanism of pH-dependent gene expression, the C. albicans homolog of pacC, designated PRR2 (for pH response regulator), was identified and cloned. pacC encodes a zinc finger-containing transcription factor that mediates pH-dependent gene expression in Aspergillus nidulans. Mutants lacking PRR2 can no longer induce the expression of alkaline-expressed genes or repress acid-expressed genes at alkaline pH. Although the mutation did not affect growth of the cells at acid or alkaline pH, the mutants exhibited medium-conditional defects in filamentation. PRR2 was itself expressed in a pH-conditional manner, and its induction at alkaline pH was controlled by PRR1. PRR1 is homologous to palF, a regulator of pacC. Thus, PRR2 expression is controlled by a pH-dependent feedback loop. The results demonstrate that the pH response pathway of Aspergillus is conserved and that this pathway has been adapted to control dimorphism in C. albicans.

FIG. 1

Amino acid sequence alignment of Prr2p from C. albicans and PacC from A. nidulans. Identical residues are boxed.

FIG. 2

Design and construction of PRR2 mutants. (A) Restriction maps of the locus as cloned in plasmid pARA1 and subcloned in pARA2, as well as the region deleted and replaced with the hisG-URA3-hisG cassette. The open reading frame of PRR2 is indicated in black. The region used as a hybridization probe is overlined. (B and C) Southern blots of genomic DNA digested with BglII (boldface in panel A) were hybridized with either PRR2 (B) or hisG (C) probes. Lanes 1 through 5, DNAs isolated from strains CAI12, CAR1, CAR14, CAR2, and CAR26 respectively.

FIG. 3

Effect of PRR2 mutations on pH-dependent gene expression. Total RNAs were isolated from the indicated strains cultured at either pH 4.0 or 7.5 and examined by Northern blot analysis. The gene sources of the hybridizing DNA probes are indicated on the left.

FIG. 4

Effect of PRR2 mutations on filamentation. The indicated strains were spotted on the indicated media and incubated at 37°C in medium 199 (pH 7.5) for 5 days, in 10% serum for 3 days, and in Spider medium for 6 days.

FIG. 5

Effect of culture pH and PRR1 mutation on PRR2 expression. Cells of either the control strain CAI12 or the PRR1 null mutant CAPM3 were cultured at either pH 4.0 or 7.5. Total RNA was isolated and examined by Northern blot analysis. The blot was hybridized with either PRR2 or ACT1 DNA.