Dominant Active Alleles of RIM101(PRR2) Bypass the pH Restriction on Filamentation of Candida albicans

Abstract

Morphological development of the fungal pathogen Candida albicans is profoundly affected by ambient pH. Acidic pH restricts growth to the yeast form, whereas neutral pH permits development of the filamentous form. Superimposed on the pH restriction is a temperature requirement of approximately 37°C for filamentation. The role of pH in development was investigated by selecting revertants of phr2Δ mutants that had gained the ability to grow at acid pH. The extragenic suppressors in two independent revertants were identified as nonsense mutations in the pH response regulatorRIM101 (PRR2) that resulted in a carboxy-terminal truncation of the open reading frame. These dominant active alleles conferred the ability to filament at acidic pH, to express PHR1, an alkaline-expressed gene, at acidic pH, and to repress the acid-expressed gene PHR2. It was also observed that both the wild-type and mutant alleles could act as multicopy suppressors of the temperature restriction on filamentation, allowing extensive filamentation at 29°C. The ability of the activated alleles to promote filamentation was dependent upon the developmental regulator EFG1. The results suggest thatRIM101 is responsible for the pH dependence of hyphal development.

ACKNOWLEDGMENTS

We especially acknowledge the expert technical assistance of Stefanie Mücksch. We thank G. R. Fink for strains JKC18 and HLC67 and D. P. Bockmühl and J. F. Ernst for strain CDB1. C. Schmidt is acknowledged for assistance with the artwork. We are grateful to S. Suerbaum and H. Karch for critical reading of the manuscript. A.P. thanks B. Maresca for continuous support.

This work was supported by grant MU1212/2-1 from the Deutsche Forschungsgemeinschaft (to F.A.M.). A.E. held a predoctoral fellowship from the Deutsche Forschungsgemeinschaft, and O.K. is supported by a student fellowship from the Studienstiftung des Deutschen Volkes. A.P., A.R., and W.A.F. were supported by Public Health Service grant GM47727 from the National Institutes of Health and the Burroughs Wellcome Fund Scholar Award in Molecular Pathogenic Mycology.