Rfg1, a protein related to the hypoxic repressor Rox1 of Saccharomyces cerevisiae, is a novel DNA binding repressor of filamentation in the pathogenic yeast, Candida albicans

I have identified a repressor of hyphal growth in the pathogenic yeast Candida albicans. C. albicans is a dimorphic yeast: it can grow as single cell blastopores in rich media, but under stressful environmental conditions (nitrogen starvation, neutral pH, hypoxia, high temperature), undergoes a switch to an invasive filamentous form, a form that is known to be associated with systemic candidosis. The gene was originally cloned in an attempt to characterize the homologue of the S. cerevisiae Rox1, a repressor of hypoxic genes. Rox1 is an HMG-domain, DNA binding protein with a repression domain that recruits the Tup1/Ssn6 general repression complex to achieve repression. The C. albicans clone also encoded an HMG protein which was capable of repression of an hypoxic gene in a S. cerevisiae rox1 deletion strain. Gel retardation experiments using the purified HMG-domain of this protein demonstrated that it was capable of binding specifically to a S. cerevisiae hypoxic operator DNA sequence. These data seemed to indicate that this gene encoded an hypoxic repressor. However, surprisingly, when a homozygous deletion was generated in C. albicans, the cells grew as hyphae even in rich media. This phenotype was rescued by the reintroduction of the wildtype gene on a plasmid, proving that the hyphal growth phenotype was due to the deletion and not a secondary mutation. Furthermore, oxygen repression of the hypoxic HEM13 gene was not affected by the deletion, nor was this putative ROX1 gene regulated positively by oxygen as is the case for the S. cerevisiae gene. On the other hand, overexpression of this gene resulted, unexpectedly, in increased filamentation compared to wildtype perhaps through sequestering other factors involved in filamentous repression. All these data indicate that this gene, now designated RFG1 for Repressor of Filamentous Growth, is a repressor of genes required for hyphal growth and not a hypoxic repressor.