1 Department of Biology, Concordia University, H4B 1R6, Montreal, Quebec, Canada.
2 Institut für Molekulare Infektionsbiologie, Universität Würzburg, 2/D15 D-97080, Würzburg, Germany.
3 The George Washington University School of Medicine and Health Sciences, 2300, I Street NW, Washington D.C., D.C. 20037. United States.
4 Department of Pathogen Biology, School of Medicine, Nantong University, Nantong, Jiangsu 226001, China.
5 Centre for Microscopy and Cellular Imaging, Concordia University, H4B 1R6, Montreal, Quebec, Canada.
6 PERFORM Centre, Concordia University, H4B 1R6, Montreal, Quebec, Canada.

Zinc cluster transcription factors are essential fungal regulators of gene expression. In the pathogen Candida albicans, the gene orf19.1604 encodes a zinc cluster transcription factor regulating filament development. Hyperactivation of orf19.1604, which we have named RHA1 for Regulator of Hyphal Activity, generates wrinkled colony morphology under non-hyphal growth conditions, triggers filament formation, invasiveness, and enhanced biofilm formation and causes reduced virulence in the mouse model of systemic infection. The strain expressing activated Rha1 shows up-regulation of genes required for filamentation and cell-wall-adhesion-related proteins. Increased expression is also seen for the hyphal-inducing transcription factors Brg1 and Ume6, while the hyphal repressor Nrg1 is downregulated. Inactivation of RHA1 reduces filamentation under a variety of filament-inducing conditions. In contrast to the partial effect of either single mutant, the double rha1 ume6 mutant strain is highly defective in both serum- and Spider-medium-stimulated hyphal development. While the loss of Brg1 function blocks serum-stimulated hyphal development, this block can be significantly bypassed by Rha1 hyperactivity, and the combination of Rha1 hyperactivity and serum addition can generate significant polarization even in brg1 ume6 double mutants. Thus, in response to external signals, Rha1 functions with other morphogenesis regulators including Brg1 and Ume6, to mediate filamentation.