Ssn6 defines a new level of regulation of white-opaque switching in Candida albicans and is required for the stochasticity of the switch

The human commensal and opportunistic pathogen Candida albicans can switch between two distinct, heritable cell types, named “white” and “opaque,” which differ in morphology, mating abilities, metabolic preferences, and in their interactions with the host immune system. Previous studies revealed a highly interconnected group of transcriptional regulators that control switching between the two cell types. Here, we identify Ssn6, the C. albicans functional homolog of the Saccharomyces cerevisiae transcriptional co-repressor Cyc8, as a new regulator of white-opaque switching. In a or α mating type strains, deletion of SSN6 results in mass switching from the white to the opaque cell type. Transcriptional profiling of ssn6 deletion mutant strains reveals that Ssn6 represses part of the opaque cell transcriptional program in white cells and the majority of the white cell transcriptional program in opaque cells. Genome-wide chromatin immunoprecipitation experiments demonstrate that Ssn6 is tightly integrated into the opaque cell regulatory circuit and that the positions to which it is bound across the genome strongly overlap with those bound by Wor1 and Wor2, previously identified regulators of white-opaque switching. This work reveals the next layer in the white-opaque transcriptional circuitry by integrating a transcriptional regulator that does not bind DNA directly but instead associates with specific combinations of DNA-bound transcriptional regulators. Three independent microarray experiments were performed for each strain. One replicate (Replicate C) used cells grown to mid-log phase in synthetic defined (SD) yeast minimal media supplemented with amino acids and uridine at 25°C. The other two replicates (Replicates A & B) used cells grown in identical conditions but with arginine supplemented into the growth media at a concentration of 2.5 mg/mL to accommodate a growth defect likely associated with the arg- genotype of our a/α strains. Each experimental strain was labeled with Cy5 and analyzed relative to a pooled reference comprised of equal portions of Cy3 labeled aa-cDNA (See Sample records for more details).