TOS9 Regulates White-Opaque Switching in Candida albicans

In Candida albicans, the a1-alpha2 complex represses white-opaque switching as well as mating. A ChIP-chip strategy was, therefore, used to screen for genes with a1-alpha2 binding sites and expression patterns consistent with a master switch gene (MSG). Of 51 genes identified with an a1-alpha2 binding site, one gene, TOS9, also referred to as EAP2, exhibited an expression pattern consistent with a MSG. TOS9 is expressed in opaque, not white a/a and alpha/alpha cells and Tos9p localizes to the opaque cell nucleus. Deletion of TOS9 blocks cells in the white phase, and misexpression in the white phase of the parent tos9+/tos9+ strain results in mass conversion to opaque. Expression of TOS9 under control of a MET promoter rescues the TOS9 null mutant tos9-/tos9- phenotype. Temperature-induced mass conversion of opaque to white in the parent strain results in the immediate cessation of TOS9 transcription, and loss of Tos9p prior to the point of commitment to white (the switch event), which occurs at the time of the second cell doubling. Misexpression of TOS9, as well as inhibition of the second round of DNA replication inhibits the temperature-induced switch from opaque to white. Based on these observations, a model is developed for the regulation and role of TOS9 in switching. Keywords: ChIP chip ChIP-chip was performed on a1 and alpha2 from C. albicans. Three biological replicates were performed for each factor in each species with one replicate representing a dye swap.