The human fungal pathogen Candida albicans exhibits remarkable phenotypic plasticity during colonization and infection of multiple niches in the human host [2]. This is exemplified by its ability to switch between two phenotypic states, 'white' and 'opaque'. This epigenetic switch occurs between isogenic cells and is both heritable and reversible. White and opaque states differ in many key aspects of Candida biology, including their interaction with immune cells, their virulence, and their response to sexual pheromones. Thus, while white cells respond to pheromone by forming communities of adherent cells known as biofilms, opaque cells respond by undergoing sexual mating [20, 17]. In both cell types, a conserved MAPK pathway facilitates pheromone signaling [5,6], yet the underlying mechanism by which cells generate different responses to pheromone is unknown. Preliminary analysis indicates that opaque cells exhibit elevated expression of three components of the MAPK signaling pathway. Aim 1 will test the hypothesis that increased expression of specific MAPK components facilitates signal transduction and is necessary for efficient mating. To achieve this goal, strains locked in the white state will be engineered to express elevated levels of key MAPK components and mating efficiency quantified. Immunoblotting will also be used to compare expression levels of MAPK proteins between opaque cells, white cells, and engineered white cells, to determine if protein levels are reflectiv of overall mating efficiency. Aim 2 will define the mechanism of sexual (pheromone-induced) biofilm formation in white cells by examining the transcriptional targets of MAPK signaling. The transcription factor Cph1 is activated in both white and opaque cells [6], yet distinct sets of genes are induced in response to pheromone in both cell types [29]. This Aim will test the hypothesis that a white-specific transcription factor works cooperatively with Cph1 to promote biofilm formation. Chromatin immunoprecipitation and DNA sequencing analysis will be performed in white and opaque cells to identify the DNA binding targets for Cph1 in both cell types. In parallel with this approach, I will screen a transcription factor overexpression library (recently constructed in the Bennett laboratory) to define those factors that promote sexual biofilm formation. Together, completion of the experiments outlined in this proposal will define how an epigenetic switch can regulate phenotypic traits, including both sexual mating and biofilm formation. Given the prevalence of C. albicans biofilms within the oral cavity [18] and on surgical implantation devices [21], the research outlined in this proposal will have direct implications in combating debilitating fungal infections in the clinic.