ABSTRACT Pseudomonas aeruginosa (Pa) frequently co-infects with other bacterial and fungal species. For example, 50% of adults with cystic fibrosis (CF) are chronically infected with Pa and Candida albicans (Ca) commonly co- infects. Antagonistic interactions between Pa and Ca can enhance the growth and virulence of both species, resulting in worse clinical outcomes. We found that the sensor kinase CbrA, which is known to regulate carbon catabolite repression and Pa metabolism, is necessary for Pa antagonism of Ca. This is in part through its control of the transcription factor PhoB which we have shown regulates the production of toxins with antifungal activity. Both CbrA and PhoB have been independently implicated in Pa fitness and virulence, but this work is the first that connects these two important regulators. We have also shown that a subset of clinical isolates that are associated with worse clinical outcomes and increased antagonism toward Ca have elevated CbrA activity and CbrA-mediated activation of PhoB. Understanding the mechanism of CbrA activation and its role in Ca antagonism will shed light on Pa polymicrobial interactions as well as in virulent clinical isolates. We propose that CbrA activity is mediated via the conserved Per-Arnt-Sim (PAS) domain via a redox-active small molecule ligand and the candidate will use genetic and biochemical approaches to test this model in Aim 1. As CbrA is conserved across other bacterial species, this will provide insight into similar regulatory pathways. In Aim 2, the candidate will investigate the role of CbrA and phosphate availability in PhoB-mediated Pa antagonism of Ca. Because PhoB activity is also controlled by extracellular phosphate concentrations, the candidate will examine the CbrA-PhoB relationship at physiological phosphate concentrations as well as the high phosphate concentrations present in many laboratory media. This candidate plans to pursue a career in academia in the area of microbial interactions in microbial communities. This work will be supported by several collaborations with biochemists and metabolomics experts, provide the candidate experience working with both a bacterium (Pa) and a fungus (Ca), and establish collaborations in critical areas including metabolite analyses and mentor relationships with researchers in the field. This project will provide opportunities for the candidate to develop their scientific and professional skills and advance their career in research.