The goal of this proposal is to understand the connection between the yeast to filament switch and fungal virulence. This switch is intimately connected to the molecules that encircle the fungal cell, beta-glucan and the linked mannoproteins (adhesins). The analysis of filamentation in the model system, Saccharomyces cerevisiae will guide the studies in the less tractable pathogen, Candida albicans. The genomes of both fungi encode many mannoproteins that confer unique adherence properties. These adhesins are required for interactions of fungal cells with each other {flocculation and filamentation), with inert surfaces (agar and plastic) and with mammalian cells. The role of a novel antisense IME4 RNA in controlling the expression of the adhesins will be determined in both organisms. The role of tyrosol, an autoregulatory molecule in triggering the switch between the yeast form and the filament form as well as its role in the interaction of fungi with phagocytic cells will be resolved by analysis of its biosynthesis, perception and role in signal transduction. The ability of macrophages to distinguish between Saccharomyces and Candida is likely to result from differential accessibility of beta-glucan on the surface of each organism to Dectin-1, the key non-opsonic fungal receptor on macrophages. Preliminary experiments show that differential beta-glucan presentation on the fungal cell surface leads to differential binding of fungi to Dectin-1 and to different elicitation of inflammatory cytokines from macrophages. The question of whether the unmasking of beta-glucan or the structure of beta-glucan is key will be resolved by systematic use of whole genome mutant libraries to identify mutants with altered beta-glucan presentation and the use of those mutants to identify the immune response. Additional experiments are designed to identify all the genes required for the mannoproteins to transit to the fungal cell wall. Our finding that hypoxia induces filamentation in Candida will be used to analyze the genes important for virulence under low oxygen tensions. The genes uncovered in our analyses are potential targets for the development of novel therapeutics against fungi, which are so devastating to those undergoing chemotherapy and afflicted with AIDS.