Inherited mutations in the gene encoding BRCA2 are associated with a predisposition to early-onset breast and ovarian cancers. The underlying basis of the tumorigenesis is thought to be linked to defects in homologous recombination which results in radiation sensitivity, mutation, and loss of genome stability. We seek to understand the molecular mechanism of BRCA2 through analysis of its ortholog Brh2 in a model microbial system, Ustilago maydis, that is amenable to molecular genetic manipulations. The powerful attributes of this system open the way for understanding BRCA2's molecular mechanism through avenues not yet approachable in the higher eukaryotic systems. We plan three lines of attack. First, we will explore the biochemical activity of Brh2 in binding DMA. Second, we will study how Brh2 and Rad51 work together to promote DNA strand exchange and will investigate how Dss1, a small regulatory protein, controls Brh2's action. Third, we will use chromatin-immunoprecipitation analysis to investigate the dynamics of Brh2, Rad51, Dss1 and associated proteins at a DNA double-strand break and will test a model for Brh2 in promoting the initial strand invasion event. Relevance: U. maydis is a laboratory microbe in which the BRCA2 system is recapitulated at the molecular level. The system thus offers a unique opportunity for experimentation on BRCA2 to a degree as yet unattainable in mammalian systems. Understanding the molecular mechanism of BRCA2, as for any gene implicated in cancer, contributes in the most fundamental way to a knowledge base illuminating the gene's action. The research program proposed here should bring new light on the molecular mechanism of action of Brh2, and by inference, BRCA2. The assembly of such knowledge is the real precursor to breast cancer therapy and cure.