Project Summary The formation of gametes in most sexually reproducing organisms involves a stage of controlled genome fragmentation and reshuffling known as meiotic recombination. Aside from promoting genetic diversity, the exchange of DNA sequences serves to tether homologous chromosomes, which is essential for controlled chromosome assortment into sperm or eggs. Meiotic recombination is initiated by the programmed induction of hundreds of DNA double-strand breaks (DSBs), which occur preferentially in chromosomal hotspots. Recent work has defined chromatin modifications that correlate with the positions of meiotic DSB hotspots. However, those modifications are poor predictors of actual hotspot usage, indicating the existence of additional determinants. The overall goal of this project is to define the molecular mechanisms governing meiotic hotspot usage. Hotspot usage will be investigated in the sexually reproducing yeast Saccharomyces cerevisiae, which has served as successful model for many aspects of meiotic recombination. Preliminary experiments in S. cerevisiae identified a conserved chromatin regulator that, when mutated, dramatically alters relative hotspot usage. The proposed experiments will determine the function of this regulator in controlling DSB levels, and take advantage of the altered DSB landscape of this mutant to determine the features of chromatin architecture and DNA topology that remain specifically associated with hot DSB hotspots. In addition, preliminary experiments have identified a characterisitic DNA topology that appears to predict hotspot activity. The proposed experiments will address if this DNA topology is related to supercoiling and will investigate the function of several topology-sensitive enzymes in influencing the activity of meiotic DSB hotspots. The proposed experiments will also use genome-wide methods to probe the three-dimensional conformation of meiotic chromosomes and determine the effects on hotspot usage when these conformations are altered. Together, the proposed experiments will provide a high-resolution view of meiotic chromosome structure and define the role of this architecture in faithfully guiding the formation of hundreds of meiotic DSBs as a prerequisite to productive gamete formation and fertility.