Differentiation requires cells to integrate external signaling cues with internal cell-type information to execute complex transcriptional programs. Mistakes in this process can lead to developmental defects and cancers. Post-translational histone modifications play central roles in orchestrating all phases of transcription. While much is known regarding how histone modifications are maintained and interpreted, a significant gap exists in our understanding of how the enzyme complexes that catalyze these modifications are controlled. The long-term goal of this project is to dissect the molecular mechanisms that regulate histone modification complexes during differentiation. The specific objective of this proposal is to investigate changes in the integrity, stability, and activity of the histone H3Lys4 methyltransferase COMPASS complex during meiotic differentiation in the budding yeast S. cerevisiae. Our central hypothesis is that there is a two- step mechanism that results in meiosis-specific COMPASS inactivation that is necessary for efficient meiotic entry and completion. The first step implicates changes in locus-specific Set1 methyltransferase recruitment as cells enter meiosis, while the second step involves meiosis- specific Set1 degradation to allow progression past the commitment point. Two Specific Aims are proposed to test this hypothesis. Aim 1 will define the role of locus-specific Set1 antagonism for meiotic entry and execution. Our preliminary and published data indicate that the Cdk8 kinase module of the RNA pol II mediator complex inhibits locus-specific Set1 recruitment. Using molecular, biochemical, and genetic approaches, we will determine if Cdk8-dependent Set1 antagonism occurs via direct or indirect mechanisms. ChIP-sequencing will determine how genome-wide Cdk8-dependent Set1 occupancy is altered as cells enter the meiotic program. Aim 2 will determine how Set1 degradation is incorporated into the meiotic program. Chemical and genetic approaches will decipher the requirement of meiotic gene expression and identify the meiotic hallmarks linked to Set1 degradation. Using genetic and molecular approaches, we will determine the consequences of stabilizing Set1 on meiotic progression and completion while monitoring COMPASS integrity and H3Lys4 me patterns. Successful completion of these Aims will have a significant impact as they will provide mechanistic detail into how COMPASS is retooled during differentiation. The strength of this project is that it merges classical techniques in yeast genetics, biochemistry, and molecular biology with contemporary approaches in genomics and computation.