Sum1 and Hst1 repress middle sporulation-specific gene expression during mitosis in Saccharomyces cerevisiae

Meiotic development in yeast is characterized by the sequential induction o f temporally distinct classes of genes. Genes that are induced at the middl e stages of the pathway share a promoter element, termed the middle sporula tion element (MSE), which interacts with the Ndt80 transcriptional activato r. We have found that a subclass of MSEs are strong repressor sites during mitosis, SUM1 and HST1, genes previously associated with transcriptional si lencing, are required for MSE-mediated repression. Sum1 binds specifically in vitro to MSEs that function as strong repressor sites lit vivo. Repressi on by Sum1 is gene specific and does not extend to neighboring genes, These results suggest that mechanisms used to silence large regions of chromatin may also be used to regulate the expression of specific genes during devel opment. NDT80 is regulated during mitosis by both the Sum1 and Ume6 repress ors. These results suggest that progression through sporulation may be cont rolled by the regulated competition between the Sum1 repressor and Ndt80 ac tivator at key MSEs.