The pachytene checkpoint in Saccharomyces cerevisiae requires the Sum1 transcriptional repressor

Saccharomyces cerevisiae mutants that fail to complete meiotic recombinatio n are blocked by the RAD17/ RAD24/MEC1 checkpoint signaling pathway in pach ytene when early sporulation genes are expressed. Middle genes are not acti vated in checkpoint-arrested cells because the Ndt80 transcription factor i s inhibited. We find that the pachytene checkpoint requires Sum1, a transcr iptional repressor that recognizes a subset of Ndt80-binding sites. Mutants lacking Sum1 or Rad17 partially bypass the block to the nuclear divisions but do not form spores, while mutants lacking both Sum1 and Rad17 completel y bypass the block and form morphologically normal spores. The level of Sum 1 protein decreases as middle genes are expressed, and this decrease is blo cked in checkpoint-arrested cells. These data suggest that Sum1 levels are regulated by the checkpoint and that progression of the meiotic divisions a nd spore differentiation can be differentially controlled by competition of the Sum1 repressor and Ndt80 activator for occupancy at key middle promote rs.