Pachytene exit controlled by reversal of Mek1-dependent phosphorylation.

During yeast meiosis, a checkpoint prevents exit from pachytene in response to defects in meiotic recombination and chromosome synapsis. This pachytene checkpoint requires two meiotic chromosomal proteins, Red1 and Mek1; Mek1 is a kinase that phosphorylates Red1. In mutants that undergo checkpoint-mediated pachytene arrest, Mek1 is active and Red1 remains phosphorylated. Activation of Mek1 requires the initiation of meiotic recombination and certain DNA damage checkpoint proteins. Mek1 kinase activity and checkpoint-induced pachytene arrest are counteracted by protein phosphatase type 1 (Glc7). Glc7 coimmunoprecipitates with Red1, colocalizes with Red1 on chromosomes, and dephosphorylates Red1 in vitro. We speculate that phosphorylated Red1 prevents exit from pachytene and that completion of meiotic recombination triggers Glc7-dependent dephosphorylation of Red1.