NDT80 and the Meiotic Recombination Checkpoint Regulate Expression of Middle Sporulation-Specific Genes in Saccharomyces cerevisiae

Distinct classes of sporulation-specific genes are sequentially expressed during the process of spore formation inSaccharomyces cerevisiae . The transition from expression of early meiotic genes to expression of middle sporulation-specific genes occurs at about the time that cells exit from pachytene and form the meiosis I spindle. To identify genes encoding potential regulators of middle sporulation-specific gene expression, we screened for mutants that expressed early meiotic genes but failed to express middle sporulation-specific genes. We identified mutant alleles ofRPD3 ,SIN3 , andNDT80 in this screen. Rpd3p, a histone deacetylase, and Sin3p are global modulators of gene expression. Ndt80p promotes entry into the meiotic divisions. We found that entry into the meiotic divisions was not required for activation of middle sporulation genes; these genes were efficiently expressed in aclb1 clb3 clb4 strain, which fails to enter the meiotic divisions due to reduced Clb-dependent activation of Cdc28p kinase. In contrast, middle sporulation genes were not expressed in admc1 strain, which fails to enter the meiotic divisions because a defect in meiotic recombination leads to aRAD17 -dependent checkpoint arrest. Expression of middle sporulation genes, as well as entry into the meiotic divisions, was restored to admc1 strain by mutation ofRAD17 . Our studies also revealed thatNDT80 was a temporally distinct, pre-middle sporulation gene and that its expression was reduced, but not abolished, on mutation ofDMC1 ,RPD3 ,SIN3 , orNDT80 itself. In summary, our data indicate that Ndt80p is required for expression of middle sporulation genes and that the activity of Ndt80p is controlled by the meiotic recombination checkpoint. Thus, middle genes are expressed only on completion of meiotic recombination and subsequent generation of an active form of Ndt80p.