NDT80 AND THE MEIOTIC RECOMBINATION CHECKPOINT REGULATE EXPRESSION OFMIDDLE SPORULATION-SPECIFIC GENES IN SACCHAROMYCES-CEREVISIAE

Distinct classes of sporulation-specific genes are sequentially expres sed duping the process of spore formation in Saccharomyces cerevisiae. The transition from expression of early meiotic genes to expression o f middle sporulation-specific genes occurs at about the time that cell s exit from pachytene and form the meiosis I spindle. To identify gene s encoding potential regulators of middle sporulation-specific gene ex pression, we screened for mutants that expressed early meiotic genes b ut failed to express middle sporulation-specific genes. We identified mutant alleles of RPD3, SIN3, and NDT80 in this screen. Rpd3p, a histo ne deacetylase, and Sin3p are global modulators of gene expression. Nd t80p promotes entry into the meiotic divisions. We found that entry in to the meiotic divisions was not required for activation of middle spo rulation genes; these genes were efficiently expressed in a clb1 clb3 clb4 strain, which fails to enter the meiotic divisions due to reduced Clb-dependent activation of Cdc28p kinase. In contrast, middle sporul ation genes were not expressed in a dmc1 strain, which fails to enter the meiotic divisions because a defect in meiotic recombination leads to a RAD17-dependent checkpoint arrest. Expression of middle sporulati on genes, as well as entry into the mciotic divisions, was restored to a dmc1 strain by mutation of RAD17. Our studies also revealed that ND T80 was a temporally distinct, pre-middle sporulation gene and that it s expression was reduced, but not abolished, on mutation of DMC1, RPD3 , SIN3, or NDT80 itself. In summary, our data indicate that Ndt80p is required for expression of middle sporulation genes and that the activ ity of Ndt80p is controlled by the meiotic recombination checkpoint. T hus, middle genes are expressed only on completion of meiotic recombin ation and subsequent generation of an active form of Ndt80p.