Figure 1

Experimental design. (a) Meiotic landmarks. The point of commitment is indicated. DSBs, double-strand breaks. (b) The regulatory network underlying the sporulation gene-expression program. Known interactions are shown. Arrows denote activation, and barred lines represent inhibition. Solid lines indicate regulation on the level of transcription while dashed lines indicate post-transcriptional regulation (for example, by protein phosphorylation). Transcription factors are shown in black and the kinase in green. The input of the cascade is shown in gray and scissors indicate degredation. IME2 activates middle gene expression, at least in part, by relieving Sum1-mediated repression of NDT80 [34]. (c) The experimental design. The sporulation process was initiated by transferring cells to sporulation medium. Cells were allowed to progress through the process for varying lengths of times, and were then transferred back to rich nutrient-containing medium. Each circle represents a time point at which genome-wide gene expression was monitored. (d) Temporal progression of sporulation. The percentages of cells that completed the first meiotic division (MI, triangles) or the second meiotic division (MII, circles) are shown in red, the percentage of asci in black and the recombination frequencies (Rec, determined by the frequency of His⁺ cells) in gray. CFU, colony-forming units. (e,f) Commitment to sporulation. (e) Cells were transferred to YPD at different stages of sporulation and were followed in YPD until 8 hours after sporulation initiation. The percentage of cells with four nuclei (determined by DAPI staining) before and after the transfer is shown. The time of transfer from sporulation medium (SPM) is indicated. (f) Cells were transferred from SPM to glucose solution (4%) at various times, as indicated. For each glucose culture, we calculated the fraction of cells that became spores, 24 hours after the initiation of the sporulation process (normalized by the sporulation efficiency at that experiment, which was 80%). Cells that were transferred early (before 5 hours in SPM) arrested in the cell cycle, as glucose alone does not support growth. At later times, cells continued the sporulation process and generated spores. Commitment occurs at around 5-6 hours in SPM.