Fig. 4. Mek1 regulates entry into meiosis I. (A) The mek1 mutant enters meiosis I faster than wildtype. Kinetics of meiotic progression of wildtype (S964; upper panel) and mek1 (S1293; lower panel) was followed by DAPI staining of nuclei. Note that the peak of meiosis I (MI) occurs 30 minutes earlier in the mek1 mutant. A representative time course is presented, but the experiment was repeated at least three times, the same results being obtained. (B) FACS analysis of the meiotic time courses shown in A. Note that the timing of premeiotic S phase (meiS) is the same in both the wildtype (upper panel) and mek1 mutant (lower panel). (C,D,E) High levels of Mek1 delay entry into meiosis I. (C) Schematic representation of the experimental procedure used for overexpression of mek1⁺ from the thiamine-repressible nmt1 promoter during meiosis in strain S1298. T, thiamine; N, nitrogen. (D) FACS analysis of meiotic time courses of S1298 in the presence (upper panel) or in the absence (lower panel) of thiamine. (E) Kinetics of meiotic progression monitored by DAPI nuclear staining during meiotic time courses of strain S1298 in the presence of thiamine (nmt-mek1⁺ OFF) or in the absence of thiamine (nmt-mek1⁺ ON). Note that the peak of meiosis I (MI) takes place 1 hour later when Mek1 is overproduced.