Two signals activate meiosis in yeast: starvation and expression of the al and a2 products of the mating-type locus. Prior studies suggest that these signals stimulate expression of an activator of meiosis, the IME1 (inducer of meiosis) product. We have cloned a gene, IME2, with properties similar to those of IME1: both genes are required for meiosis, and both RNAs are induced in mneiotic cells. Elevated dosage of IME1 or IME2 stimulates the meiotic recombination pathway without starvation; thus, the IME products may be part of the switch that activates meiosis. IME1 was found to be required for IME2 expression, and a multicopy IME2 plasmid permitted meiosis in an imel deletion mutant. Accordingly, we propose that the IME1 product stimulates meiosis mainly through activation of IME2 expression.

Diploid a/ac Saccharomyces cerevisiae cells cease mitotic growth and enter meiosis in response to starvation. Expression of meiotic genes depends on the IMEI gene product, which accumulates only in meiotic cells. We report here an analysis of the regulatory region of IME2, an IMEl-dependent meiotic gene. Deletion and substitution studies identified a 48-bp IMEl-dependent upstream activation sequence (UAS). Activity of the UAS also requires the RIMJJ, RIM15, and RIM16 gene products, which are required for expression of the chromosomal IME2 promoter and for meiosis.

The yeast meiotic activator IME1 stimulates transcription of many early meiotic genes. These genes share a 5 sequence called URS1. URS1 sites function as repression sites in cells that lack IME1; we show here that URS1 sites are weak activation sequences in cells that express IME1. Repression through URS1 sites is known to depend upon the URS1-binding protein UME6. We have identified a UME6 allele (previously called rim16-12) that causes a defect in IME1-dependent activation of meiotic genes but not in repression through URS1 sites. In contrast, a ume6 null mutation causes defects in both IME1-dependent activation and in repression through URS1 sites. A LexA-UME6 fusion protein is an IME1-dependent transcriptional activator, whereas a LexA-UME6 fusion carrying the rim16-12 substitution cannot activate transcription. These findings argue that IME1 activates meiotic genes by converting UME6 from a negative regulator to a positive regulator; the rim16-12 mutant protein is defective in conversion to a positive regulator.The yeast UME6 protein and URS1 site are components of a sequence-specific repression system. URS1 sites are widely distributed in genetic regulatory regions (29), where they function as repression sites in growing, nonmeiotic cells (4,10,12,17,31). Repression of URS1-containing genes requires the UME6 gene product (16,27). UME6 is a zinc cluster protein that binds specifically to a URS1 site in vitro (27). A second protein, the RPA1/2/3 heterotrimer (RP-A), also binds to URS1 sites (11). Binding of RP-A is independent of UME6 (16), and binding of recombinant UME6 is independent of RP-A (27). The functional relationship among RP-A, URS1, and other URS1-binding proteins is presently unclear. However, genetic analysis indicates that UME6 is a repressor or part of a repression complex that acts through URS1 sites in nonmeiotic cells (16,27).Three observations made in nonmeiotic cells indicate that the URS1/UME6 system participates in repression of early meiotic genes. First, URS1-like sites are found upstream of almost all early meiotic genes (4; see reference 14 for a compilation). Second, mutations that remove or disrupt these URS1 sites cause increased expression in nonmeiotic cells (1,4,31). Third, ume6 loss-of-function mutations cause increased early meiotic gene expression in nonmeiotic cells (1,26,27). Thus, repression by the URS1/UME6 system prevents inappropriate meiotic gene expression in nonmeiotic cells.Entry into meiosis is accompanied by elevated expression of IME1 (7); the IME1 gene product is required for expression of almost all meiotic genes (see reference 14 for a review). Two observations indicate that the URS1/UME6 system may be required for IME1 to activate early meiotic genes. First, mutations in the URS1 sites near the meiotic genes SPO13, HOP1, and IME2 cause reduced expression during meiosis (1,4,31). At HOP1 and IME2, mutations in nearby positive sites (called the UAS H [UAS, upstream activating sequence] and T 4 C sites) also cause reduced expression during meiosis (1, 31). S...

The Saccharomyces cerevisiae RIM15 gene was identified previously through a mutation that caused reduced ability to undergo meiosis. We report here an analysis of the cloned RIM15 gene, which specifies a 1,770-residue polypeptide with homology to serine/threonine protein kinases. Rim15p is most closely related to Schizosaccharomyces pombe cek1 ؉ . Analysis of epitope-tagged derivatives indicates that Rim15p has autophosphorylation activity. Deletion of RIM15 causes reduced expression of several early meiotic genes (IME2, SPO13, and HOP1) and of IME1, which specifies an activator of early meiotic genes. However, overexpression of IME1 does not permit full expression of early meiotic genes in a rim15⌬ mutant. Ime1p activates early meiotic genes through its interaction with Ume6p, and analysis of Rim15p-dependent regulatory sites at the IME2 promoter indicates that activation through Ume6p is defective. Two-hybrid interaction assays suggest that Ime1p-Ume6p interaction is diminished in a rim15 mutant. Glucose inhibits Ime1p-Ume6p interaction, and we find that Rim15p accumulation is repressed in glucose-grown cells. Thus, glucose repression of Rim15p may be responsible for glucose inhibition of Ime1p-Ume6p interaction.The budding yeast Saccharomyces cerevisiae undergoes meiosis and spore formation in response to both genetic and nutritional signals (reviewed in references 7 and 14). The genetic signal that permits meiosis is the presence of the repressor a1-␣2, which determines the a/␣ cell type (reviewed in reference 13). Thus, a/␣ cells are able to sporulate; a and ␣ cells are not. The nutritional signals that permit meiosis are nitrogen limitation and the absence of glucose (reviewed in reference 26). These signals effectively restrict meiosis to nongrowing cells.The pathway that transmits genetic control over meiosis is well understood. a1-␣2 represses RME1, which specifies an inhibitor of meiosis (29), and stimulates IME4, which specifies a positive regulator of meiosis (40). Effects of Rme1p and Ime4p are exerted on expression of IME1, which specifies a positive regulator of meiosis. Overexpression of IME1 permits cells lacking a1-␣2 to enter meiosis (18; reviewed in references 14 and 27). Thus, genetic control over meiosis is ultimately exerted through control of IME1 expression.The pathways that sense and transmit nutritional signals act through several mechanisms (see reference 27). For example, IME1 transcript levels are repressed by glucose and by nitrogen (18). However, overexpression of IME1 does not permit cells to enter meiosis in the presence of glucose or nitrogen (19,21,41,47). Thus, nutritional control over meiosis is exerted through control of IME1 expression and through additional mechanisms.Analysis of Ime1p activity and function has revealed some of these additional mechanisms. Ime1p is required to activate expression of many meiotic or sporulation-specific genes (reviewed in reference 27). Ime1p acts through the URS1 site (3), which lies in the 5Ј regulatory region of almost all early meiotic...

SK-1: 7hr 9hr 10hr BR2495: 13 hr 17 hr 20 hr FIG. 1. Time course of meiotic events. The main phases of the sporulation program are indicated in relation to the times at which early, middle, and late genes are expressed. The time after starvation at which each phase occurs is indicated for SK-1-derived strains (71) and BR2495-derived strains (65). The relative times of early, middle, and late meiotic gene expression are from references 114, 57, and 65. SC, synaptonemal complex.