In the yeast Saccharomyces cerevisiae, sporulation occurs in response to nutritional and genetic signals. The process is initiated when nutrient availability limits mitotic growth, but only in AMTa/MA Ta diploid cells.Under these conditions, the cells express an activator of meiosis (IME1), which is required for the expression of early sporulation-specific genes. We describe a new gene, IME4, whose activity is essential for IME1 transcript accumulation and sporulation. The IME4 transcript was induced in starved A 4Ta/MA Ta diploids but not in other cell types. In addition, excess IME4 promoted sporulation in mat-insufficient cells. Thus, IME4 appears to activate IME1 in response to cell type and nutritional signals. We have also explored the interactions between IME4 and two genes that are known to regulate IMEI expression. Normally, cells that lack complete AMT information cannot sporulate; when such strains lack RME1 activity or contain the semidominant RES]-] mutation, however, they can express IMEI and sporulate to low levels. Our results show that mat-insufficient strains containing rmel::LEU2 or RES]-] bypass mutations still retain AMT control of IME4 expression. Even though IME4 levels remained low, the rmel::LEU2 and RES)-) mutations allowed IME1 accumulation, implying that these mutations do not require IME4 to exert their effects. In accord with this interpretation, the RES1-1 mutation allowed IME1 accumulation in Af4Ta/MATc strains that contain ime4::LEU2 alleles. These strains still sporulated poorly, suggesting that IME4 plays a role in sporulation in addition to promoting IME1 transcript accumulation. IME4 is located between ADES and LYS5 on chromosome VII.Each of the Saccharomyces cerevisiae cell types is capable of a unique set of responses to environmental conditions (reviewed in references 6, 12, 13, 15, and 25.) These responses are governed, ultimately, by the combinations of AMT (mating type) products that are expressed in particular cells. In wild-type diploids, the combination of the AMTa and MAoTa alleles leads to the formation of a specific repressor, the al-a2 heterodimer, that is unique to these cells (8-10, 28, 46). As a result of the interaction of this molecule with its many targets, MATa/M oTa cells do not mate. Instead, diploidMATa/MAT4T cells are able to undergo meiosis and spore formation when nitrogen limitation restricts mitotic growth.Genetic studies have led to the identification of several key molecules that regulate the initiation of sporulation in response to the AMT genotype and the nutritional status of the cells (22,27,31,42,52). Of central importance is an activator of meiosis, the product of the IMEI gene, which is present at high levels only in starved MATa/MATat diploids (11,22,29). IMEI expression leads to the accumulation of a second activator, IME2, and to the activation of additional sporulation-specific genes (29,42,43,52

In the yeast Saccharomyces cerevisiae, sporulation occurs in response to nutritional and genetic signals. The process is initiated when nutrient availability limits mitotic growth, but only in MATa/MAT alpha diploid cells. Under these conditions, the cells express an activator of meiosis (IME1), which is required for the expression of early sporulation-specific genes. We describe a new gene, IME4, whose activity is essential for IME1 transcript accumulation and sporulation. The IME4 transcript was induced in starved MATa/MAT alpha diploids but not in other cell types. In addition, excess IME4 promoted sporulation in mat-insufficient cells. Thus, IME4 appears to activate IME1 in response to cell type and nutritional signals. We have also explored the interactions between IME4 and two genes that are known to regulate IME1 expression. Normally, cells that lack complete MAT information cannot sporulate; when such strains lack RME1 activity or contain the semidominant RES1-1 mutation, however, they can express IME1 and sporulate to low levels. Our results show that mat-insufficient strains containing rme1::LEU2 or RES1-1 bypass mutations still retain MAT control of IME4 expression. Even though IME4 levels remained low, the rme1::LEU2 and RES1-1 mutations allowed IME1 accumulation, implying that these mutations do not require IME4 to exert their effects. In accord with this interpretation, the RES1-1 mutation allowed IME1 accumulation in MATa/MAT alpha strains that contain ime4::LEU2 alleles. These strains still sporulated poorly, suggesting that IME4 plays a role in sporulation in addition to promoting IME1 transcript accumulation. IME4 is located between ADE5 and LYS5 on chromosome VII.

The RES1-1 mutation was isolated on the basis of its ability to allow MATa/MAT alpha diploid Saccharomyces cerevisiae cells to express a late sporulation-regulated gene, SPR3, in the presence of excess copies of RME1. RME1 is a repressor of meiosis that is normally expressed in cells that lack the a1/alpha 2 repressor encoded by MAT. The RES1-1 mutation also supports sporulation in mat-insufficient diploids. This phenotype does not result from a failure to express RME1 and is not due to activation of the silent copies of mating type information. RES1-1 activates sporulation by allowing IME1 accumulation in all cell types, irrespective of the presence of the MAT products. IME1 is still responsive to RME1 in RES1-1 cells, since double mutants (rme1 RES1-1) that are deficient at MAT can sporulate better than either single mutant. RES1-1 is not an allele of IME1.