In the yeast Saccharomyces cerevisiae, meiosis and spore formation require the induction of sporulationspecific genes. Two genes are thought to activate the sporulation program: IMEI and IME2 (inducer of meiosis). Both genes are induced upon entry into meiosis, and IMEI is required for IME2 expression. We report here that IMEI is essential for expression of four sporulation-specific genes. In contrast, IME2 is not absolutely essential for expression of the sporulation-specific genes, but contributes to their rapid induction. Expression of IME2 from a heterologous promoter permits the expression of these sporulation-specific genes, meiotic recombination, and spore formation in the absence of IMEI. We propose that the IMEI and IME2 products can each activate sporulation-specific genes independently. In addition, the IMEI product stimulates sporulation-specific gene expression indirectly through activation of IME2 expression.Sporulation in the yeast Saccharomyces cerevisiae is a program of cellular differentiation that includes genetic recombination, meiotic divisions, and spore formation (7). Sporulation is induced by starvation and is restricted to one type of cell, the a/a cell. The other two types of cells, a and a cells, express an inhibitor of meiosis that blocks sporulation (14). Several genes that are essential for sporulation are expressed only in sporulating cells: they are induced by starvation in a/a cells, not in a or a cells (1,4,13,15,16,20,23). Control of sporulation is thus achieved, at least in part, through control of sporulation-specific gene expression.Two genes are thought to play a central role in the decision to enter meiosis and to sporulate: IMEI and IME2 (inducer of meiosis [11,18]). Null mutations in either gene block sporulation, and an increased dosage of either gene can partially relieve both genetic and environmental controls over sporulation. (A third gene with these properties, IME3, has recently been characterized [L. Neigeborn and A. P. Mitchell, manuscript in preparation].) IME1 and IME2 transcript levels increase more than 30-fold upon entry into meiosis, and mutations that alter genetic or nutritional regulation of meiosis alter IME1 and IME2 expression in parallel. The IME1 product is required for IME2 expression, and the presence of IME2 on a multicopy plasmid can partially relieve the requirement for IME1 in sporulation (18). We thus proposed that genetic and environmental signals govern IME1 expression and that the IME1 product activates meiosis through activation of IME2 expression.Because the IME2 plasmid suppressed an imel deletion weakly, we suggested that the IMEI product may have a second role in meiosis, in addition to activation of IME2.In the present study, we examined sporulation-specific gene expression in strains with deletions of IMEI or IME2 and in a strain that expresses IME2 in the absence of IMEI.Our results indicate that each IME product plays a unique role in activation of sporulation-specific genes and that the IMEI product also stimulates sporulation-s...
Two signals are required for meiosis and spore formation in the yeast Saccharomyces cerevisiae: starvation and the MAT products al and a2, which determine the a/a cell type. These signals lead to increased expression of the IMEI (inducer of meiosis) gene, which is required for sporulation and sporulation-specific gene expression. We report here the sequence of the IME1 gene and the consequences of IMEI expression from the GAL] promoter. The deduced IMEI product is a 360-amino-acid protein with a tyrosine-rich C-terminal region. Expression Sporulation of the yeast Saccharomyces cerevisiae is a cellular differentiation pathway (reviewed in references 12 and 26). It is normally restricted to one type of cell, the a/a cell, and is induced by nitrogen starvation. These circumstances lead to arrest of the mitotic cell cycle, to expression of sporulation-specific genes, and to initiation of the sporulation program. Cells engage in meiotic DNA synthesis, recombination, and two meiotic divisions. Each of the four meiotic products is packaged into a spore, and the four spores of the cell are encased in a sac, the ascus. Sporulation thus includes meiosis and spore formation.One of the earliest unique events in starved a/a cells is elevated accumulation of IME1 RNA (22,28,38; reviewed in reference 27). The IME1 product is thought to play a pivotal role in activating meiosis, because multicopy IME1 plasmids permit sporulation in cells that lack the determinants of a/a cell type, the MATal and MATa2 gene products, and also permit meiotic recombination in the absence of nitrogen starvation (22,38 Sporulation is accompanied by express'ion of a unique set of genes, the sporulation-specific genes. Some of these genes are essential for particular meiotic events, and others have no essential role in sporulation under laboratory conditions (1,6,11,14,17,21,25,26,33,34,42,45,47). These genes fall into early, middle, and late expression classes (1,23,25,26 The imel-12, ime2-2, gal80, ho::LYS2, his4-G, and his4-N mutations have previously been described, as have the auxotrophic markers in these strains (28,38). We note that the ho::LYS2 insertion confers a weak Lys' phenotype.The a/a diploid (strain 545) was derived from an a/a diploid (strain 537) after mild UV irradiation by screening colonies for mating-factor production through a halo assay (46). Engebrecht and Roeder observed that a/a and a/a diploids in the SK1 background were able to sporulate at a low level (strains J254 and J256 [11]). In side-by-side comparisons, we confirmed that J254 and J256 were able to sporulate and that our strain 545 was unable to sporulate. Fourteen four-spored tetrads were analyzed from an a/a/a/a tetraploid derived from crossing strains 545 and J256. Nine segregants were able to mate and able to sporulate weakly; 25 segregants were able to mate but unable to sporulate. These observations indicate that the difference in sporulation abilities 6103 on May 10, 2018 by guest
IME1 is required in yeast for meiosis and for expression of IME2 and other early meiotic genes. IME1 is a 360-amino acid polypeptide with central and C-terminal tyrosine-rich regions. We report here that a fusion protein composed of the lexA DNA-binding domain and IME1 activates transcription in vivo of a reporter gene containing upstream lexA binding sites. Activation by the fusion protein shares several features with natural IME1 activity: both are dependent on the RIM11 gene product; both are impaired by the same ime1 missense mutations; both are restored by intragenic suppressors. The central tyrosine-rich region is sufficient to activate transcription when fused to lexA. Deletion of this putative activation domain results in a defective IME1 derivative. Function of the deletion derivative is restored by fusion to the acidic Herpesvirus VP16 activation domain. The C-terminal tyrosine-rich region is dispensable for transcriptional activation; rather it renders activation dependent upon starvation and RIM11. Immunofluorescence studies indicate that an IME1-lacZ fusion protein is concentrated in the nucleus. These observations are consistent with a model in which IME1 normally stimulates IME2 expression by providing a transcriptional activation domain at the IME2 5' regulatory region.
Two signals are required for meiosis and spore formation in the yeast Saccharomyces cerevisiae: starvation and the MAT products a1 and alpha 2, which determine the a/alpha cell type. These signals lead to increased expression of the IME1 (inducer of meiosis) gene, which is required for sporulation and sporulation-specific gene expression. We report here the sequence of the IME1 gene and the consequences of IME1 expression from the GAL1 promoter. The deduced IME1 product is a 360-amino-acid protein with a tyrosine-rich C-terminal region. Expression of PGAL1-IME1 in vegetative a/alpha cells led to moderate accumulation of four early sporulation-specific transcripts (IME2, SPO11, SPO13, and HOP1); the transcripts accumulated 3- to 10-fold more after starvation. Two sporulation-specific transcripts normally expressed later (SPS1 and SPS2) did not accumulate until PGAL1-IME1 strains were starved, and the intact IME1 gene was not activated by PGAL1-IME1 expression. In a or alpha cells, which lack alpha 2 or a1, expression of PGAL1-IME1 led to the same pattern of IME2 and SPO13 expression as in a/alpha cells, as measured with ime2::lacZ and spo13::lacZ fusions. Thus, in wild-type strains, the increased expression of IME1 in starved a/alpha cells can account entirely for cell type control, but only partially for nutritional control, of early sporulation-specific gene expression. PGAL1-IME1 expression did not cause growing cells to sporulate but permitted efficient sporulation of amino acid-limited cells, which otherwise sporulated poorly. We suggest that IME1 acts primarily as a positive regulator of early sporulation-specific genes and that growth arrest is an independent prerequisite for execution of the sporulation program.
In the yeast Saccharomyces cerevisiae, meiosis and spore formation require the induction of sporulation-specific genes. Two genes are thought to activate the sporulation program: IME1 and IME2 (inducer of meiosis). Both genes are induced upon entry into meiosis, and IME1 is required for IME2 expression. We report here that IME1 is essential for expression of four sporulation-specific genes. In contrast, IME2 is not absolutely essential for expression of the sporulation-specific genes, but contributes to their rapid induction. Expression of IME2 from a heterologous promoter permits the expression of these sporulation-specific genes, meiotic recombination, and spore formation in the absence of IME1. We propose that the IME1 and IME2 products can each activate sporulation-specific genes independently. In addition, the IME1 product stimulates sporulation-specific gene expression indirectly through activation of IME2 expression.
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