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Meiosis is the developmental program by which diploid organisms produce haploid gametes capable of sexual reproduction. Here we describe the yeast gene AMA1, a new member of the Cdc20 protein family that regulates the multisubunit ubiquitin ligase termed the anaphase promoting complex͞cyclosome (APC͞C). AMA1 is developmentally regulated in that its transcription and splicing occur only in meiotic cells. The meiosis-specific processing of AMA1 mRNA depends on the previously described MER1 splicing factor. Several results indicate that Ama1p is required for APC͞C function during meiosis. First, coimmunoprecipitation assays indicate that Ama1p associates with the APC͞C in vivo. Second, Ama1p is required for the degradation of the B-type cyclin Clb1p, an APC͞C substrate in both meiotic and mitotic cells. Third, ectopic overexpression of AMA1 is able to stimulate ubiquitination of Clb1p in vitro and degradation of Clb1p in vivo. Mutants lacking AMA1 revealed that it is required for the first meiotic division but not the mitotic-like meiosis II. In addition, ama1 mutants are defective for both spore wall assembly and the expression of late meiotic genes. In conclusion, this study indicates that Ama1p directs a meiotic APC͞C that functions solely outside mitotic cell division. The requirement of Ama1p only for meiosis I and spore morphogenesis suggests a function for APC͞C Ama1 specifically adapted to germ cell development. G ametogenesis requires the execution of several interrelated events including genetic exchange, haploidization, and cellular differentiation. Haploidization is achieved through two consecutive nuclear divisions, meiosis I (reductional) and meiosis II (equational). During the reductional division, replicated sister chromatids stay attached and segregate as a single unit to the same pole. The second meiotic division resembles mitosis in that the centromeres of replicated sisters bind to spindles emanating from opposite poles and separate at anaphase II. Finally, during gametogenesis, differentiation programs instruct the formation of specialized cells that are capable of sexual reproduction. In yeast, the haploid products are encapsulated in spores, which have the capacity to mate after they germinate and reenter the mitotic cell cycle (1).Several studies have indicated that the basic mitotic cell cycle machinery is required for many aspects of meiosis (reviewed in ref.2). For example, the budding yeast mitotic cell cycle is driven by the cyclin-dependent protein kinase Cdc28p (3). Cdc28p is activated by a conserved family of proteins termed cyclins (4) with the four B-type cyclins (Clb1-4p) regulating the G 2 ͞M transition. Similarly, the normal execution of meiosis I and II also requires the Cdc28p-Clb1p and Cdc28p-Clb4p kinases (5-7). However, the production of haploid products during meiosis requires two events that are strictly prohibited by mitotic checkpoint pathways (8). First, replicated sister chromatids stay paired during meiosis I rather than segregate to the opposite poles as they do in ...
Meiosis is the developmental program by which diploid organisms produce haploid gametes capable of sexual reproduction. Here we describe the yeast gene AMA1, a new member of the Cdc20 protein family that regulates the multisubunit ubiquitin ligase termed the anaphase promoting complex͞cyclosome (APC͞C). AMA1 is developmentally regulated in that its transcription and splicing occur only in meiotic cells. The meiosis-specific processing of AMA1 mRNA depends on the previously described MER1 splicing factor. Several results indicate that Ama1p is required for APC͞C function during meiosis. First, coimmunoprecipitation assays indicate that Ama1p associates with the APC͞C in vivo. Second, Ama1p is required for the degradation of the B-type cyclin Clb1p, an APC͞C substrate in both meiotic and mitotic cells. Third, ectopic overexpression of AMA1 is able to stimulate ubiquitination of Clb1p in vitro and degradation of Clb1p in vivo. Mutants lacking AMA1 revealed that it is required for the first meiotic division but not the mitotic-like meiosis II. In addition, ama1 mutants are defective for both spore wall assembly and the expression of late meiotic genes. In conclusion, this study indicates that Ama1p directs a meiotic APC͞C that functions solely outside mitotic cell division. The requirement of Ama1p only for meiosis I and spore morphogenesis suggests a function for APC͞C Ama1 specifically adapted to germ cell development. G ametogenesis requires the execution of several interrelated events including genetic exchange, haploidization, and cellular differentiation. Haploidization is achieved through two consecutive nuclear divisions, meiosis I (reductional) and meiosis II (equational). During the reductional division, replicated sister chromatids stay attached and segregate as a single unit to the same pole. The second meiotic division resembles mitosis in that the centromeres of replicated sisters bind to spindles emanating from opposite poles and separate at anaphase II. Finally, during gametogenesis, differentiation programs instruct the formation of specialized cells that are capable of sexual reproduction. In yeast, the haploid products are encapsulated in spores, which have the capacity to mate after they germinate and reenter the mitotic cell cycle (1).Several studies have indicated that the basic mitotic cell cycle machinery is required for many aspects of meiosis (reviewed in ref.2). For example, the budding yeast mitotic cell cycle is driven by the cyclin-dependent protein kinase Cdc28p (3). Cdc28p is activated by a conserved family of proteins termed cyclins (4) with the four B-type cyclins (Clb1-4p) regulating the G 2 ͞M transition. Similarly, the normal execution of meiosis I and II also requires the Cdc28p-Clb1p and Cdc28p-Clb4p kinases (5-7). However, the production of haploid products during meiosis requires two events that are strictly prohibited by mitotic checkpoint pathways (8). First, replicated sister chromatids stay paired during meiosis I rather than segregate to the opposite poles as they do in ...
A transcriptional analysis of the response of Saccharomyces cerevisiae strain BY4743 to controlled air-drying (desiccation) and subsequent rehydration under minimal glucose conditions was performed. Expression of genes involved in fatty acid oxidation and the glyoxylate cycle was observed to increase during drying and remained in this state during the rehydration phase. When the BY4743 expression profile for the dried sample was compared to that of a commercially prepared dry active yeast, strikingly similar expression changes were observed. The fact that these two samples, dried by different means, possessed very similar transcriptional profiles supports the hypothesis that the response to desiccation is a coordinated event independent of the particular conditions involved in water removal. Similarities between "stationary-phase-essential genes" and those upregulated during desiccation were also noted, suggesting commonalities in different routes to reduced metabolic states. Trends in extracellular and intracellular glucose and trehalose levels suggested that the cells were in a "holding pattern" during the rehydration phase, a concept that was reinforced by cell cycle analyses. Application of a "redescription mining" algorithm suggested that sulfur metabolism is important for cell survival during desiccation and rehydration.Water is essential for life, and thus the removal of even a small portion of the water from a cell is a severe, often lethal stress due to changes in cell volume and shape, condensation and crowding of the cytoplasm, membrane phase transitions, loss of supercoiling of DNA, oxidative damage, and metabolic arrest (5, 59). Organisms that can withstand the removal of virtually all of their cellular water possess a unique physiological adaptation that may have arisen very early in evolution, perhaps leading to a widespread distribution of primitive cells (50). The cellular responses to desiccation and subsequent rehydration are distinct from one another and are equally complex stresses (57). Survival of dehydration damage in some, but not all, organisms is correlated with the intracellular accumulation of compatible solutes, as well as vitrification of the cytoplasm. The mechanisms of action are described by the "water replacement hypothesis" and the glass transition theory, respectively, for which there is considerable experimental support (13-15).Given the historical and economic significance of dried yeast and the current needs of the medical and biodefense communities for stabilized biologicals, it is surprising that there has been no genome-wide analyses of the response of yeast to water deficit. In an effort to fill this void, we analyzed the transcriptional response of yeast to desiccation and rehydration under glucose-limiting conditions. In addition, we compared the transcriptional profile of a dried lab strain (BY4743) with that of a dry active yeast purchased locally. Metabolite profiling was also performed to assess the roles that glucose and trehalose levels may have in the overall ...
During sporulation in Saccharomyces cerevisiae, the four haploid nuclei are encapsulated within multilayered spore walls. Glucan, the major constituent of the spore wall, is synthesized by 1,3--glucan synthase, which is composed of a putative catalytic subunit encoded by FKS1 and FKS2. Although another homolog, encoded by FKS3, was identified by homology searching, its function is unknown. In this report, we show that FKS2 and FKS3 are required for spore wall assembly. The ascospores of fks2 and fks3 mutants were enveloped by an abnormal spore wall with reduced resistance to diethyl ether, elevated temperatures, and ethanol. However, deletion of the FKS1 gene did not result in a defective spore wall. The construction of fusion genes that expressed Fks1p and Fks2p under the control of the FKS2 promoter revealed that asci transformed with FKS2p-driven Fks1p and Fks2p were resistant to elevated temperatures, which suggests that the expression of FKS2 plays an important role in spore wall assembly. The expression of FKS1p-driven Fks3p during vegetative growth did not affect 1,3--glucan synthase activity in vitro but effectively suppressed the growth defect of the temperature-sensitive fks1 mutant by stabilizing Rho1p, which is a regulatory subunit of glucan synthase. Based on these results, we propose that FKS2 encodes the primary 1,3--glucan synthase in sporulation and that FKS3 is required for normal spore wall formation because it affects the upstream regulation of 1,3--glucan synthase.Sporulation in the budding yeast Saccharomyces cerevisiae provides a model system for studying the developmental processes of many eukaryotic cells. Sporulation in the a/␣ diploid cells is triggered by carbon starvation and is followed by meiosis and the formation of asci that contain four haploid spores encapsulated within a spore wall (25, 33). The spore wall, which consists of four distinct layers, has been shown to play a central role in protecting the cell from environmental damage. The inner two layers consist of -glucan and mannan (4), components that are similar to those found in the vegetative cell wall. In contrast, the outer layer consists of chitosan, a polymer of -1,4-linked glucosamine, and the outermost layer consists of dityrosine, both of which are specific to the spore wall (3,4,5,6,43). Previous investigations of spore wall formation have focused on the specific components of the spore wall, such as chitosan and dityrosine. The mechanism of assembly of spore walls, including the synthesis of the inner two layers, remains unclear. We speculate that -glucan is also largely responsible for spore resistance to environmental damage, since glucan is the major constituent of the spore wall (4) and provides rigidity to the cell wall during vegetative growth (36).In yeast, glucan is constituted predominantly by 1,3--glucan, which is synthesized by 1,3--glucan synthase (GS), which in turn consists of a catalytic and a regulatory subunit. Two genes for the putative catalytic subunit of GS in budding yeast have been...
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