Isolation and functional analysis of sporulation-induced transcribed sequences from Saccharomyces cerevisiae

The modest beginnings of one genome project

One of the top things on a geneticist's wish list has to be a set of mutants for every gene in their particular organism. Such a set was produced for the yeast, Saccharomyces cerevisiae near the end of the 20th century by a consortium of yeast geneticists. However, the functional genomic analysis of one chromosome, its smallest, had already begun more than 25 years earlier as a project that was designed to define most or all of that chromosome's essential genes by temperature-sensitive lethal mutations. When far fewer than expected genes were uncovered, the relatively new field of molecular cloning enabled us and indeed, the entire community of yeast researchers to approach this problem more definitively. These studies ultimately led to cloning, genomic sequencing, and the production and phenotypic analysis of the entire set of knockout mutations for this model organism as well as a better concept of what defines an essential function, a wish fulfilled that enables this model eukaryote to continue at the forefront of research in modern biology.

SSP1, a gene necessary for proper completion of meiotic divisions and spore formation in Saccharomyces cerevisiae

During meiosis, a diploid cell undergoes two rounds of nuclear division following one round of DNA replication to produce four haploid gametes. In yeast, haploid meiotic products are packaged into spores. To gain new insights into meiotic development and spore formation, we followed differential expression of genes in meiotic versus vegetatively growing cells in the yeast Saccharomyces cerevisiae. Our results indicate that there are at least five different classes of transcripts representing genes expressed at different stages of the sporulation program. Here we describe one of these differentially expressed genes, SSP1, which plays an essential role in meiosis and spore formation. SSP1 is expressed midway through meiosis, and homozygous ssp1 diploid cells fail to sporulate. In the ssp1 mutant, meiotic recombination is normal but viability declines rapidly. Both meiotic divisions occur at the normal time; however, the fraction of cells completing meiosis is significantly reduced, and nuclei become fragmented soon after meiosis II. The ssp1 defect does not appear to be related to a microtubule-cytoskeletal-dependent event and is independent of two rounds of chromosome segregation. The data suggest that Ssp1 is likely to function in a pathway that controls meiotic nuclear divisions and coordinates meiosis and spore formation.

Identification of a developmentally regulated septin and involvement of the septins in spore formation in Saccharomyces cerevisiae

The Saccharomyces cerevisiae CDC3, CDC10, CDC11, and CDC12 genes encode a family of related proteins, the septins, which are involved in cell division and the organization of the cell surface during vegetative growth. A search for additional S. cerevisiae septin genes using the polymerase chain reaction identified SPR3, a gene that had been identified previously on the basis of its sporulation-specific expression. The predicted SPR3 product shows 25-40% identity in amino acid sequence to the previously known septins from S. cerevisiae and other organisms. Immunoblots confirmed the sporulation-specific expression of Spr3p and showed that other septins are also present at substantial levels in sporulating cells. Consistent with the expression data, deletion of SPR3 in either of two genetic backgrounds had no detectable effect on exponentially growing cells. In one genetic background, deletion of SPR3 produced a threefold reduction in sporulation efficiency, although meiosis appeared to be completed normally. In this background, deletion of CDC10 had no detectable effect on sporulation. In the other genetic background tested, the consequences of the two deletions were reversed. Immunofluorescence observations suggest that Spr3p, Cdc3p, and Cdc11p are localized to the leading edges of the membrane sacs that form near the spindle-pole bodies and gradually extend to engulf the nuclear lobes that contain the haploid chromosome sets, thus forming the spores. Deletion of SPR3 does not prevent the localization of Cdc3p and Cdc11p, but these proteins appear to be less well organized, and the intensity of their staining is reduced. Taken together, the results suggest that the septins play important but partially redundant roles during the process of spore formation.

Molecular cloning of chromosome I DNA from Saccharomyces cerevisiae: analysis of the genes in the FUN38-MAK16-SPO7 region

Transcribed regions on a 42-kb segment of chromosome I from Saccharomyces cerevisiae were mapped. Polyadenylated transcripts corresponding to eight previously characterized genes (MAK16, LTE1, CCR4, FUN30, FUN31, TPD3, DEP1, and CYS3) and eight new genes were identified. All transcripts were present at one to four copies per cell except for one which was significantly less abundant. This region has been sequenced, and the sizes, locations, and orientations of the transcripts were in nearly perfect agreement with the open reading frames. Disruptions in eight genes identified solely on the basis of a transcribed region, FUN38, FUN25, FUN26, FUN28, FUN30, FUN31, FUN33, and FUN34, indicated that all were nonessential for growth on rich medium at 30 degrees C. Disruption of FUN30, a gene closely related to RAD16 and RAD54, surprisingly resulted in increased resistance to UV irradiation. No additional phenotypes, other than slow growth, were observed for all other mutants. The distribution of essential genes on chromosome I is discussed.

Control of meiotic gene expression in Saccharomyces cerevisiae

Sporulation of the yeast Saccharomyces cerevisiae is restricted to one type of cell, the a/alpha cell, and is initiated after starvation for nitrogen in the absence of a fermentable carbon source. More than 25 characterized genes are expressed only during sporulation and are referred to as meiotic genes or sporulation-specific genes. These genes are in the early, middle, and late expression classes. Most early genes have a 5' regulatory site, URS1, and one of two additional sequences, UASH or a T4C site. URS1 is required both to repress meiotic genes during vegetative growth and to activate these genes during meiosis. UASH and the T4C site also contribute to meiotic expression. A different type of site, the NRE, is found in at least two late genes. The NRE behaves as a repression site in vegetative cells and is neutral in meiotic cells. Many regulatory genes that either repress or activate meiotic genes have been identified. One group of regulators affects the expression of IME1, which specifies a positive regulator of meiotic genes and is expressed at the highest levels in meiotic cells. A second group of regulators acts in parallel with or downstream of IME1 to influence meiotic gene expression. This group includes UME6, which is required both for repression through the URS1 site in vegetative cells and for IME1-dependent activation of an upstream region containing URS1 and T4C sites. IME1 may activate meiotic genes by modifying a UME6-dependent repression complex at a URS1 site. Several additional mechanisms restrict functional expression of some genes to meiotic cells. Translation of IME1 has been proposed to occur only in meiotic cells; several meiotic transcripts are more stable in acetate medium than in glucose medium; and splicing of MER2 RNA depends on a meiosis-specific gene, MER1.

SSG1, a gene encoding a sporulation-specific 1,3-beta-glucanase in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the meiotic process is accompanied by a large increase in 1,3-beta-glucan-degradative activity. The molecular cloning of the gene (SSG1) encoding a sporulation-specific exo-1,3-beta-glucanase was achieved by screening a genomic library with a DNA probe obtained by polymerase chain reaction amplification using synthetic oligonucleotides designed according to the nucleotide sequence predicted from the amino-terminal region of the purified protein. DNA sequencing indicates that the SSG1 gene specifies a 445-amino-acid polypeptide (calculated molecular mass, 51.8 kDa) showing extensive similarity to the extracellular exo-1,3-beta-glucanases encoded by the EXG1 gene (C. R. Vazquez de Aldana, J. Correa, P. San Segundo, A. Bueno, A. R. Nebreda, E. Mendez, and F. del Rey, Gene 97:173-182, 1991). The N-terminal domain of the putative precursor is a very hydrophobic segment with structural features resembling those of signal peptides of secreted proteins. Northern (RNA) analysis reveals a unique SSG1-specific transcript, 1.7 kb long, which can be detected only in sporulating diploids (MATa/MAT alpha) but does not appear in vegetatively growing cells or in nonsporulating diploids (MAT alpha/MAT alpha) when incubated under nitrogen starvation conditions. The meiotic time course of SSG1 induction indicates that the gene is transcribed only in the late stages of the process, beginning at the time of meiosis I and reaching a maximum during spore formation. Homozygous ssg1/ssg1 mutant diploids are able to complete sporulation, although with a significant delay in the appearance of mature asci.

The Saccharomyces cerevisiae SPR1 gene encodes a sporulation-specific exo-1,3-beta-glucanase which contributes to ascospore thermoresistance

A number of genes have been shown to be transcribed specifically during sporulation in Saccharomyces cerevisiae, yet their developmental function is unknown. The SPR1 gene is transcribed during only the late stages of sporulation. We have sequenced the SPR1 gene and found that it has extensive DNA and protein sequence homology to the S. cerevisiae EXG1 gene which encodes an exo-1,3-beta-glucanase expressed during vegetative growth (C. R. Vasquez de Aldana, J. Correa, P. San Segundo, A. Bueno, A. R. Nebrada, E. Mendez, and F. del Ray, Gene 97:173-182, 1991). We show that spr1 mutant cells do not hydrolyze p-nitrophenyl-beta-D-glucoside or laminarin in a whole-cell assay for exo-1,3-beta-glucanases. In addition to the absence of this enzymatic activity, spr1 mutant spores exhibit reduced thermoresistance relative to isogenic wild-type spores. These observations are consistent with the notion that SPR1 encodes a sporulation-specific exo-1,3-beta-glucanase.

Role of IME1 expression in regulation of meiosis in Saccharomyces cerevisiae

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.

An RME1-independent pathway for sporulation control in Saccharomyces cerevisiae acts through IME1 transcript accumulation

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.

Role of IME1 expression in regulation of meiosis in Saccharomyces cerevisiae

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.

Functional analysis of the sporulation-specific SPR6 gene of Saccharomyces cerevisiae

The SPR6 gene of Saccharomyces cerevisiae encodes a moderately abundant RNA that is present at high levels only during sporulation. The gene contains a long open reading frame that could encode a hydrophilic protein approximately 21 kDa in size. This protein is probably produced by the yeast, because the lacZ gene of Escherichia coli is expressed during sporulation when fused to SPR6 in the expected reading frame. SPR6 is inessential for sporulation; mutants that lack SPR6 activity sporulate normally and produce viable ascospores. Nonetheless, the SPR6 gene encodes a function that is relevant to sporulating cells; the wild-type allele can enhance sporulation in strains that are defective for several SPR functions. SPR6 is located on chromosome V, 14.4 centimorgans centromere-distal to MET6.

Complementary transcripts from two genes necessary for normal meiosis in the yeast Saccharomyces cerevisiae

The SPO12 gene, which is required for meiosis I chromosome division during sporulation of the yeast Saccharomyces cerevisiae, has been isolated. DNA sequencing has identified an open reading frame of 173 codons that encodes the putative SPO12 protein and has no significant sequence similarities to known genes. The last 15 amino acids of this putative protein have a high negative charge, which appears to be required for function. A second sporulation-specific gene, designated SPO16, was found adjacent to SPO12 and shown to be necessary for efficient spore formation. The two genes are encoded on opposite DNA strands with only 103 nucleotides between the termination codons. Up to 700 nucleotides of the SPO12 and SPO16 transcripts are complementary, and the 3' untranslated region of the longest SPO16 transcript is complementary to all or nearly all of the SPO12 mRNA. A strain homozygous for an insertion which removes the complementarity between the SPO12 and SPO16 mRNAs has an efficiency of sporulation, number of spores per ascus, and spore viability identical to those of a wild-type strain. The complementarity therefore has either no function or only a subtle function in meiosis and sporulation.

Complementary transcripts from two genes necessary for normal meiosis in the yeast Saccharomyces cerevisiae

The SPO12 gene, which is required for meiosis I chromosome division during sporulation of the yeast Saccharomyces cerevisiae, has been isolated. DNA sequencing has identified an open reading frame of 173 codons that encodes the putative SPO12 protein and has no significant sequence similarities to known genes. The last 15 amino acids of this putative protein have a high negative charge, which appears to be required for function. A second sporulation-specific gene, designated SPO16, was found adjacent to SPO12 and shown to be necessary for efficient spore formation. The two genes are encoded on opposite DNA strands with only 103 nucleotides between the termination codons. Up to 700 nucleotides of the SPO12 and SPO16 transcripts are complementary, and the 3' untranslated region of the longest SPO16 transcript is complementary to all or nearly all of the SPO12 mRNA. A strain homozygous for an insertion which removes the complementarity between the SPO12 and SPO16 mRNAs has an efficiency of sporulation, number of spores per ascus, and spore viability identical to those of a wild-type strain. The complementarity therefore has either no function or only a subtle function in meiosis and sporulation.

Positive control of sporulation-specific genes by the IME1 and IME2 products in Saccharomyces cerevisiae

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.

Positive control of sporulation-specific genes by the IME1 and IME2 products in Saccharomyces cerevisiae

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.

Two DNA repair and recombination genes in Saccharomyces cerevisiae, RAD52 and RAD54, are induced during meiosis

The DNA repair and recombination genes of Saccharomyces cerevisiae, RAD52 and RAD54, were transcriptionally induced approximately 10- to 15-fold in sporulating MATa/alpha cells. Congenic MATa/a cells, which did not sporulate, did not show similar increases. Assays of beta-galactosidase activity in strains harboring either a RAD52- or RAD54-lacZ gene fusion indicated that this induction occurred at a time concomitant with a commitment to meiotic recombination, as measured by prototroph formation from his1 heteroalleles.

A transcriptional cascade governs entry into meiosis in Saccharomyces cerevisiae

Two signals activate meiosis in yeast: starvation and expression of the a1 and alpha 2 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 meiotic 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 ime1 deletion mutant. Accordingly, we propose that the IME1 product stimulates meiosis mainly through activation of IME2 expression.

Two DNA repair and recombination genes in Saccharomyces cerevisiae, RAD52 and RAD54, are induced during meiosis

The DNA repair and recombination genes of Saccharomyces cerevisiae, RAD52 and RAD54, were transcriptionally induced approximately 10- to 15-fold in sporulating MATa/alpha cells. Congenic MATa/a cells, which did not sporulate, did not show similar increases. Assays of beta-galactosidase activity in strains harboring either a RAD52- or RAD54-lacZ gene fusion indicated that this induction occurred at a time concomitant with a commitment to meiotic recombination, as measured by prototroph formation from his1 heteroalleles.

A transcriptional cascade governs entry into meiosis in Saccharomyces cerevisiae

Two signals activate meiosis in yeast: starvation and expression of the a1 and alpha 2 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 meiotic 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 ime1 deletion mutant. Accordingly, we propose that the IME1 product stimulates meiosis mainly through activation of IME2 expression.

Dependence of inessential late gene expression on early meiotic events in Saccharomyces cerevisiae

SPR3 is one of at least nine genes which are expressed in sporulating Saccharomyces cerevisiae cells at the time of meiosis I. We show below that strains homozygous for null alleles of SPR3 are capable of normal meiosis and the production of viable ascospores. We have also monitored SPR3 expression in a series of strains that are defective in meiotic development, using an SPR3:lacZ fusion carried on a single copy plasmid. beta-Galactosidase activity occurred at wild-type levels in diploid strains homozygous for mutations in spo13, rad50, rad57 and cdc9, but was greatly reduced in strains carrying cdc8 or spo7 defects. We conclude that SPR3 expression is a valid monitor of early meiotic development, even though the gene is inessential for the sporulation process.

The SPS100 gene of Saccharomyces cerevisiae is activated late in the sporulation process and contributes to spore wall maturation

We previously described the use of a differential hybridization screen of a genomic DNA library of Saccharomyces cerevisiae to identify sporulation-specific (SPS) genes (A. Percival-Smith and J. Segall, Mol. Cell. Biol. 4:142-150, 1984). This initial screen identified 14 SPS genes that are first expressed 6 to 8 h after transfer of cells to sporulation medium. Accumulation of transcripts corresponding to these genes becomes maximal at 8 to 12 h of sporulation, the time at which meiotic events are nearing completion, and by 15 h of sporulation, transcript levels are beginning to decrease. In the present study two additional SPS genes, first expressed at 12 h of sporulation, were isolated. The steady-state level of transcripts corresponding to these two genes, termed SPS100 and SPS101, remains unchanged from 15 to 35 h, a time coincident with spore wall maturation. The nature of the putative 34.2-kilodalton protein encoded by the SPS100 gene is consistent with its being a component of the glycoprotein matrix of the spore wall; the protein contains a potential signal sequence and cleavage site and numerous sites for potential glycosylation. A MATa sps100/MAT alpha sps100 strain was found to be indistinguishable from the wild-type strain when assessed for efficiency of ascus formation and spore viability. However, a more detailed analysis of the mutant strain revealed that the SPS100 gene product serves a protective role during the early stages of spore wall formation. The time at which resistance to ether could first be detected in developing spores was delayed by 5 h in the mutant strain relative to the wild-type strain. This phenotype is presumably a reflection of a defect in spore wall maturation. This study has confirmed that temporally distinct classes of sporulation-specific genes are sequentially activated during the process of meiosis and spore formation and has shown that the SPS100 gene, identified on the basis of its developmental-specific expression pattern, contributes to spore development.

The SPS100 gene of Saccharomyces cerevisiae is activated late in the sporulation process and contributes to spore wall maturation

We previously described the use of a differential hybridization screen of a genomic DNA library of Saccharomyces cerevisiae to identify sporulation-specific (SPS) genes (A. Percival-Smith and J. Segall, Mol. Cell. Biol. 4:142-150, 1984). This initial screen identified 14 SPS genes that are first expressed 6 to 8 h after transfer of cells to sporulation medium. Accumulation of transcripts corresponding to these genes becomes maximal at 8 to 12 h of sporulation, the time at which meiotic events are nearing completion, and by 15 h of sporulation, transcript levels are beginning to decrease. In the present study two additional SPS genes, first expressed at 12 h of sporulation, were isolated. The steady-state level of transcripts corresponding to these two genes, termed SPS100 and SPS101, remains unchanged from 15 to 35 h, a time coincident with spore wall maturation. The nature of the putative 34.2-kilodalton protein encoded by the SPS100 gene is consistent with its being a component of the glycoprotein matrix of the spore wall; the protein contains a potential signal sequence and cleavage site and numerous sites for potential glycosylation. A MATa sps100/MAT alpha sps100 strain was found to be indistinguishable from the wild-type strain when assessed for efficiency of ascus formation and spore viability. However, a more detailed analysis of the mutant strain revealed that the SPS100 gene product serves a protective role during the early stages of spore wall formation. The time at which resistance to ether could first be detected in developing spores was delayed by 5 h in the mutant strain relative to the wild-type strain. This phenotype is presumably a reflection of a defect in spore wall maturation. This study has confirmed that temporally distinct classes of sporulation-specific genes are sequentially activated during the process of meiosis and spore formation and has shown that the SPS100 gene, identified on the basis of its developmental-specific expression pattern, contributes to spore development.

Transcriptional regulation of sporulation genes in yeast

The relative transcription rates of three sporulation-regulated genes of yeast (SPR1, SPR2 and SPR3) were determined at intervals during sporulation, using a filter binding assay. The binding of in vivo labeled RNA to the corresponding DNAs increased 3- to 12-fold at the time of meiosis I, in parallel with the accumulation of the SPR transcripts. SPR1 and SPR3 mRNA abundance increased from less than 0.7 to 130 and 90 copies per cell, respectively, between the time of shift to sporulation medium and the initiation of spore formation. This represented a 150-to 200-fold increase in the steady-state levels of these RNAs. Similarly, the levels of beta-galactosidase present in sporulating cells harboring fusions between SPR3 and Escherichia coli lacZ increased at least 700-fold. We conclude that SPR1, SPR2 and SPR3 transcription is modulated during sporulation, possibly in response to earlier events in the process.

Isolation, DNA sequence, and regulation of a meiosis-specific eukaryotic recombination gene

The SPO11 gene, required for meiotic recombination in Saccharomyces cerevisiae, has been cloned by direct selection for complementation of the spo11-1 phenotype: lack of meiotic recombination and low spore viability. DNA sequencing indicates that the gene encodes a 398-amino acid protein having a predicted molecular mass of 45.3 kDa. There is no significant similarity between the SPO11 protein and other protein sequences, including those from genes known to be involved in DNA recombination or repair. Strains bearing a disruption allele are viable, indicating that SPO11 is dispensable for mitotic growth. RNA analyses demonstrate that SPO11 produces a 1.5-kilobase transcript that is developmentally regulated and expressed early in the sporulation process.

Increased copy number of the 5' end of the SPS2 gene inhibits sporulation of Saccharomyces cerevisiae

We found that the introduction into a yeast cell of a high-copy-number plasmid containing the 5' end of the SPS2 gene, a sporulation-specific gene of Saccharomyces cerevisiae, led to a reduction in the efficiency of spore formation. The plasmid pAP290, which contains the sequence from -138 to +152 of the SPS2 gene, caused a fivefold reduction in spore formation; the presence of the plasmid had no effect on transcription of the chromosomal SPS2 gene. A plasmid containing only the sequence upstream of the TATA box of the SPS2 gene (-350 to -68) was unable to inhibit the completion of sporulation, whereas the downstream sequence, from -70 to +404, although unable by itself to inhibit sporulation, could do so when provided with an upstream fragment containing the CYC1 upstream activation sequence. Deletion of 22 base pairs from pAP290, which introduced a frameshift after codon 17 of the SPS2 gene and reduced the open reading frame to 26 amino acids, generated a plasmid (pAP290 delta Pst) which could no longer inhibit sporulation. The SPS2 inserts of pAP290 and pAP290 delta Pst were found to direct equivalent levels of sporulation-specific transcription. We conclude from these results that the presence of both the SPS2 promoter (or a substitute promoter) and the initial coding sequence of the SPS2 gene is required in the high-copy-number plasmid to generate the asporogenous phenotype. We speculate that the accumulation of a protein containing the amino-terminal portion of the SPS2 gene product, synthesized from the transcripts of the truncated plasmid-borne copies of the SPS2 gene, prevents ascus formation.

Developmental regulation of SPO13, a gene required for separation of homologous chromosomes at meiosis I

Previous studies have demonstrated that the SPO13 gene is required for chromosome separation during meiosis I in Saccharomyces cerevisiae. In the presence of the spo13-1 nonsense mutation, MATa/MAT alpha diploid cells complete a number of events typical of meiosis I including premeiotic DNA synthesis, genetic recombination, and spindle formation. Disjunction of homologous chromosomes, however, fails to occur. Instead, cells proceed through a single meiosis II-like division and form two diploid spores. In this paper, we report the cloning of this essential meiotic gene and an analysis of its transcription during vegetative growth and sporulation. Disruptions of SPO13 in haploid and diploid cells show that it is dispensible for mitotic cell division. Diploids homozygous for the disruptions behave similarly to spo13-1 mutants; they sporulate at wild-type levels and produce two-spored asci. The DNA region complementing spo13-1 encodes two overlapping transcripts, which have the same 3' end but different 5' ends. The major transcript is 400 bases shorter than the larger, less abundant one. The shorter RNA is sufficient to complement the spo13-1 mutation. While both transcripts are undetectable or just barely detectable in vegetative cultures, they each undergo a greater than 70-fold induction early during sporulation, reaching a maximum level about the time of the first meiotic division. In synchronously sporulating populations, the transcripts nearly disappear before the completion of ascus formation. Nonsporulating cells homozygous for the mating-type locus show a small increase in abundance (less than 5% of the increase in sporulating cells) of both transcripts in sporulation medium. These results indicate that expression of the SPO13 gene is developmentally regulated and starvation alone, independent of the genotype at MAT, can trigger initial induction.

Increased copy number of the 5' end of the SPS2 gene inhibits sporulation of Saccharomyces cerevisiae

We found that the introduction into a yeast cell of a high-copy-number plasmid containing the 5' end of the SPS2 gene, a sporulation-specific gene of Saccharomyces cerevisiae, led to a reduction in the efficiency of spore formation. The plasmid pAP290, which contains the sequence from -138 to +152 of the SPS2 gene, caused a fivefold reduction in spore formation; the presence of the plasmid had no effect on transcription of the chromosomal SPS2 gene. A plasmid containing only the sequence upstream of the TATA box of the SPS2 gene (-350 to -68) was unable to inhibit the completion of sporulation, whereas the downstream sequence, from -70 to +404, although unable by itself to inhibit sporulation, could do so when provided with an upstream fragment containing the CYC1 upstream activation sequence. Deletion of 22 base pairs from pAP290, which introduced a frameshift after codon 17 of the SPS2 gene and reduced the open reading frame to 26 amino acids, generated a plasmid (pAP290 delta Pst) which could no longer inhibit sporulation. The SPS2 inserts of pAP290 and pAP290 delta Pst were found to direct equivalent levels of sporulation-specific transcription. We conclude from these results that the presence of both the SPS2 promoter (or a substitute promoter) and the initial coding sequence of the SPS2 gene is required in the high-copy-number plasmid to generate the asporogenous phenotype. We speculate that the accumulation of a protein containing the amino-terminal portion of the SPS2 gene product, synthesized from the transcripts of the truncated plasmid-borne copies of the SPS2 gene, prevents ascus formation.

Developmental regulation of SPO13, a gene required for separation of homologous chromosomes at meiosis I

Previous studies have demonstrated that the SPO13 gene is required for chromosome separation during meiosis I in Saccharomyces cerevisiae. In the presence of the spo13-1 nonsense mutation, MATa/MAT alpha diploid cells complete a number of events typical of meiosis I including premeiotic DNA synthesis, genetic recombination, and spindle formation. Disjunction of homologous chromosomes, however, fails to occur. Instead, cells proceed through a single meiosis II-like division and form two diploid spores. In this paper, we report the cloning of this essential meiotic gene and an analysis of its transcription during vegetative growth and sporulation. Disruptions of SPO13 in haploid and diploid cells show that it is dispensible for mitotic cell division. Diploids homozygous for the disruptions behave similarly to spo13-1 mutants; they sporulate at wild-type levels and produce two-spored asci. The DNA region complementing spo13-1 encodes two overlapping transcripts, which have the same 3' end but different 5' ends. The major transcript is 400 bases shorter than the larger, less abundant one. The shorter RNA is sufficient to complement the spo13-1 mutation. While both transcripts are undetectable or just barely detectable in vegetative cultures, they each undergo a greater than 70-fold induction early during sporulation, reaching a maximum level about the time of the first meiotic division. In synchronously sporulating populations, the transcripts nearly disappear before the completion of ascus formation. Nonsporulating cells homozygous for the mating-type locus show a small increase in abundance (less than 5% of the increase in sporulating cells) of both transcripts in sporulation medium. These results indicate that expression of the SPO13 gene is developmentally regulated and starvation alone, independent of the genotype at MAT, can trigger initial induction.

Molecular cloning of chromosome I DNA from Saccharomyces cerevisiae: isolation and characterization of the CDC24 gene and adjacent regions of the chromosome

Molecular cloning techniques were used to isolate and characterize the DNA including and surrounding the CDC24 and PYK1 genes on the left arm of chromosome I of the yeast Saccharomyces cerevisiae. A plasmid that complemented a temperature-sensitive cdc24 mutation was isolated from a yeast genomic DNA library in a shuttle vector. Plasmids containing pyk1-complementing DNA were obtained from other investigators. Several lines of evidence (including one-step gene replacement experiments) demonstrated that the complementing plasmids contained the bona fide CDC24 and PYK1 genes. These sequences were then used to isolate additional DNA from chromosome I by probing a yeast genomic DNA library in a lambda vector. A total of 28 kilobases (kb) of contiguous DNA surrounding the CDC24 and PYK1 genes was isolated, and a restriction map was determined. Electron microscopy of R-loop-containing DNA and RNA blot hybridization analyses indicated that an 18-kb segment contained at least seven transcribed regions, only three of which corresponded to previously known genes (CDC24, PYK1, and CYC3). Southern blot hybridization experiments suggested that none of the genes in this region was duplicated elsewhere in the yeast genome. The centers of CDC24 and PYK1 were only approximately 7.5 kb apart, although the genetic map distance between them is approximately 13 centimorgans. As previous studies with S. cerevisiae have indicated that 1 centimorgan generally corresponds to approximately 3 kb, the region between CDC24 and PYK1 appears to undergo meiotic recombination at an unusually high frequency.

Molecular cloning of chromosome I DNA from Saccharomyces cerevisiae: isolation and characterization of the CDC24 gene and adjacent regions of the chromosome

Molecular cloning techniques were used to isolate and characterize the DNA including and surrounding the CDC24 and PYK1 genes on the left arm of chromosome I of the yeast Saccharomyces cerevisiae. A plasmid that complemented a temperature-sensitive cdc24 mutation was isolated from a yeast genomic DNA library in a shuttle vector. Plasmids containing pyk1-complementing DNA were obtained from other investigators. Several lines of evidence (including one-step gene replacement experiments) demonstrated that the complementing plasmids contained the bona fide CDC24 and PYK1 genes. These sequences were then used to isolate additional DNA from chromosome I by probing a yeast genomic DNA library in a lambda vector. A total of 28 kilobases (kb) of contiguous DNA surrounding the CDC24 and PYK1 genes was isolated, and a restriction map was determined. Electron microscopy of R-loop-containing DNA and RNA blot hybridization analyses indicated that an 18-kb segment contained at least seven transcribed regions, only three of which corresponded to previously known genes (CDC24, PYK1, and CYC3). Southern blot hybridization experiments suggested that none of the genes in this region was duplicated elsewhere in the yeast genome. The centers of CDC24 and PYK1 were only approximately 7.5 kb apart, although the genetic map distance between them is approximately 13 centimorgans. As previous studies with S. cerevisiae have indicated that 1 centimorgan generally corresponds to approximately 3 kb, the region between CDC24 and PYK1 appears to undergo meiotic recombination at an unusually high frequency.