GermOnline, a cross-species community knowledgebase on germ cell differentiation

GermOnline provides information and microarray expression data for genes involved in mitosis and meiosis, gamete formation and germ line development across species. The database has been developed, and is being curated and updated, by life scientists in cooperation with bioinformaticists. Information is contributed through an online form using free text, images and the controlled vocabulary developed by the GeneOntology Consortium. Authors provide up to three references in support of their contribution. The database is governed by an international board of scientists to ensure a standardized data format and the highest quality of GermOnline's information content. Release 2.0 provides exclusive access to microarray expression data from Saccharomyces cerevisiae and Rattus norvegicus, as well as curated information on approximately 700 genes from various organisms. The locus report pages include links to external databases that contain relevant annotation, microarray expression and proteome data. Conversely, the Saccharomyces Genome Database (SGD), S.cerevisiae GeneDB and Swiss-Prot link to the budding yeast section of GermOnline from their respective locus pages. GermOnline, a fully operational prototype subject-oriented knowledgebase designed for community annotation and array data visualization, is accessible at http://www.germonline.org. The target audience includes researchers who work on mitotic cell division, meiosis, gametogenesis, germ line development, human reproductive health and comparative genomics.

A screen for genes required for meiosis and spore formation based on whole-genome expression

BACKGROUND

Meiosis is the process by which gametes are generated with half the ploidy of somatic cells. This reduction is achieved by three major differences in chromosome behavior during meiosis as compared to mitosis: the production of chiasmata by recombination, the protection of centromere-proximal sister chromatid cohesion, and the monoorientation of sister kinetochores during meiosis I. Mistakes in any of these processes lead to chromosome missegregation.

RESULTS

To identify genes involved in meiotic chromosome behavior in Saccharomyces cerevisiae, we deleted 301 open reading frames (ORFs) which are preferentially expressed in meiotic cells according to microarray gene expression data. To facilitate the detection of chromosome missegregation mutants, chromosome V of the parental strain was marked by GFP. Thirty-three ORFs were required for the formation of wild-type asci, eight of which were needed for proper chromosome segregation. One of these (MAM1) is essential for the monoorientation of sister kinetochores during meiosis I. Two genes (MND1 and MND2) are implicated in the recombination process and another two (SMA1 and SMA2) in prospore membrane formation.

CONCLUSIONS

Reverse genetics using gene expression data is an effective method for identifying new genes involved in specific cellular processes.

Identification of the Sin3-binding site in Ume6 defines a two-step process for conversion of Ume6 from a transcriptional repressor to an activator in yeast

The DNA-binding protein Ume6 is required for both repression and activation of meiosis-specific genes, through interaction with the Sin3 corepressor and Rpd3 histone deacetylase and the meiotic activator Ime1. Here we show that fusion of a heterologous activation domain to Ume6 is unable to convert it into a constitutive activator of early meiotic gene transcription, indicating that an additional function is needed to overcome repression at these promoters. Mutations in UME6 allowing the fusion to activate lie in a predicted amphipathic alpha helix and specifically disrupt interaction with Sin3 but not with Teal, an activator of Ty transcription also found to interact with Ume6 in a two-hybrid screen. The mutations cause a loss of repression by Ume6 and precisely identify the Ume6 Sin3-binding domain, which we show interacts with the paired amphipathic helix 2 region of Sin3. Analysis of these mutants indicates that conversion of Ume6 to an activator involves two genetically distinct steps that act to relieve Sin3-mediated repression and provide an activation domain to Ume6. The mutants further demonstrate that premature expression and lack of subsequent rerepression of Ume6-Sin3-regulated genes are not deleterious to meiotic progression and suggest that the essential role of Sin3 in meiosis is independent of Ume6. The model for Ume6 function arising from these studies indicates that Ume6 is similar in many respects to metazoan regulators that utilize Sin3, such as the Myc-Mad-Max system and nuclear hormone receptors, and provides new insights into the control of transcriptional repression and activation by the Ume6-URS1 regulatory complex in yeast.

The core meiotic transcriptome in budding yeasts

We used high-density oligonucleotide microarrays to analyse the genomes and meiotic expression patterns of two yeast strains, SK1 and W303, that display distinct kinetics and efficiencies of sporulation. Hybridization of genomic DNA to arrays revealed numerous gene deletions and polymorphisms in both backgrounds. The expression analysis yielded approximately 1,600 meiotically regulated genes in each strain, with a core set of approximately 60% displaying similar patterns in both strains. Most of these (95%) are MATa/MATalpha-dependent and are not similarly expressed in near-isogenic meiosis-deficient controls. The transcript profiles correlate with the distribution of defined meiotic promoter elements and with the time of known gene function.

Spo1, a phospholipase B homolog, is required for spindle pole body duplication during meiosis in Saccharomyces cerevisiae

The SPO1 gene was cloned and shown to encode an early meiotic transcript specifying a nuclear protein with extensive similarity to fungal and vertebrate phospholipase enzymes. Alteration of a conserved serine residue in the putative phospholipase active site, and presence of the spo1-1 temperature-sensitive mutation, which resides near this site, each result in loss of SPO1 function. The phenotype of a complete deletion indicates that SPO1 is dispensable for vegetative growth, premeiotic DNA synthesis and meiotic recombination. In contrast, it is required for Meiosis I (MI) and Meiosis II (MII) chromosome segregation and spore formation. In a null mutant approximately 75% of cells arrest early at MI spindle pole body (SPB) duplication, approximately 20% arrest at MII, and approximately 5% arrest at spore formation. Progression beyond the first arrest point suggests the existence of functions partially redundant to Spo1 and that Spo1 is required at multiple stages. At present SPO1 is the only known gene required for SPB duplication in meiosis but not in mitosis. Its product may thus play a regulatory (rather than a structural) role in SPB function. The transcriptional program in the spo1 null is similar to the wild type early in meiosis but is significantly delayed at later stages of sporulation. A single gene, CWP1, was recovered as a multicopy suppressor of the spo1 null. CWP1 encodes a cell wall protein with a glycolipid moiety. We propose that, when modified by other lipases, this moiety may substitute for the product(s) of Spo1p lipase activity in meiosis. Based on the similarity of Spo1p to phospholipase B enzymes, its unique role in SPB duplication, and pleiotropic effects on MII, late gene expression and spore formation, we propose that the Spo1 protein participates in a novel meiotic pathway that functions through the SPB to coordinate nuclear division with spore development.

Recombination can partially substitute for SPO13 in regulating meiosis I in budding yeast

Recombination and chromosome synapsis bring homologous chromosomes together, creating chiasmata that ensure accurate disjunction during reductional division. SPO13 is a key gene required for meiosis I (MI) reductional segregation, but dispensable for recombination, in Saccharomyces cerevisiae. Absence of SPO13 leads to single-division meiosis where reductional segregation is largely eliminated, but other meiotic events occur relatively normally. This phenotype allows haploids to produce viable meiotic products. Spo13p is thought to act by delaying nuclear division until sister centromeres/chromatids undergo proper cohesion for segregation to the same pole at MI. In the present study, a search for new spo13-like mutations that allow haploid meiosis recovered only new spo13 alleles. Unexpectedly, an unusual reduced-expression allele (spo13-23) was recovered that behaves similarly to a null mutant in haploids but to a wild-type allele in diploids, dependent on the presence of recombining homologs rather than on a diploid genome. This finding demonstrates that in addition to promoting accurate homolog disjunction, recombination can also function to partially substitute for SPO13 in promoting sister cohesion. Analysis of various recombination-defective mutants indicates that this contribution of recombination to reductional segregation requires full levels of crossing over. The implications of these results regarding SPO13 function are discussed.

The SPO1 gene product required for meiosis in yeast has a high similarity to phospholipase B enzymes

The SPO1 gene of Saccharomyces cerevisiae has been cloned and sequenced. The Spo1 protein reveals significant similarity with fungal phospholipase B (PLB) enzymes. Features of the SPO1 gene sequence are presented.

UME6 is a central component of a developmental regulatory switch controlling meiosis-specific gene expression

The UME6 gene of Saccharomyces cerevisiae was identified as a mitotic repressor of early meiosis-specific gene expression. It encodes a Zn2Cys6 DNA-binding protein which binds to URS1, a promoter element needed for both mitotic repression and meiotic induction of early meiotic genes. This paper demonstrates that a complete deletion of UME6 causes not only vegetative derepression of early meiotic genes during vegetative growth but also a significant reduction in induction of meiosis-specific genes, accompanied by a severe defect in meiotic progression. After initiating premeiotic DNA synthesis the vast majority of cells (approximately 85%) become arrested in prophase and fail to execute recombination; a minority of cells (approximately 15%) complete recombination and meiosis I, and half of these form asci. Quantitative analysis of the same early meiotic transcripts that are vegetatively derepressed in the ume6 mutant, SPO11, SPO13, IME2, and SPO1, indicates a low level of induction in meiosis above their vegetative derepressed levels. In addition, the expression of later meiotic transcripts, SPS2 and DIT1, is significantly delayed and reduced. The expression pattern of early meiotic genes in ume6-deleted cells is strikingly similar to that of early meiotic genes with promoter mutations in URS1. These results support the view that UME6 and URS1 are part of a developmental switch that controls both vegetative repression and meiotic induction of meiosis-specific genes.

UME6, a negative regulator of meiosis in Saccharomyces cerevisiae, contains a C-terminal Zn2Cys6 binuclear cluster that binds the URS1 DNA sequence in a zinc-dependent manner

UME6 is a protein of 836 amino acids from Saccharomyces cerevisiae that acts as a repressor and activator of several early meiotic genes. UME6 contains, near the C-terminus, the amino acid sequence-771C-X2-C-X6-C-X6-C-X2-C-X6-C-, in which the spacings of the six Cys residues are identical to those found in 39 N-terminal Cys-rich DNA binding subdomains of fungal transcription factors. This sequence has been shown in GAL4 and other proteins to form a zinc binuclear cluster. In spite of the different location, the C-rich sequence, cloned and over-produced within the last 111 amino acid residues of UME6, UME6(111), forms a binuclear cluster and exhibits a Zn-dependent binding to the URS1 DNA sequence. The latter, TAGCCGCCGA, is required for the repression or activation of meiosis-specific genes by UME6. UME6(111) contains 1.8 +/- 0.4 mol Zn/mol protein and the Zn can be exchanged for Cd to yield a protein containing 1.9 +/- 0.1 mol Cd/mol protein. At 5 degrees C, 113Cd2UME6(111) shows two 113Cd NMR signals, with chemical shifts of 699 and 689 ppm, similar to those observed for 113Cd2GAL4(149). The magnitude of these chemical shifts suggests that each 113Cd nucleus is coordinated to four -S- ligands, compatible with a 113Cd2 cluster structure in which two thiolates from bridging ligands. The entire UME6 gene has been cloned and overexpressed and binds more tightly to the URS1 sequence than the zinc binuclear cluster domain alone. DNase I footprints of UME6 on URS1-containing DNA show that the protein protects the phosphodiesters of the 5'-CCGCCG-3' region within the URS1 sequence.

SPO13 negatively regulates the progression of mitotic and meiotic nuclear division in Saccharomyces cerevisiae

The meiosis-specific yeast gene SPO13 has been previously shown to be required to obtain two successive divisions in meiosis. We report here that vegetative expression of this gene causes a CDC28-dependent cell-cycle arrest at mitosis. Overexpression of SPO13 during meiosis causes a transient block to completion of the meiosis I division and suppresses the inability of cdc28ts strains to execute meiosis II. The spo13 defect can be partially suppressed by conditions that slow progression of the first meiotic division. Based on the results presented below, we propose that SPO13 acts as a meiotic timing function by transiently blocking progression through the meiosis I division, thereby allowing (1) coordination of the first division with assembly of the reductional segregation apparatus, and (2) subsequent entry into a second round of segregation to separate replicated sister chromatids without an intervening S-phase.

Reversal of cell determination in yeast meiosis: postcommitment arrest allows return to mitotic growth

When yeast from the early stages of meiosis are transferred from sporulation to growth medium, they can reenter the mitotic cell cycle directly. In contrast, cells from later stages of meiosis (after the initiation of the first nuclear division) will complete meiosis and sporulation despite the shift to growth medium, a phenomenon known as "commitment to meiosis." This study reports the surprising finding that when the normal progression of meiosis is arrested, cells from later stages of meiosis can return to growth. Cells were arrested after the first or second meiotic division by three independent means: the spo14 mutation, the spo3-1 mutation, and a high-temperature arrest of wild-type cells. In every case, the arrested cells were able to form buds after transfer to growth medium. These cells, however, experienced a delay upon return to growth relative to uncommitted cells. We propose that the commitment phenomenon results from a transient delay of mitotic growth, which occurs specifically during meiosis, and that commitment does not involve an irreversible inhibition of mitosis as previously thought.

The yeast UME5 gene regulates the stability of meiotic mRNAs in response to glucose

We reported previously that early meiotic transcripts are highly unstable. These mRNAs exhibit half-lives of approximately 3 min when expressed during vegetative growth in glucose medium and are stabilized twofold during sporulation in acetate medium. Two genes, UME2 and UME5, that regulate the stability of meiosis-specific transcripts have been identified. The wild-type UME5 gene, which has been analyzed in detail, decreases the stability of all meiotic mRNAs tested approximately twofold when expressed during vegetative growth but has no effect on the half-lives of a number of vegetative mRNAs examined. The UME5 gene is dispensable for mitotic and meiotic development. Cells in which the entire UME5 gene has been deleted are viable, although the generation time is slightly longer and sporulation is less efficient. The UME5 transcript is constitutively expressed, and its stability is not autoregulated. The UME5 gene encodes a predicted 63-kDa protein with homology to the family of CDC28 serine/threonine-specific protein kinases. The kinase activity appears to be central to the function of the UME5 protein, since alteration of a highly conserved amino acid in the kinase domain results in a phenotype identical to that of a ume5 deletion. Genetic epistasis studies suggest that the UME2 and UME5 gene products act in the same pathway to regulate meiotic transcript stability. This pathway is independent of deadenylation and translation, two factors known to be important in regulating mRNA turnover. Significantly, the UME5-mediated destabilization of meiotic mRNAs occurs in glucose- but not in acetate-containing medium. Thus, the UME5 gene appears to participate in a glucose signal transduction pathway governing message stability.

UME6 is a key regulator of nitrogen repression and meiotic development

This report describes the identification, cloning, and molecular analysis of UME6 (CAR80/CARGRI), a key transcriptional regulator of early meiotic gene expression. Loss of UME6 function results in the accumulation of fully derepressed levels (70- to 100-fold increase above basal level) of early meiotic transcripts during vegetative growth. In contrast, mutations in five previously identified UME loci (UME1 to UME5), result in low to moderate derepression (2- to 10-fold increase) of early meiotic genes. The behavior of insertion and deletion alleles indicates that UME6 is dispensable for mitotic division but is required for meiosis and spore germination. Despite the high level of meiotic gene expression during vegetative growth, the generation times of ume6 mutant haploid and diploid cells are only slightly reduced. However, both ascus formation and spore viability are affected more severely. The UME6 gene encodes a 91-kD protein that contains a C6 zinc cluster motif similar to the DNA-binding domain of GAL4. The integrity of this domain is required for UME6 function. It has been reported recently that a mutation in CAR80 fails to complement an insertion allele of UME6. CAR80 is a gene required for nitrogen repression of the arginine catabolic enzymes. Here, through sequence analysis, we demonstrate that UME6 and CAR80 are identical. Analyses of UME6 mRNA during both nitrogen starvation and meiotic development indicate that its transcription is constitutive, suggesting that regulation of UME6 activity occurs at a post-transcriptional level.

Meiotic recombination in yeast

Over the past several years, the yeast Saccharomyces cerevisiae has proven to be an extremely useful model system for understanding how cells acquire high recombinational ability during meiosis. Due to recent advances in the physical monitoring of DNA intermediates during meiosis, new cytological methods for visualization of chromosomes during pairing and exchange, and progress in the identification and analysis of recombination-defective mutants, a general picture of the order and dependencies of specific recombination events is now emerging.

Regulatory mechanisms in meiosis

Meiosis can be viewed both as a process of cell differentiation and as a modification of the mitotic cell cycle. Here we describe recent progress in defining a variety of regulatory mechanisms that govern the meiotic divisions. Studies in the yeast Saccharomyces cerevisiae and in higher organisms have led to complementary insights into these controls.

Early meiotic transcripts are highly unstable in Saccharomyces cerevisiae

Meiosis in Saccharomyces cerevisiae requires the induction of a large number of genes whose mRNAs accumulate at specific times during meiotic development. This study addresses the role of mRNA stability in the regulation of meiosis-specific gene expression. Evidence is provided below demonstrating that the levels of meiotic mRNAs are exquisitely regulated by both transcriptional control and RNA turnover. The data show that (i) early meiotic transcripts are extremely unstable when expressed during either vegetative growth or sporulation, and (ii) transcriptional induction, rather than RNA turnover, is the predominant mechanism responsible for meiosis-specific transcript accumulation. When genes encoding the early meiotic mRNAs are fused to other promoters and expressed during vegetative growth, their mRNA half-lives, of under 3 min, are among the shortest known in S. cerevisiae. Since these mRNAs are only twofold more stable when expressed during sporulation, we conclude that developmental regulation of mRNA turnover can be eliminated as a major contributor to meiosis-specific mRNA accumulation. The rapid degradation of the early mRNAs at all stages of the yeast life cycle, however, suggests that a specific RNA degradation system operates to maintain very low basal levels of these transcripts during vegetative growth and after their transient transcriptional induction in meiosis. Studies to identify specific cis-acting elements required for the rapid degradation of early meiotic transcripts support this idea. A series of deletion derivatives of one early meiosis-specific gene, SPO13, indicate that its mRNA contains determinants, located within the coding region, which contribute to the high instability of this transcript. Translation is another component of the degradation mechanism since frameshift and nonsense mutations within the SPO13 mRNA stabilize the transcript.

RPD1 (SIN3/UME4) is required for maximal activation and repression of diverse yeast genes

We show that the extent of transcriptional regulation of many, apparently unrelated, genes in Saccharomyces cerevisiae is dependent on RPD1 (and RPD3 [M. Vidal and R. F. Gaber, Mol. Cell. Biol. 11:6317-6327, 1991]). Genes regulated by stimuli as diverse as external signals (PHO5), cell differentiation processes (SPO11 and SPO13), cell type (RME1, FUS1, HO, TY2, STE6, STE3, and BAR1), and genes whose regulatory signals remain unknown (TRK2) depend on RPD1 to achieve maximal states of transcriptional regulation. RPD1 enhances both positive and negative regulation of these genes: in rpd1 delta mutants, higher levels of expression are observed under repression conditions and lower levels are observed under activation conditions. We show that several independent genetic screens, designed to identify yeast transcriptional regulators, have detected the RPD1 locus (also known as SIN3, SD11, and UME4). The inferred RPD1 protein contains four regions predicted to take on helix-loop-helix-like secondary structures and three regions (acidic, glutamine rich, and proline rich) reminiscent of the activating domains of transcriptional activators.

Nucleotide sequence and promoter analysis of SPO13, a meiosis-specific gene of Saccharomyces cerevisiae

The SPO13 gene, required for meiosis I segregation in Saccharomyces cerevisiae, produces two developmentally regulated transcripts (1.0 and 1.4 kilobases) that differ in length at their 5' ends. The shorter transcript is sufficient to complement the spo13-1 mutation and contains a major open reading frame encoding a highly basic protein of 33.4 kilodaltons. A fragment upstream (-170 to -8) of the open reading frame confers meiosis-specific transcription on a spo13-HIS3 fusion. Deletions at the 5' end of spo13-lacZ fusions define a region between -140 and -80 that is essential for meiosis-specific expression. This region acts in an orientation-independent manner and is responsive to the MAT-RME regulatory cascade. It contains a 10-base-pair sequence, TAGCCGCCGA, found in a number of meiosis-specific genes, that appears to be required for SPO13 expression. This sequence is identical to URS1, a ubiquitous mitotic repressor element.

Identification of negative regulatory genes that govern the expression of early meiotic genes in yeast

Mutations in Saccharomyces cerevisiae have been identified that derepress early meiotic genes functioning in separable pathways required for normal meiotic development. The phenotypes of these ume (unscheduled meiotic gene expression) mutations suggest that their wild-type alleles encode negative regulators acting downstream of both the cell-type and nutritional controls of meiosis. These newly defined loci do not affect either general transcription or transcription of meiotic genes expressed later in meiosis and spore formation.

Evidence for two pathways of meiotic intrachromosomal recombination in yeast

This study shows that RAD50, a yeast DNA repair gene required for meiotic interchromosomal exchange between homologs, also is required for meiotic intrachromosomal recombination. However, only intrachromosomal events in nonribosomal DNA are dependent on RAD50; those in ribosomal DNA (rRNA-encoding DNA) occur in the absence of this gene. Furthermore, nonribosomal DNA sequences retain their RAD50-dependence even when inserted into the ribosomal DNA array. We argue that these data provide evidence for at least two pathways of meiotic intrachromosomal recombination whose activity depends on the specific sequences involved or their structural context in the chromosome. In contrast to its role in meiosis, RAD50 is not required for either inter- or intrachromosomal spontaneous mitotic recombination.

A new role for a yeast transcriptional silencer gene, SIR2, in regulation of recombination in ribosomal DNA

The yeast SIR2 gene is involved in regulating nucleosome phasing and transcription in the mating type system. We have found that SIR2 also plays another important role in the cell. Specifically, in wild-type SIR2 strains recombination between the tandemly repeated ribosomal RNA genes is depressed. In sir2 mutants, both mitotic and meiotic intrachromosomal recombination increase 10- to 15-fold. In striking contrast to its effect on rDNA, the SIR2 gene does not affect intrachromosomal recombination between non-rDNA gene duplications. Furthermore, in the absence of the SIR2 gene product, rDNA acquires a partial dependency on recombination gene functions (RAD50 and RAD52) that are normally dispensable for exchange in the rDNA array. Thus, SIR2 may function in excluding the rDNA region from the general recombination system. Here we demonstrate that SIR2's effect is not restricted to controlling mating type expression, but rather that SIR2 functions in a more general way in the genome.

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.

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.

Meiotic exchange within and between chromosomes requires a common Rec function in Saccharomyces cerevisiae

We used haploid yeast cells that express both the MATa and MAT alpha mating-type alleles and contain the spo13-1 mutation to characterize meiotic recombination within single, unpaired chromosomes in Rec+ and Rec- Saccharomyces cerevisiae. In Rec+ haploids, as in diploids, intrachromosomal recombination in the ribosomal DNA was detected in 2 to 6% of meiotic divisions, and most events were unequal reciprocal sister chromatid exchange (SCE). By contrast, intrachromosomal recombination between duplicated copies of the his4 locus occurred in approximately 30% of haploid meiotic divisions, a frequency much higher than that reported in diploids; only about one-half of the events were unequal reciprocal SCE. The spo11-1 mutation, which virtually eliminates meiotic exchange between homologs in diploid meiosis, reduced the frequency of intrachromosomal recombination in both the ribosomal DNA and the his4 duplication during meiosis by 10- to greater than 50-fold. This Rec- mutation affected all forms of recombination within chromosomes: unequal reciprocal SCE, reciprocal intrachromatid exchange, and gene conversion. Intrachromosomal recombination in spo11-1 haploids was restored by transformation with a plasmid containing the wild-type SPO11 gene. Mitotic intrachromosomal recombination frequencies were unaffected by spo11-1. This is the first demonstration of a gene product required for recombination between homologs as well as recombination within chromosomes during meiosis.

The role of the SPO11 gene in meiotic recombination in yeast

Several complementary experimental approaches were used to demonstrate that the SPO11 gene is specifically required for meiotic recombination. First, sporulating cultures of spo11-1 mutant diploids were examined for landmark biochemical, cytological and genetic events of meiosis and ascosporogenesis. Cells entered sporulation with high efficiency and showed a near-doubling of DNA content. Synaptonemal complexes, hallmarks of intimate homologous pairing, and polycomplex structures appeared during meiotic prophase. Although spontaneous mitotic intra- and intergenic recombination occurred at normal levels, no meiotic recombination was observed. Whereas greater than 50% of cells completed both meiotic divisions, packaging of the four meiotic products into mature ascospores took place in only a small subset of asci. Haploidization occurred in less than 1% of viable colony-forming units. Second, the Rec- meiotic defect conferred by spo11-1 was confirmed by dyad analysis of spores derived from spo13-1 single-division meiosis in which recombination is not a requirement for viable ascospore production. Diploids homozygous for the spo13-1 mutation undergo meiotic levels of exchange followed by a single predominantly equational division and form asci containing two near-diploid spores. With the introduction of the spo11-1 mutation, high spore viability was retained, whereas intergenic recombination was reduced by more than 100-fold.

Chromosomes XIV and XVII of Saccharomyces cerevisiae constitute a single linkage group

We present several lines of evidence that chromosomes XIV and XVII of Saccharomyces cerevisiae are not independent chromosomes, but rather constitute a single linkage group. Studies which made use of a new mapping method based on the haploidization-without-recombination meiotic phenotype of the spoll mutant initially indicated that markers on chromosomes XIV and XVII were linked. Tetrad analysis was used to establish gene-gene distances, and a new chromosome XIV map incorporating markers originally assigned to chromosome XVII was derived. During the course of trisomic segregation studies, we discovered that a 2n + 2 homothallic diploid, originally believed to be tetrasomic for chromosome XVII (now XIV), carries two normal chromosome XIV homologs and two aberrant homologs which appear to be deficient for a large portion of the right arm of XIV. The previous evidence that established chromosome XVII as an independent linkage group is discussed in the light of these findings.

Meiosis in haploid yeast

Haploid yeast cells normally contain either the MATa or MATalpha mating-type allele and cannot undergo meiosis and spore formation. If both mating-type alleles are present as a consequence of chromosome III disomy (MATa/MATalpha), haploids initiate meiosis but do not successfully form spores, probably because the haploid chromosome complement is irregularly partitioned during meiotic nuclear division. We have demonstrated that the ochre-suppressible mutation spo13-1 enables haploid yeast cells disomic for chromosome III and heterozygous at the mating-type locus to complete meiosis and spore formation, yielding two haploid spores. Previous studies have shown that the absence of the wild-type SPO13 gene function permits diploid cells to bypass homologous chromosome segregation at meiosis I and proceed directly to meiosis II. During spo13-1 haploid meiosis, cells enter prophase of meiosis I. Genetic recombination, monitored on the chromosome III disome, occurs at levels similar to those seen in diploids, indicating that the level of exchange between homologs is an autonomous property of individual chromosomes and not dependent on exchange elsewhere in the genome. Exchange is then followed by a single meiosis II equational chromosome division. Recombination in spo13-1 haploids is blocked by the spo11-1 mutation, which also eliminates recombination between homologous chromosomes during conventional diploid meiosis. We conclude that Spo(+) haploids expressing both a and alpha mating-type information attempt a SPO13-dependent meiosis I division, and that this division, in the absence of paired homologous chromosomes, is responsible for the failure of such haploids to complete normal gametogenesis. Our observations support the conclusion that initiation and completion of meiosis II and spore formation are not dependent on either completion of meiosis I or the presence of a diploid chromosome complement.

A new mapping method employing a meiotic rec-mutant of yeast

A rapid new mapping method has been developed for localizing a dominant or recessive mutation to a particular chromosome of yeast. The procedure utilizes the ability of strains homozygous for the spo11-1 mutation to undergo chromosome segregation without appreciable recombination during sporulation. The level of sporulation in spo11-1/spo11-1 diploids is reduced and asci are often immature or abnormal in appearance; spore viability is less than 1%. The first step of the mapping procedure is the construction of a haploid spo11-1 strain carrying a recessive drug-resistance marker and the unmapped mutation(s). This strain is crossed to a set of three spo11-1 mapping tester strains containing, among them, a recessive marker on each chromosome. The resulting spo11-1/spo11-1 diploids are sporulated and plated on drug-containing medium. Viable meiotic products that express the drug-resistance marker due to chromosome haploidization are selectively recovered. These meiotic products are haploid for most, but generally not all, chromosomes. The level of disomy for individual chromosomes averages 19%. Each of the recessive chromosomal markers is expressed in approximately a third of the drug-resistant segregants. Ninety-eight percent of these segregants show no evidence of intergenic recombination. Thus, two markers located on the same chromosome, but on different homologs, are virtually never expressed in the same drug-resistant clone. The utility of this mapping procedure is demonstrated by confirming the chromosomal location of seven known markers, as well as by the assignment of a previously unmapped mutation, spo12-1, to chromosome VIII. In addition, the analysis of the products of spo11-1 meiosis indicates that several markers previously assigned to either chromosome XIV or chromosome XVII are actually on the same chromosome.

Recombinationless meiosis in Saccharomyces cerevisiae

We have utilized the single equational meiotic division conferred by the spo13-1 mutation of Saccharomyces cerevisiae (S. Klapholtz and R. E. Esposito, Genetics 96:589-611, 1980) as a technique to study the genetic control of meiotic recombination and to analyze the meiotic effects of several radiation-sensitive mutations (rad6-1, rad50-1, and rad52-1) which have been reported to reduce meiotic recombination (Game et al., Genetics 94:51-68, 1980); Prakash et al., Genetics 94:31-50, 1980). The spo13-1 mutation eliminates the meiosis I reductional segregation, but does not significantly affect other meiotic events (including recombination). Because of the unique meiosis it confers, the spo13-1 mutation provides an opportunity to recover viable meiotic products in a Rec- background. In contrast to the single rad50-1 mutant, we found that the double rad50-1 spo13-1 mutant produced viable ascospores after meiosis and sporulation. These spores were nonrecombinant: meiotic crossing-over was reduced at least 150-fold, and no increase in meiotic gene conversion was observed over mitotic background levels. The rad50-1 mutation did not, however, confer a Rec- phenotype in mitosis; rather, it increased both spontaneous crossing-over and gene conversion. The spore inviability conferred by the single rad6-1 and rad52-1 mutations was not eliminated by the presence of the spo13-1 mutation. Thus, only the rad50 gene has been unambiguously identified by analysis of viable meiotic ascospores as a component of the meiotic recombination system.

Recombination and chromosome segregation during the single division meiosis in SPO12-1 and SPO13-1 diploids

This paper reports a study of chromosome segregation and recombination during sporulation of spo12-1 and spo13-1 diploid strains of S. cerevisiae. These strains undergo a single division to form asci containing two diploid or near-diploid spores. The segregation of centromere-linked markers in the two-spored (dyad) products indicates that the division is generally equational. However, in a small percentage of the spo12-1 and spo13-1 cells, it appears that a meiosis I-like division occurs. Aberrant segregation of the MAT locus on chromosome III, yielding a monosomic and a trisomic spores pair, occurs in 12% of all dyads. The segregation patterns of markers at various distances from their centromeres and several pairs of markers on the same chromosome indicate that recombination takes place in both strains at nearly standard meiotic levels.

Isolation of SPO12-1 and SPO13-1 from a natural variant of yeast that undergoes a single meiotic division

ATCC4117 is a strain of S. cerevisiae that undergoes a single nuclear division during sporulation to produce asci containing two diploid ascopores (Grewal and Miller 1972). All clones derived from these spores are sporulation-capable and, like the parental strain, form two-spored asci. In this paper, we describe the genetic analysis of ATCC4117. In tetraploid hybrids of vegetative cells of the ATCC4117 diploid and a/a or alpha/alpha diploids, the production of two-spored asci is recessive. From these tetraploids, we have isolated two recessive alleles, designated spo12-1 and spo13-1, each of which alone results in the production of asci with two diploid or near-diploid spores. These alleles are unlinked and segregate as single nuclear genes. spo12-1 is approximately 22 cM from its centromere; spo13-1 has been localized to within 1 cM of arg4 on chromosome VIII. This analysis also revealed that ATCC4117 carries a diploidization gene allelic to or closely linked to HO, modifiers that reduce the number of haploid spores per ascus and alleles affecting the total level of sporulation.

The RAD52 gene is required for homothallic interconversion of mating types and spontaneous mitotic recombination in yeast

The rad52-1 mutation prevents homothallic mating type interconversion and reduces mitotic recombination in yeast. It has been previously reported that rad52-1 abolishes meiotic recombination. These data suggest either that a generalized recombination function(s) is required for mating type switching or that generalized recombination and specific homothallic functions are jointly controlled by the RAD52 gene. The rad52-1 mutation affects the interconversion of the two yeast mating types (a and alpha) differently, suggesting that the interconversion process is not equivalent for both mating types. This type of asymmetry is not predicted by current models of homothallic switching.

Meiosis in a temperature-sensitive DNA-synthesis mutant and in an apomictic yeast strain (Saccharomyces cerevisiae)

It is shown that in the temperature-sensitive yeast mutant (Saccharomyces cerevisiae) spo 11 at the restrictive temperature of 34 degrees C. (1) premeiotic DNA synthesis is nearly completely blocked; (2) the nucleus enters meiotic prophase indicated by the formation of axial cores and polysynaptonemal complexes; (3) the kinetic apparatus functions normally at meiosis I and II; (4) early spore formation occurs in nearly all cells but it is variable and all spores eventually degenerate. It is concluded that chromosome replication is not a prerequisite for the functions listed above. The apomictic yeast strain 4117 produces 2 diploid spores. It is shown that a diploid which produces 2-spored asci, synthesized from 4117, no. 5, and an adenine requiring strain (1) has a normal meiotic prophase with abundant synaptonemal complexes; (2) has only one meiotic spindle; (3) has spores which form red clones more frequently than normal or u.v.-treated vegetative cells form ade/ade red sectors through mitotic recombination. It is concluded that this apomictic yeast has maintained meiotic prophase, but that one of the two meiotic divisions is suppressed.

The effect of ochre suppression on meiosis and ascospore formation in Saccharomyces

The effect of altered tyrosyl-tRNAs on the developmental process of sporulation was examined. Mutations in eight independent loci resulting in tyrosine-inserting nonsense suppressor were tested for their effects on sporulation. Different levels of inhibition were found ranging from SUP3-omicron, which caused the greatest reduction of sporulation (7-17% of wild type), to SUP11-omicron which caused no reduction in sporulation. Since the SUP3-omicron mutation exhibited the greatest effect, it was studied in detail. Although SUP3-omicron is a dominant nonsense suppressor, its effect on sporulation is recessive. Expression of the sporulation deficiency is dependent upon the stage of transfer from glucose growth medium (i.e., log, early stationary, etc.) to sporulation medium. SUP3-omicron/SUP3-omicron diploid cells transferred from log or early stationary phase are capable of sporulation, whereas cells transferred after early stationary phase (i.e., after adaptation to respiration) exhibit poor sporulative ability. Sporulation events were examined under restrictive conditions to observe those events completed by SUP3-omicron/SUP3-omicron diploids. The early events of sporulation occur in these cells. Later events are completed by progressively fewer cells. Premeiotic DNA synthesis occurred in approximately 40% of the cells, nuclear segregation occurred in 20%, and finally, only 2% formed asci. The fact that fewer late-sporulation events occur under restrictive conditions can be explained by increased efficiency of suppression.

Antimutator activity during mitosis by a meiotic mutant of yeast

Diploids homozygous for the mutation spo7-1 do not exhibit net premeiotic DNA synthesis at 34 degrees C and are defective in commitment to recombination following exposure to sporulation medium. The spo7-1 mutation confers antimutator activity during mitosis at 34 degrees C, indicating that the SPO7 gene product is involved in both mitotic and premeiotic DNA metabolism. Strains bearing spo7-1 are slightly more sensitive to killing by ultraviolet light than the wild type but are proficient in UV induced mutation and mitotic intragenic recombination. The mitotic antimutator activity of spo7-1 is directed against a class of forward mutations known to occur more frequently during mitosis than meiosis.

Single-strand scissions of chromosomal DNA during commitment to recombination at meiosis

Diploid cells of the yeast Saccharomyces cerevisiae induced to undergo meiosis accumulate single-strand scissions in both template and newly synthesized DNA during commitment to genetic recombination. No evidence for accumulation of double-strand breaks during meiosis was obtained. When commitment to recombination is at the full meiotic level there are approximately 70 to 200 single-strand scissions per meiotic cell in which approximately 150 recombination events have been reported to occur.

Genetic recombination and commitment to meiosis in Saccharomyces

Diploid cells of the yeast Saccharomyces cerevisiae become committed to recombination at meiotic levels without becoming committed to the meiotic disjunction of chromosomes. These two events of the meiotic process can be separated by removing cells from a meiosis-inducing medium and returning them to a medium that promotes vegetative cell division. Cells removed at an appropriate time remain diploid, revert to mitosis, and display recombination with meiotic-like frequencies. Those removed after this time are committed to the completion of meiosis. Diploids of three conditional sporulation-deficient mutants (spo1-1, spo2-1, and spo3-1) have been examined for recombination at restrictive temperatures. All exhibit commitment to recombination without commitment to meiotic disjunction as in the wild type. Cells of spo1-1/spo1-1 do not replicate the spindle pole body for meiosis I; thus, recombination ability can be acquired by cells that do not proceed beyond this cytological stage.

Conditional mutants of meiosis in yeast

Three temperature-sensitive mutants, spo1-1, spo2-1, and spo3-1, were characterized with respect to their behavior in sporulation medium at a restrictive temperature. The time of expression of the functions defective in the mutants was determined by temperature-shift experiments during the sporulation process. In addition, each mutant was examined for the following: (i) its ability to undergo the nuclear divisions of meiosis; (ii) deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and protein synthesis; (iii) protein turnover; and (iv) colony-forming ability after exposure to sporulation medium. Mutant spo1-1 is defective in a function which confers a temperature-sensitive period which extends over 32% of the sporulation cycle. The temperature-sensitive period of mutant spo2-1 occupies 34% of the cycle, whereas the temperature-sensitive period of mutant spo3-1 extends over 2% of the sporulation cycle. Cytological evidence indicates that all three mutants initiate but do not complete the meiotic nuclear divisions. The DNA content of sporulation cultures of mutants spo1-1 and spo3-1 did not increase to the wild-type level; DNA synthesis in spo2-1 was normal. All three strains exhibit a loss of colony-forming ability during incubation in sporulation medium at the restrictive temperature. RNA and protein synthesis and protein turnover occur in the mutants.

Acetate utilization and macromolecular synthesis during sporulation of yeast

Acetate utilization and macromolecule synthesis during sporulation (meiosis) of Saccharomyces cerevisiae were studied. When diploid cells are transferred from glucose nutrient medium to acetate sporulation medium at early stationary phase, respiration of the exogenously supplied acetate proceeds without any apparent lag. At the completion of ascospore development, 62% of the acetate carbon consumed has been respired, 22% remains in the soluble pool, and 16% is incorporated into lipids, protein, nucleic acids, and other cell components. Measurements of the rate of protein synthesis during sporulation reveal two periods of maximal synthetic activity: an early phase coincidental with increases in deoxyribonucleic acid, ribonucleic acid, and protein cellular content and a later phase during ascospore formation. Experiments in which protein synthesis was inhibited at intervals during sporulation indicate that protein synthesis is required both for the initiation and completion of ascus development.