IME4, a gene that mediates MAT and nutritional control of meiosis in Saccharomyces cerevisiae

Combined analysis of expression data and transcription factor binding sites in the yeast genome

BACKGROUND

The analysis of gene expression using DNA microarrays provides genome wide profiles of the genes controlled by the presence or absence of a specific transcription factor. However, the question arises of whether a change in the level of transcription of a specific gene is caused by the transcription factor acting directly at the promoter of the gene or through regulation of other transcription factors working at the promoter.

RESULTS

To address this problem we have devised a computational method that combines microarray expression and site preference data. We have tested this approach by identifying functional targets of the a1-alpha2 complex, which represses haploid-specific genes in the yeast Saccharomyces cerevisiae. Our analysis identified many known or suspected haploid-specific genes that are direct targets of the a1-alpha2 complex, as well as a number of previously uncharacterized targets. We were also able to identify a number of haploid-specific genes which do not appear to be direct targets of the a1-alpha2 complex, as well as a1-alpha2 target sites that do not repress transcription of nearby genes. Our method has a much lower false positive rate when compared to some of the conventional bioinformatic approaches.

CONCLUSIONS

These findings show advantages of combining these two forms of data to investigate the mechanism of co-regulation of specific sets of genes.

Induction of sporulation in Saccharomyces cerevisiae leads to the formation of N6-methyladenosine in mRNA: a potential mechanism for the activity of the IME4 gene

N6-methyladenosine (m6A) is present at internal sites in mRNA isolated from all higher eukaryotes, but has not previously been detected in the mRNA of the yeast Saccharomyces cerevisiae. This nucleoside modification occurs only in a sequence- specific context that appears to be conserved across diverse species. The function of this modification is not fully established, but there is some indirect evidence that m6A may play a role in the efficiency of mRNA splicing, transport or translation. The S.cerevisiae gene IME4, which is important for induction of sporulation, is very similar to the human gene MT-A70, which has been shown to be a critical subunit of the human mRNA [N6-adenosine]-methyltransferase. This observation led to the hypothesis that yeast sporulation may be dependent upon methylation of yeast mRNA, mediated by Ime4p. In this study we show that induction of sporulation leads to the appearance of low levels of m6A in yeast mRNA and that this modification requires IME4. Moreover, single amino acid substitutions in the putative catalytic residues of Ime4p lead to severe sporulation defects in a strain whose sporulation ability is completely dependent on this protein. Collectively, these data suggest very strongly that the activation of sporulation by Ime4p is the result of its proposed methyltransferase activity and provide the most direct evidence to date of a physiologic role of m6A in a gene regulatory pathway.

Genomic evidence for a complete sexual cycle in Candida albicans

Candida albicans is a diploid fungus that has become a medically important opportunistic pathogen in immunocompromised individuals. We have sequenced the C. albicans genome to 10.4-fold coverage and performed a comparative genomic analysis between C. albicans and Saccharomyces cerevisiae with the objective of assessing whether Candida possesses a genetic repertoire that could support a complete sexual cycle. Analyzing over 500 genes important for sexual differentiation in S. cerevisiae, we find many homologues of genes that are implicated in the initiation of meiosis, chromosome recombination, and the formation of synaptonemal complexes. However, others are striking in their absence. C. albicans seems to have homologues of all of the elements of a functional pheromone response pathway involved in mating in S. cerevisiae but lacks many homologues of S. cerevisiae genes for meiosis. Other meiotic gene homologues in organisms ranging from filamentous fungi to Drosophila melanogaster and Caenorhabditis elegans were also found in the C. albicans genome, suggesting potential alternative mechanisms of genetic exchange.

Snf1 kinase connects nutritional pathways controlling meiosis in Saccharomyces cerevisiae

Glucose inhibits meiosis in Saccharomyces cerevisiae at three different steps (IME1 transcription, IME2 transcription, and entry into late stages of meiosis). Because many of the regulatory effects of glucose in yeast are mediated through the inhibition of Snf1 kinase, a component of the glucose repression pathway, we determined the role of SNF1 in regulating meiosis. Deleting SNF1 repressed meiosis at the same three steps that were inhibited by glucose, suggesting that glucose blocks meiosis by inhibiting Snf1. For example, the snf1Delta mutant completely failed to induce IME1 transcripts in sporulation medium. Furthermore, even when this block was bypassed by expression of IME1 from a multicopy plasmid, IME2 transcription and meiotic initiation occurred at only 10 to 20% of the levels seen in wild-type cells. The addition of glucose did not further inhibit IME2 transcription, suggesting that Snf1 is the primary mediator of glucose controls on IME2 expression. Finally, in snf1Delta cells in which both blocks on meiotic initiation were bypassed, early stages of meiosis (DNA replication and commitment to recombination) occurred, but later stages (chromosome segregation and spore formation) did not, suggesting that Snf1 controls later stages of meiosis independently from the two controls on meiotic initiation. Because Snf1 is known to activate the expression of genes required for acetate metabolism, it may also serve to connect glucose and acetate controls on meiotic differentiation.

Multiple and distinct activation and repression sequences mediate the regulated transcription of IME1, a transcriptional activator of meiosis-specific genes in Saccharomyces cerevisiae

IME1 encodes a transcriptional activator required for the transcription of meiosis-specific genes and initiation of meiosis in Saccharomyces cerevisiae. The transcription of IME1 is repressed in the presence of glucose, and a low basal level of IME1 RNA is observed in vegetative cultures with acetate as the sole carbon source. Upon nitrogen depletion a transient induction in the transcription of IME1 is observed in MATa/MATalpha diploids but not in MAT-insufficient strains. In this study we demonstrate that the transcription of IME1 is controlled by an extremely unusual large 5' region, over 2,100 bp long. This area is divided into four different upstream controlling sequences (UCS). UCS2 promotes the transcription of IME1 in the presence of a nonfermentable carbon source. UCS2 is flanked by three negative regions: UCS1, which exhibits URS activity in the presence of nitrogen, and UCS3 and UCS4, which repress the activity of UCS2 in MAT-insufficient cells. UCS2 consists of alternate positive and negative elements: three distinct constitutive URS elements that prevent the function of any upstream activating sequence (UAS) under all growth conditions, a constitutive UAS element that promotes expression under all growth conditions, a UAS element that is active only in vegetative media, and two discrete elements that function as UASs in the presence of acetate. Sequence analysis of IME1 revealed the presence of two almost identical 30- to 32-bp repeats. Surprisingly, one repeat, IREd, exhibits constitutive URS activity, whereas the other repeat, IREu, serves as a carbon-source-regulated UAS element. The RAS-cyclic AMP-dependent protein kinase cAPK pathway prevents the UAS activity of IREu in the presence of glucose as the sole carbon source, while the transcriptional activators Msn2p and Msn4p promote the UAS activity of this repeat in the presence of acetate. We suggest that the use of multiple negative and positive elements is essential to restrict transcription to the appropriate conditions and that the combinatorial effect of the entire region leads to the regulated transcription of IME1.

Stimulation of yeast meiotic gene expression by the glucose-repressible protein kinase Rim15p

The Saccharomyces cerevisiae RIM15 gene was identified previously through a mutation that caused reduced ability to undergo meiosis. We report here an analysis of the cloned RIM15 gene, which specifies a 1,770-residue polypeptide with homology to serine/threonine protein kinases. Rim15p is most closely related to Schizosaccharomyces pombe cek1+. Analysis of epitope-tagged derivatives indicates that Rim15p has autophosphorylation activity. Deletion of RIM15 causes reduced expression of several early meiotic genes (IME2, SPO13, and HOP1) and of IME1, which specifies an activator of early meiotic genes. However, overexpression of IME1 does not permit full expression of early meiotic genes in a rim15delta mutant. Ime1p activates early meiotic genes through its interaction with Ume6p, and analysis of Rim15p-dependent regulatory sites at the IME2 promoter indicates that activation through Ume6p is defective. Two-hybrid interaction assays suggest that Ime1p-Ume6p interaction is diminished in a rim15 mutant. Glucose inhibits Ime1p-Ume6p interaction, and we find that Rim15p accumulation is repressed in glucose-grown cells. Thus, glucose repression of Rim15p may be responsible for glucose inhibition of Ime1p-Ume6p interaction.

Nutritional regulation of late meiotic events in Saccharomyces cerevisiae through a pathway distinct from initiation

The IME1 gene is essential for initiation of meiosis in the yeast Saccharomyces cerevisiae, although it is not required for growth. Here we report that in stationary-phase cultures containing low concentration of glucose, cells overexpressing IME1 undergo the early meiotic events, including DNA replication, commitment to recombination, and synaptonemal complex formation and dissolution. In contrast, later meiotic events, such as chromosome segregation, commitment to meiosis, and spore formation, do not occur. Thus, nutrients can repress the late stages of meiosis independently of their block of initiation. Cells arrested at this midpoint in meiosis are relatively stable and can resume meiotic differentiation if transferred to sporulation conditions. Resumption of meiosis does not require repression of IME1 expression, since IME1 RNA levels stay high after transfer of the arrested cells to sporulation medium. These results suggest that meiosis in S. cerevisiae is a paradigm of a differentiation pathway regulated by signal transduction at both early and late stages.

A 15-base-pair element activates the SPS4 gene midway through sporulation in Saccharomyces cerevisiae

Sporulation of the yeast Saccharomyces cerevisiae represents a simple developmental process in which the events of meiosis and spore wall formation are accompanied by the sequential activation of temporally distinct classes of genes. In this study, we have examined expression of the SPS4 gene, which belongs to a group of genes that is activated midway through sporulation. We mapped the upstream boundary of the regulatory region of SPS4 by monitoring the effect of sequential deletions of 5'-flanking sequence on expression of plasmid-borne versions of SPS4 introduced into a MATa/MAT alpha delta sps4/delta sps4 strain. This analysis indicated that the 5' boundary of the regulatory region was within 50 bp of the putative TATA box of the gene. By testing various oligonucleotides that spanned this boundary and the downstream sequence for their ability to activate expression of a heterologous promoter, we found that a 15-bp sequence sufficed to act as a sporulation-specific upstream activation sequence. This 15-bp fragment, designated UASSPS4, activated expression of a CYC1-lacZ reporter gene midway through sporulation and was equally active in both orientations. Extending the UAS fragment to include the adjacent 14-bp enhanced its activity 10-fold. We show that expression of SPS4 is regulated in a manner distinct from that of early meiotic genes: mutation of UME6 did not lead to vegetative expression of SPS4, and sporulation-specific expression was delayed by mutation of IME2. In vivo and in vitro assays suggested that a factor present in vegetative cells bind to the UASSPS4 element. We speculate that during sporulation this factor is modified to serve as an activator of the SPS4 gene or, alternatively, that it recruits an activator to the promoter.

Analysis of RIM11, a yeast protein kinase that phosphorylates the meiotic activator IME1

Many yeast genes that are essential for meiosis are expressed only in meiotic cells. Known regulators of early meiotic genes include IME1, which is required for their expression, and SIN3 and UME6, which prevent their expression in nonmeiotic cells. We report here the molecular characterization of the RIM11 gene, which we find is required for expression of several early meiotic genes. A close functional relationship between RIM11 and IME1 is supported by two observations. First, sin3 and ume6 mutations are epistatic to rim11 mutations; prior studies have demonstrated their epistasis to ime1 mutations. Second, overexpression of RIM11 can suppress an ime1 missense mutation (ime1-L321F) but not an ime1 deletion. Sequence analysis indicates that RIM11 specifies a protein kinase related to rat glycogen synthase kinase 3 and the Drosophila shaggy/zw3 gene product. Three partially defective rim11 mutations alter residues involved in ATP binding or catalysis, and a completely defective rim11 mutation alters a tyrosine residue that corresponds to the site of an essential phosphorylation for glycogen synthase kinase 3. Immune complexes containing a hemagglutinin (HA) epitope-tagged RIM11 derivative, HA-RIM11, phosphorylate two proteins, p58 and p60, whose biological function is undetermined. In addition, HA-RIM11 immune complexes phosphorylate a functional IME1 derivative but not the corresponding ime1-L321F derivative. We propose that RIM11 stimulates meiotic gene expression through phosphorylation of IME1.

Analysis of RIM11, a yeast protein kinase that phosphorylates the meiotic activator IME1

Many yeast genes that are essential for meiosis are expressed only in meiotic cells. Known regulators of early meiotic genes include IME1, which is required for their expression, and SIN3 and UME6, which prevent their expression in nonmeiotic cells. We report here the molecular characterization of the RIM11 gene, which we find is required for expression of several early meiotic genes. A close functional relationship between RIM11 and IME1 is supported by two observations. First, sin3 and ume6 mutations are epistatic to rim11 mutations; prior studies have demonstrated their epistasis to ime1 mutations. Second, overexpression of RIM11 can suppress an ime1 missense mutation (ime1-L321F) but not an ime1 deletion. Sequence analysis indicates that RIM11 specifies a protein kinase related to rat glycogen synthase kinase 3 and the Drosophila shaggy/zw3 gene product. Three partially defective rim11 mutations alter residues involved in ATP binding or catalysis, and a completely defective rim11 mutation alters a tyrosine residue that corresponds to the site of an essential phosphorylation for glycogen synthase kinase 3. Immune complexes containing a hemagglutinin (HA) epitope-tagged RIM11 derivative, HA-RIM11, phosphorylate two proteins, p58 and p60, whose biological function is undetermined. In addition, HA-RIM11 immune complexes phosphorylate a functional IME1 derivative but not the corresponding ime1-L321F derivative. We propose that RIM11 stimulates meiotic gene expression through phosphorylation of IME1.

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.

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.

Mating type regulates the radiation-associated stimulation of reciprocal translocation events in Saccharomyces cerevisiae

Both ultraviolet (UV) and ionizing radiation were observed to stimulate mitotic, ectopic recombination between his3 recombinational substrates, generating reciprocal translocations in Saccharomyces cerevisiae (yeast). The stimulation was greatest in diploid strains competent for sporulation and depends upon both the ploidy of the strain and heterozygosity at the MATlocus. The difference in levels of stimulation between MATa/MAT alpha diploid and MAT alpha haploid strains increases when cells are exposed to higher levels of UV radiation (sevenfold at 150 J/m2), whereas when cells are exposed to higher levels of ionizing radiation (23.4 krad), only a twofold difference is observed. When the MAT alpha gene was introduced by DNA transformation into a MATa/mat alpha::LEU2+ diploid, the levels of radiation-induced ectopic recombination approach those obtained in a strain that is heterozygous at MAT. Conversely, when the MATa gene was introduced by DNA transformation into a MAT alpha haploid, no enhanced stimulation of ectopic recombination was observed when cells were irradiated with ionizing radiation but a threefold enhancement was observed when cells were irradiated with UV. The increase in radiation-stimulated ectopic recombination resulting from heterozygosity at MAT correlated with greater spontaneous ectopic recombination and higher levels of viability after irradiation. We suggest that MAT functions that have been previously shown to control the level of mitotic, allelic recombination (homolog recombination) also control the level of mitotic, radiation-stimulated ectopic recombination between short dispersed repetitive sequences on non-homologous chromosomes.

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.

Molecular characterization of the yeast meiotic regulatory gene RIM1

In the yeast Saccharomyces cerevisiae, genetic studies suggest that the RIM1 gene encodes a positive regulator of meiosis. rim1 mutations cause reduced expression of IME1, which is required for expression of many meiotic genes, and thus lead to a partial defect in meiosis and spore formation. We report the sequence of RIM1 and functional analysis of its coding region. The RIM1 gene product (RIM1) contains three regions similar to C2H2 zinc fingers. Serine substitutions for cysteine in each of the putative zinc fingers abolish RIM1 function. The carboxyl-terminus of RIM1 is enriched in acidic amino acids and is required for full RIM1 activity. RIM1 also contains two putative cAMP-dependent protein kinase (cAPK) phosphorylation sites. At one site, substitution of alanine for serine does not affect RIM1 activity; at the other site, this substitution impairs activity. This analysis of RIM1 suggests that the protein may function as a transcriptional activator. We have used the cloned RIM1 gene to create a complete rim1 deletion. This null allele, like previously isolated rim1 mutations, causes a partial meiotic defect. In addition to RIM1, maximum IME1 expression requires the MCK1 and IME4 gene products. Defects associated with rim1, mck1, and ime4 mutations in expression of a meiotic reporter gene (ime2-lacZ) and in sporulation are additive. These findings suggest that RIM1 acts independently of MCK1 and IME4 to stimulate IME1 expression.

Post-transcriptional regulation of IME1 determines initiation of meiosis in Saccharomyces cerevisiae

The IME1 gene of Saccharomyces cerevisiae is required for initiation of meiosis. Transcription of IME1 is detected under conditions which are known to induce initiation of meiosis, namely starvation for nitrogen and glucose, and the presence of MATa1 and MAT alpha 2 gene products. In this paper we show that IME1 is also subject to translational regulation. Translation of IME1 mRNA is achieved either upon nitrogen starvation, or upon G1 arrest. In the presence of nutrients, constitutively elevated transcription of IME1 is also sufficient for the translation of IME1 RNA. Four different conditions were found to cause expression of Ime1 protein in vegetative cultures: elevated transcription levels due to the presence of IME1 on a multicopy plasmid; elevated transcription provided by a Gal-IME1 construct; G1 arrest due to alpha-factor treatment; G1 arrest following mild heat-shock treatment of cdc28 diploids. Using these conditions, we obtained evidence that starvation is required not only for transcription and efficient translation of IME1, but also for either the activation of Ime1 protein or for the induction/activation of another factor that, either alone or in combination with Ime1, induces meiosis.