Accessibility of alpha 2-repressed promoters to the activator Gal4

Using synthetic bacterial enhancers to reveal a looping-based mechanism for quenching-like repression

We explore a model for 'quenching-like' repression by studying synthetic bacterial enhancers, each characterized by a different binding site architecture. To do so, we take a three-pronged approach: first, we compute the probability that a protein-bound dsDNA molecule will loop. Second, we use hundreds of synthetic enhancers to test the model's predictions in bacteria. Finally, we verify the mechanism bioinformatically in native genomes. Here we show that excluded volume effects generated by DNA-bound proteins can generate substantial quenching. Moreover, the type and extent of the regulatory effect depend strongly on the relative arrangement of the binding sites. The implications of these results are that enhancers should be insensitive to 10-11 bp insertions or deletions (INDELs) and sensitive to 5-6 bp INDELs. We test this prediction on 61 σ(54)-regulated qrr genes from the Vibrio genus and confirm the tolerance of these enhancers' sequences to the DNA's helical repeat.

The Cyc8-Tup1 complex inhibits transcription primarily by masking the activation domain of the recruiting protein

The yeast Tup1-Cyc8 corepressor complex is recruited to promoters by DNA-binding repressors, but the mechanisms by which it inhibits expression of genes involved in various stress pathways are poorly understood. Conditional and rapid depletion of Tup1 from the nucleus leads to concurrent nucleosome depletion and histone acetylation, recruitment of coactivators (Swi/Snf, SAGA, and Mediator), and increased transcriptional activity. Conversely, coactivator dissociation occurs rapidly upon rerepression by Cyc8-Tup1, although coactivator association and transcription can be blocked even in the absence of nucleosomes. The coactivators are recruited to the sites where Tup1 was located prior to depletion, indicating that the repressor proteins that recruit Tup1 function as activators in its absence. Last, Cyc8-Tup1 can interact with activation domains in vivo. Thus, Cyc8-Tup1 regulates transcription primarily by masking and inhibiting the transcriptional activation domains of the recruiting proteins, not by acting as a corepressor. We suggest that the corepressor function of Cyc8-Tup1 makes only a modest contribution to expression of target genes, specifically to keep expression levels below the nonactivated state.

Constitutive Promoter Occupancy by the MBF-1 Activator and Chromatin Modification of the Developmental Regulated Sea Urchin α-H2A Histone Gene

The tandemly repeated sea urchin alpha-histone genes are developmentally regulated. These genes are transcribed up to the early blastula stage and permanently silenced as the embryos approach gastrulation. As previously described, expression of the alpha-H2A gene depends on the binding of the MBF-1 activator to the 5' enhancer, while down-regulation relies on the functional interaction between the 3' sns 5 insulator and the GA repeats located upstream of the enhancer. As persistent MBF-1 binding and enhancer activity are detected in gastrula embryos, we have studied the molecular mechanisms that prevent the bound MBF-1 from trans-activating the H2A promoter at this stage of development. Here we used chromatin immunoprecipitation to demonstrate that MBF-1 occupies its site regardless of the transcriptional state of the H2A gene. In addition, we have mapped two nucleosomes specifically positioned on the enhancer and promoter regions of the repressed H2A gene. Interestingly, insertion of a 26 bp oligonucleotide between the enhancer and the TATA box, led to up-regulation of the H2A gene at gastrula stage, possibly by changing the position of the TATA nucleosome. Finally, we found association of histone de-acetylase and de-acetylation and methylation of K9 of histone H3 on the promoter and insulator of the repressed H2A chromatin. These data argue for a role of a defined positioned nucleosome in the promoter and histone tail post-translational modifications, in the 3' insulator and 5' regulatory regions, in the repression of the alpha-H2A gene despite the presence of the MBF-1 activator bound to the enhancer.

Effect of sequence-directed nucleosome disruption on cell-type-specific repression by alpha2/Mcm1 in the yeast genome

In Saccharomyces cerevisiae, a-cell-specific genes are repressed in MATalpha cells by alpha2/Mcm1, acting in concert with the Ssn6-Tup1 corepressors and the Isw2 chromatin remodeling complex, and nucleosome positioning has been proposed as one mechanism of repression. However, prior studies showed that nucleosome positioning is not essential for repression by alpha2/Mcm1 in artificial reporter plasmids, and the importance of the nucleosome positioning remains questionable. We have tested the function of positioned nucleosomes through alteration of genomic chromatin at the a-cell-specific gene BAR1. We report here that a positioned nucleosome in the BAR1 promoter is disrupted in cis by the insertion of diverse DNA sequences such as poly(dA) . poly(dT) and poly(dC-dG) . poly(dC-dG), leading to inappropriate partial derepression of BAR1. Also, we show that isw2 mutation causes loss of nucleosome positioning in BAR1 in MATalpha cells as well as partial disruption of repression. Thus, nucleosome positioning is required for full repression, but loss of nucleosome positioning is not sufficient to relieve repression completely. Even though disruption of nucleosome positioning by the cis- and trans-acting modulators of chromatin has a modest effect on the level of transcription, it causes significant degradation of the alpha-mating pheromone in MATalpha cells, thereby affecting its cell type identity. Our results illustrate a useful paradigm for analysis of chromatin structural effects at genomic loci.

Redundant mechanisms are used by Ssn6-Tup1 in repressing chromosomal gene transcription in Saccharomyces cerevisiae

The Ssn6-Tup1 corepressor complex regulates many genes in Saccharomyces cerevisiae. Three mechanisms have been proposed to explain its repression functions: 1) nucleosome positioning by binding histone tails; 2) recruitment of histone deacetylases; and 3) direct interference with the general transcription machinery or activators. It is unclear if Ssn6-Tup1 utilizes each of these mechanisms at a single gene in a redundant manner or each individually at different loci. A systematic analysis of the contribution of each mechanism at a native promoter has not been reported. Here we employed a genetic strategy to analyze the contributions of nucleosome positioning, histone deacetylation, and Mediator interference in the repression of chromosomal Tup1 target genes in vivo. We exploited the fact that Ssn6-Tup1 requires the ISW2 chromatin remodeling complex to establish nucleosome positioning in vivo to disrupt chromatin structure without affecting other Tup1 repression functions. Deleting ISW2, the histone deacetylase gene HDA1, or genes encoding Mediator subunits individually caused slight or no derepression of RNR3 and HUG1. However, when Mediator mutations were combined with Deltaisw2 or Deltahda1 mutations, enhanced transcription was observed, and the strongest level of derepression was observed in triple Deltaisw2/Deltahda1/Mediator mutants. The increased transcription in the mutants was not due to the loss of Tup1 at the promoter and correlated with increased TBP cross-linking to promoters. Thus, Tup1 utilizes multiple redundant mechanisms to repress transcription of native genes, which may be important for it to act as a global corepressor at a wide variety of promoters.

The essential transcription factor Reb1p interacts with the CLB2 UAS outside of the G2/M control region

Regulation of CLB2 is important both for completion of the normal vegetative cell cycle in Saccharomyces cerevisiae and for departure from the vegetative cell cycle upon nitrogen deprivation. Cell cycle-regulated transcription of CLB2 in the G2/M phase is known to be brought about by a set of proteins including Mcm1p, Fkh2/1p and Ndd1p that associate with a 35 bp G2/M-specific sequence common to a set of co-regulated genes. CLB2 transcription is regulated by additional signals, including by nitrogen levels, by positive feedback from the Clb2-Cdc28 kinase, and by osmotic stress, but the corresponding regulatory sequences and proteins have not been identified. We have found that the essential Reb1 transcription factor binds with high affinity to a sequence upstream of CLB2, within a region implicated previously by others in regulated expression, but upstream of the known G2/M-specific site. CLB2 sequence from the region around the Reb1p site blocks activation by the Gal4 protein when positioned downstream of the Gal4-binding site. Since a mutation in the Reb1p site abrogates this effect, we suggest that Reb1p is likely to occupy this site in vivo.

Recruitment of O-GlcNAc transferase to promoters by corepressor mSin3A: coupling protein O-GlcNAcylation to transcriptional repression

Transcription factors and RNA polymerase II can be modified by O-linked N-acetylglucosamine (O-GlcNAc) monosaccharides at serine or threonine residues, yet the precise functional roles of this modification are largely unknown. Here, we show that O-GlcNAc transferase (OGT), the enzyme that catalyzes this posttranslational modification, interacts with a histone deacetylase complex by binding to the corepressor mSin3A. Functionally, OGT and mSin3A cooperatively repress transcription in parallel with histone deacetylation. We propose that mSin3A targets OGT to promoters to inactivate transcription factors and RNA polymerase II by O-GlcNAc modification, which acts in concert with histone deacetylation to promote gene silencing in an efficient and specific manner.

Tup1p represses Mcm1p transcriptional activation and chromatin remodeling of an a-cell-specific gene

In yeast, a number of regulatory proteins expressed only in specific cell types interact with general transcription factors in a combinatorial manner to control expression of cell-type-specific genes. We report a detailed analysis of activation and repression events that occur at the promoter of the a-cell-specific STE6 gene fused to a beta-galactosidase gene in a yeast minichromosome, as well as factors that control the chromatin structure of this promoter both in the minichromosome and in the genomic STE6 locus. Mcm1p results in chromatin remodeling and is responsible for all transcriptional activity from the STE6 promoter in both wild-type a and alpha cells. Matalpha2p cooperates with Tup1p to block both chromatin remodeling and Mcm1p-associated activation. While Matalpha2p represses only Mcm1p, the Tup1p-mediated repression involves both Mcm1p-dependent and -independent mechanisms. Swi/Snf and Gcn5p, required for full induction of the STE6 gene, do not contribute to chromatin remodeling. We suggest that Tup1p can contribute to repression by blocking transcriptional activators, in addition to interacting with transcription machinery and stabilizing chromatin.

Turning genes off by Ssn6-Tup1: a conserved system of transcriptional repression in eukaryotes

The Ssn6-Tup1 repressor forms one of the largest and most important gene-regulatory circuits in budding yeast. This circuit, which appears conserved in flies, worms and mammals, exemplifies how a 'global' repressor (i.e. a repressor that regulates many genes in the cell) can be highly selective in the genes it represses. It also explains how, given the appropriate signal, specific subsets of these genes can be derepressed. Ssn6-Tup1 seems especially robust, bringing about a high level of repression irrespective of its precise placement on DNA or of specific features of the DNA control regions of its target genes. This high degree of repression probably results from several distinct mechanisms acting together.

Groucho/transducin-like enhancer of split (TLE) family members interact with the yeast transcriptional co-repressor SSN6 and mammalian SSN6-related proteins: implications for evolutionary conservation of transcription repression mechanisms

The yeast proteins TUP1 and SSN6 form a transcription repressor complex that is recruited to different promoters via pathway-specific DNA-binding proteins and regulates the expression of a variety of genes. TUP1 is functionally related to invertebrate and vertebrate transcriptional repressors of the Groucho/transducin-like Enhancer of split (TLE) family. The aim was to examine whether similar mechanisms underlie the transcription repression functions of TUP1 and Groucho/TLEs by determining whether TLE family members can interact with yeast SSN6 and mammalian SSN6-like proteins. It is shown in the present work that SSN6 binds to TLE1 and mediates transcriptional repression when expressed in mammalian cells. Moreover, TLE1 and TLE2 interact with two mammalian proteins related to SSN6, designated as the products of the ubiquitously transcribed tetratricopeptide-repeat genes on the Y (or X) chromosomes (UTY/X). These findings suggest that mammalian TLE and UTY/X proteins may mediate repression mechanisms similar to those performed by TUP1-SSN6 in yeast.

Yeast carbon catabolite repression

Glucose and related sugars repress the transcription of genes encoding enzymes required for the utilization of alternative carbon sources; some of these genes are also repressed by other sugars such as galactose, and the process is known as catabolite repression. The different sugars produce signals which modify the conformation of certain proteins that, in turn, directly or through a regulatory cascade affect the expression of the genes subject to catabolite repression. These genes are not all controlled by a single set of regulatory proteins, but there are different circuits of repression for different groups of genes. However, the protein kinase Snf1/Cat1 is shared by the various circuits and is therefore a central element in the regulatory process. Snf1 is not operative in the presence of glucose, and preliminary evidence suggests that Snf1 is in a dephosphorylated state under these conditions. However, the enzymes that phosphorylate and dephosphorylate Snf1 have not been identified, and it is not known how the presence of glucose may affect their activity. What has been established is that Snf1 remains active in mutants lacking either the proteins Grr1/Cat80 or Hxk2 or the Glc7 complex, which functions as a protein phosphatase. One of the main roles of Snf1 is to relieve repression by the Mig1 complex, but it is also required for the operation of transcription factors such as Adr1 and possibly other factors that are still unidentified. Although our knowledge of catabolite repression is still very incomplete, it is possible in certain cases to propose a partial model of the way in which the different elements involved in catabolite repression may be integrated.

Genomic footprinting of the yeast zinc finger protein Rme1p and its roles in repression of the meiotic activator IME1

The zinc finger protein Rme1p is a negative regulator of the meiotic activator IME1 in Saccharomyces cerevisiae . Prior studies have shown that Rme1p binds in vitro to a site near nt -2030 in the IME1 upstream region, but a genomic mutation in that site has little effect on repression of IME1 . To identify Rme1p binding sites in vivo , we have examined the binding of Rme1p to genomic sites through in vivo footprinting. We show that Rme1p binds to two sites in the IME1 upstream region, near nt -1950 and -2030. Mutations in both binding sites abolish repression of chromosomal IME1 by Rme1p, whereas a mutation in either single site causes partial derepression. Therefore, both Rme1p binding sites are essential for repression of IME1 . Prior studies have shown that repression by Rme1p depends upon RGR1 and SIN4 , which specify RNA polymerase II mediator subunits that are required for normal nucleosome density. We find that RGR1 and SIN4 are not simply required for Rme1p to bind to DNA in vivo . These results suggest that Rme1p functions directly as a repressor of IME1 and that Rgr1p and Sin4p are required for DNA-bound Rme1p to exert repression.

Gal4p-mediated chromatin remodeling depends on binding site position in nucleosomes but does not require DNA replication

Biochemical studies have demonstrated decreased binding of various proteins to DNA in nucleosome cores as their cognate sites are moved from the edge of the nucleosome to the pseudodyad (center). However, to date no study has addressed whether this structural characteristic of nucleosomes modulates the function of a transcription factor in living cells, where processes of DNA replication and chromatin modification or remodeling could significantly affect factor binding. Using a sensitive, high-resolution methyltransferase assay, we have monitored the ability of Gal4p in vivo to interact with a nucleosome at positions that are known to be inaccessible in nucleosome cores in vitro. Gal4p efficiently bound a single cognate site (UASG) centered at 41 bp from the edge of a positioned nucleosome, perturbing chromatin structure and inducing transcription. DNA binding and chromatin perturbation accompanying this interaction also occurred in the presence of hydroxyurea, indicating that DNA replication is not necessary for Gal4p-mediated nucleosome disruption. These data extend previous studies, which demonstrated DNA replication-independent chromatin remodeling, by showing that a single dimer of Gal4p, without the benefit of cooperative interactions that occur at complex wild-type promoters, is competent for invasion of a preestablished nucleosome. When the UASG was localized at the nucleosomal pseudodyad, relative occupancy by Gal4p, nucleosome disruption, and transcriptional activation were substantially compromised. Therefore, despite the increased nucleosome binding capability of Gal4p in cells, the precise translational position of a factor binding site in one nucleosome in an array can affect the ability of a transcriptional regulator to overcome the repressive influence of chromatin.

Functional relationships of Srb10-Srb11 kinase, carboxy-terminal domain kinase CTDK-I, and transcriptional corepressor Ssn6-Tup1

The Srb10-Srb11 protein kinase of Saccharomyces cerevisiae is a cyclin-dependent kinase (cdk)-cyclin pair which has been found associated with the carboxy-terminal domain (CTD) of RNA polymerase II holoenzyme forms. Previous genetic findings implicated the Srb10-Srb11 kinase in transcriptional repression. Here we use synthetic promoters and LexA fusion proteins to test the requirement for Srb10-Srb11 in repression by Ssn6-Tup1, a global corepressor. We show that srb10delta and srb11delta mutations reduce repression by DNA-bound LexA-Ssn6 and LexA-Tup1. A point mutation in a conserved subdomain of the kinase similarly reduced repression, indicating that the catalytic activity is required. These findings establish a functional link between Ssn6-Tup1 and the Srb10-Srb11 kinase in vivo. We also explored the relationship between Srb10-Srb11 and CTD kinase I (CTDK-I), another member of the cdk-cyclin family that has been implicated in CTD phosphorylation. We show that mutation of CTK1, encoding the cdk subunit, causes defects in transcriptional repression by LexA-Tup1 and in transcriptional activation. Analysis of the mutant phenotypes and the genetic interactions of srb10delta and ctk1A suggests that the two kinases have related but distinct roles in transcriptional control. These genetic findings, together with previous biochemical evidence, suggest that one mechanism of repression by Ssn6-Tup1 involves functional interaction with RNA polymerase II holoenzyme.

Genetics of transcriptional regulation in yeast: connections to the RNA polymerase II CTD

Transcriptional regulation is important in all eukaryotic organisms for cell growth, development, and responses to environmental change. Saccharomyces cerevisiae, or bakers' yeast, has provided a powerful system for genetic analysis of transcriptional regulation, and findings from the study of this model system have proven broadly applicable to higher organisms. Transcriptional regulation requires the interactions of regulatory proteins with various components of the transcription machinery. Recently, genetic analysis of a diverse set of transcriptional regulatory responses has converged with studies of the function of the RNA polymerase II carboxy-terminal domain (CTD) to reveal regulatory roles for proteins associated with the CTD. These proteins, designated Srb/mediator proteins, are broadly involved in both positive and negative regulatory responses in vivo. This review focuses on the connections between genetic analysis of transcriptional regulation and the functions of the Srb/mediator proteins associated with the RNA polymerase II CTD.

Amino termini of histones H3 and H4 are required for a1-alpha2 repression in yeast

The Saccharomyces cerevisiae alpha2 repressor controls two classes of cell-type-specific genes in yeast through association with different partners. alpha2-Mcm1 complexes repress a cell-specific gene expression in haploid alpha cells and diploid a/alpha cells, while a1-alpha2 complexes repress haploid-specific genes in diploid cells. In both cases, repression is mediated through Ssn6-Tu1 corepressor complexes that are recruited via direct interactions with alpha2. We have previously shown that nucleosomes are positioned adjacent to the alpha2-Mcm1 operator under conditions of repression and that Tupl interacts directly with histones H3 and H4. Here, we examine the role of chromatin in a1-alpha2 repression to determine if chromatin is a general feature of repression by Ssn6-Tup1. We find that mutations in the amino terminus of histone H4 cause a 4- to 11-fold derepression of a reporter gene under a1-alpha2 control, while truncation of the H3 amino terminus has a more modest (3-fold or less) effect. Strikingly, combination of the H3 truncation with an H4 mutation causes a 40-fold decrease in repression, clearly indicating a central role for these histones in a1-alpha2-mediated repression. However, in contrast to the ordered positioning of nucleosomes adjacent to the alpha2-Mcm1 operator, nucleosomes are not positioned adjacent to the a1-alpha2 operator in diploid cells. Our data indicate that chromatin is important to Ssn6-Tup1-mediated repression but that the degrees of chromatin organization directed by these proteins differ at different promoters.

Interplay of yeast global transcriptional regulators Ssn6p-Tup1p and Swi-Snf and their effect on chromatin structure

Transcriptional regulation in yeast involves a number of general trans-acting factors affecting chromatin structure. The Swi-Snf complex is required for expression of a large number of genes and has the ability to remodel chromatin in vitro. The Ssn6p-Tup1p repressor complex may be involved in chromatin organization through the interaction with pathway-specific DNA-binding proteins. To study the interplay of these factors and their effect on chromatin we have analyzed SUC2 chromatin structure in wild-type cells and in strains bearing combinations of ssn6/tup1 and swi1 mutations. We have mapped nucleosome positioning of the repressed gene in wild-type cells using primer extension methodology, allowing base pair resolution, and have analyzed details of chromatin remodeling in the derepressed state. In ssn6 or tup1 mutants under repressing conditions the observed changes in SUC2 chromatin structure may be suppressed by the swi1 mutation, suggesting that Ssn6p-Tup1p is not required for the establishment of nucleosome positioning at the SUC2 promoter. Our data indicate the involvement of chromatin remodeling factors distinct from the Swi-Snf complex in SUC2 transcriptional regulation and suggest that Swi-Snf may antagonize Ssn6p-Tup1p by controlling remodeling activity. We also show that a relatively high level of SUC2 transcription can coexist with positioned nucleosomes.

Cell type-specific chromatin organization of the region that governs directionality of yeast mating type switching

Switching of mating type in Saccharomyces cerevisiae is directional; MAT alpha cells recombine to transfer information from HMRa while MATa cells switch using the silent cassette at HML alpha. Genetic analysis recently has defined a 700 bp recombination enhancer approximately 29 kb from the left end of chromosome III that is necessary for directionality. The chromatin structure of this region differs strikingly in a- and alpha-cells. Mat alpha2p organizes a 3.7 kb chromatin domain that opposes interaction of trans-acting proteins with the enhancer. In a-cells lacking the alpha2 repressor, two footprinted regions flank an approximately 100 bp section having a unique DNA structure. This structural signature probably reflects interactions of proteins that result in directional mating type switching.

Transcriptional repression at a distance through exclusion of activator binding in vivo

The yeast repressor Rme1p acts from distant binding sites to block transcription of the chromosomal IME1 gene. Rme1p can also repress the heterologous CYC1 promoter when Rme1p binding sites are placed 250-300 bp upstream of CYC1 transcriptional activator binding sites (UAS1 and UAS2). Here, in vivo footprinting studies indicate that Rme1p acts over this distance by preventing the binding of the CYC1 transcriptional activators to UAS1 and UAS2. Inhibition of activator binding by Rme1p has the same genetic requirements as repression: both depend upon sequences flanking the Rme1p binding sites and upon Rgr1p and Sin4p, two subunits of the RNA polymerase II-associated Mediator complex that are required for normal nucleosome density. Thus Rme1p may alter chromatin to prevent binding of transcriptional activators to distant DNA sequences.