Swapping the gene-specific and regional silencing specificities of the Hst1 and Sir2 histone deacetylases

Sir2 and Hst1 are NAD(+)-dependent histone deacetylases of budding yeast that are related by strong sequence similarity. Nevertheless, the two proteins promote two mechanistically distinct forms of gene repression. Hst1 interacts with Rfm1 and Sum1 to repress the transcription of specific middle-sporulation genes. Sir2 interacts with Sir3 and Sir4 to silence genes contained within the silent-mating-type loci and telomere chromosomal regions. To identify the determinants of gene-specific versus regional repression, we created a series of Hst1::Sir2 hybrids. Our analysis yielded two dual-specificity chimeras that were able to perform both regional and gene-specific repression. Regional silencing by the chimeras required Sir3 and Sir4, whereas gene-specific repression required Rfm1 and Sum1. Our findings demonstrate that the nonconserved N-terminal region and two amino acids within the enzymatic core domain account for cofactor specificity and proper targeting of these proteins. These results suggest that the differences in the silencing and repression functions of Sir2 and Hst1 may not be due to differences in enzymatic activities of the proteins but rather may be the result of distinct cofactor specificities.

N-terminal arm of Mcm1 is required for transcription of a subset of genes involved in maintenance of the cell wall

The yeast Mcm1 protein is a member of the MADS box family of transcription factors that interacts with several cofactors to differentially regulate genes involved in cell-type determination, mating, cell cycle control and arginine metabolism. Residues 18 to 96 of the protein, which form the core DNA-binding domain of Mcm1, are sufficient to carry out many Mcm1-dependent functions. However, deletion of residues 2 to 17, which form the nonessential N-terminal (NT) arm, confers a salt-sensitive phenotype, suggesting that the NT arm is required for the activation of salt response genes. We used a strategy that combined information from the mutational analysis of the Mcm1-binding site with microarray expression data under salt stress conditions to identify a new subset of Mcm1-regulated genes. Northern blot analysis showed that the transcript levels of several genes encoding associated with the cell wall, especially YGP1, decrease significantly upon deletion of the Mcm1 NT arm. Deletion of the Mcm1 NT arm results in a calcofluor white-sensitive phenotype, which is often associated with defects in transcription of cell wall genes. In addition, the deletion makes cells sensitive to CaCl2 and alkaline pH. We found that the defect caused by removal of the NT arm is not due to changes in Mcm1 protein level, stability, DNA-binding affinity, or DNA bending. This suggests that residues 2 to 17 of Mcm1 may be involved in recruiting a cofactor to the promoters of these genes to activate transcription.

Analysis of the meiotic role of the mitochondrial ribosomal proteins Mrps17 and Mrpl37 in Saccharomyces cerevisiae

Sporulation in the yeast Saccharomyces cerevisiae is a complex and tightly regulated pathway that involves the induction of a large number of genes. We have identified MRPS17 in a cDNA library enriched for sporulation-specific genes. Homology searches show that the first one-third of Mrps17 has strong sequence similarity to bacterial S17 proteins, suggesting that Mrps17 is a potential mitochondrial ribosomal protein. This is further supported by the fact that mrps17Delta cells are respiratory-deficient and that a Mrps17-GFP fusion localizes to the mitochondria. We have confirmed by Northern blot analysis that both MRPS17 and MRPL37 are strongly induced during the middle stages of sporulation and that this induction is dependent on the presence of a middle sporulation element (MSE) in the promoters of these genes. Interestingly, we found that Mrps17 and Mrpl37, but not other mitochondrial ribosomal proteins, accumulate during the middle stages of sporulation. These results suggest that Mrps17 and Mrpl37 may have additional meiosis-specific roles.

Repression of the yeast HO gene by the MATalpha2 and MATa1 homeodomain proteins

The HO gene in Saccharomyces cerevisiae is regulated by a large and complex promoter that is similar to promoters in higher order eukaryotes. Within this promoter are 10 potential binding sites for the a1-alpha2 heterodimer, which represses HO and other haploid-specific genes in diploid yeast cells. We have determined that a1-alpha2 binds to these sites with differing affinity, and that while certain strong-affinity sites are crucial for repression of HO, some of the weak-affinity sites are dispensable. However, these weak-affinity a1-alpha2-binding sites are strongly conserved in related yeast species and have a role in maintaining repression upon the loss of strong-affinity sites. We found that these weak sites are sufficient for a1-alpha2 to partially repress HO and recruit the Tup1-Cyc8 (Tup1-Ssn6) co-repressor complex to the HO promoter. We demonstrate that the Swi5 activator protein is not bound to URS1 in diploid cells, suggesting that recruitment of the Tup1-Cyc8 complex by a1-alpha2 prevents DNA binding by activator proteins resulting in repression of HO.

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.

Characterization of critical interactions between Ndt80 and MSE DNA defining a novel family of Ig-fold transcription factors

The Ndt80 protein of the yeast Saccharomyces cerevisiae is the founding member of a new sub-family of proteins in the Ig-fold superfamily of transcription factors. The crystal structure of Ndt80 bound to DNA shows that it makes contacts through several loops on one side of the protein that connect beta-strands which form the beta-sandwich fold common to proteins in this superfamily. However, the DNA-binding domain of Ndt80 is considerably larger than many other members of the Ig-fold superfamily and it appears to make a larger number of contacts with the DNA than these proteins. To determine the contribution of each of these contacts and to examine if the mechanism of Ndt80 DNA binding was similar to other members of the Ig-fold superfamily, amino acid substitutions were introduced at each residue that contacts the DNA and assayed for their effect on Ndt80 activity. Many of the mutations caused significant decreases in DNA-binding affinity and transcriptional activation. Several of these are in residues that are not found in other sub-families of Ig-fold proteins. These additional contacts are likely responsible for Ndt80's ability to bind DNA as a monomer while most other members require additional domains or cofactors to recognize their sites.

Alpha1-induced DNA bending is required for transcriptional activation by the Mcm1-alpha1 complex

The yeast Mcm1 protein is a founding member of the MADS-box family of transcription factors that is involved in the regulation of diverse sets of genes through interactions with distinct cofactor proteins. Mcm1 interacts with the Matalpha1 protein to activate the expression of the alpha-cell type-specific genes. To understand the requirement of the cofactor alpha1 for Mcm1-alpha1-dependent transcriptional activation we analyzed the recruitment of Mcm1 to the promoters of alpha-specific genes in vivo and found that Mcm1 is able to bind to the promoters of alpha-specific genes in the absence of alpha1. This suggests the function of alpha1 is more complex than simply recruiting Mcm1. Several MADS-box transcription factors, including Mcm1, induce DNA bending and there is evidence the proper bend may be required for transcriptional activation. We analyzed Mcm1-dependent bending of a Mcm1-alpha1 binding site in the presence and absence of alpha1 and found that Mcm1 alone shows a reduced DNA-bend at this site compared with other Mcm1 binding sites. However, the addition of alpha1 markedly increases the DNA-bend and we present evidence this bend is required for full transcriptional activation. These results support a model in which proper DNA-bending by the Mcm1-alpha1 complex is required for transcriptional activation.

Sfp1 plays a key role in yeast ribosome biogenesis

Sfp1, an unusual zinc finger protein, was previously identified as a gene that, when overexpressed, imparted a nuclear localization defect. sfp1 cells have a reduced size and a slow growth phenotype. In this study we show that SFP1 plays a role in ribosome biogenesis. An sfp1 strain is hypersensitive to drugs that inhibit translational machinery. sfp1 strains also have defects in global translation as well as defects in rRNA processing and 60S ribosomal subunit export. Microarray analysis has previously shown that ectopically expressed SFP1 induces the transcription of a large subset of genes involved in ribosome biogenesis. Many of these induced genes contain conserved promoter elements (RRPE and PAC). Our results show that activation of transcription from a reporter construct containing two RRPE sites flanking a single PAC element is SFP1 dependent. However, we have been unable to detect direct binding of the protein to these elements. This suggests that regulation of genes containing RRPEs is dependent upon Sfp1 but that Sfp1 may not directly bind to these conserved promoter elements; rather, activation may occur through an indirect mechanism.

Depletion of H2A-H2B dimers in Saccharomyces cerevisiae triggers meiotic arrest by reducing IME1 expression and activating the BUB2-dependent branch of the spindle checkpoint

In the yeast Saccharomyces cerevisiae, diploid strains carrying homozygous hta1-htb1Delta mutations express histone H2A-H2B dimers at a lower level than do wild-type cells. Although this mutation has only minor effects on mitotic growth, it causes an arrest in sporulation prior to the first meiotic division. In this report, we show that the hta1-htb1Delta mutant exhibits reduced expression of early and middle-sporulation-specific genes and that the meiotic arrest of the hta1-htb1Delta mutant can be partially bypassed by overexpression of IME1. Additionally, deletions of BUB2 or BFA1, components of one branch of the spindle checkpoint pathway, bypass the meiotic arrest. Mutations in the other branch of the pathway or in the pachytene checkpoint are unable to suppress the meiotic block. These observations indicate that depletion of the H2A-H2B dimer blocks sporulation by at least two mechanisms: disruption of the expression of meiotic regulatory genes and activation of the spindle checkpoint. Our results show that the failure to progress through the meiotic pathway is not the result of global chromosomal alterations but that specific aspects of meiosis are sensitive to depletion of the H2A-H2B dimer.

Sum1 and Ndt80 proteins compete for binding to middle sporulation element sequences that control meiotic gene expression

A key transition in meiosis is the exit from prophase and entry into the nuclear divisions, which in the yeast Saccharomyces cerevisiae depends upon induction of the middle sporulation genes. Ndt80 is the primary transcriptional activator of the middle sporulation genes and binds to a DNA sequence element termed the middle sporulation element (MSE). Sum1 is a transcriptional repressor that binds to MSEs and represses middle sporulation genes during mitosis and early sporulation. We demonstrate that Sum1 and Ndt80 have overlapping yet distinct sequence requirements for binding to and acting at variant MSEs. Whole-genome expression analysis identified a subset of middle sporulation genes that was derepressed in a sum1 mutant. A comparison of the MSEs in the Sum1-repressible promoters and MSEs from other middle sporulation genes revealed that there are distinct classes of MSEs. We show that Sum1 and Ndt80 compete for binding to MSEs and that small changes in the sequence of an MSE can yield large differences in which protein is bound. Our results provide a mechanism for differentially regulating the expression of middle sporulation genes through the competition between the Sum1 repressor and the Ndt80 activator.

Rfm1, a novel tethering factor required to recruit the Hst1 histone deacetylase for repression of middle sporulation genes

Transcriptional repression is often correlated with the alteration of chromatin structure through modifications of the nucleosomes in the promoter region, such as by deacetylation of the N-terminal histone tails. This is presumed to make the promoter region inaccessible to other regulatory factors and the general transcription machinery. To accomplish this, histone deacetylases are recruited to specific promoters via DNA-binding proteins and tethering factors. We have previously reported the requirement for the NAD(+)-dependent histone deacetylase Hst1 and the DNA-binding protein Sum1 for vegetative repression of many middle sporulation genes in Saccharomyces cerevisiae. Here we report the identification of a novel tethering factor, Rfm1, that is required for Hst1-mediated repression. Rfm1 interacts with both Sum1 and Hst1 and is required for the Sum1-Hst1 interaction. DNA microarray and Northern blot analyses showed that Rfm1 is required for repression of the same subset of Sum1-repressed genes that require Hst1. These results suggest that Rfm1 is a specificity factor that targets the Hst1 deacetylase to a subset of Sum1-regulated genes.

Crystallographic studies of a novel DNA-binding domain from the yeast transcriptional activator Ndt80

The Ndt80 protein is a transcriptional activator that plays a key role in the progression of the meiotic divisions in the yeast Saccharomyces cerevisiae. Ndt80 is strongly induced during the middle stages of the sporulation pathway and binds specifically to a promoter element called the MSE to activate transcription of genes required for the meiotic divisions. Here, the preliminary structural and functional studies to characterize the DNA-binding activity of this protein are reported. Through deletion analysis and limited proteolysis studies of Ndt80, a novel 32 kDa DNA-binding domain that is sufficient for DNA-binding in vitro has been defined. Crystals of the DNA-binding domain of Ndt80 in two distinct lattices have been obtained, for which diffraction data extend to 2.3 A resolution.

Crystal structure of the DNA-binding domain from Ndt80, a transcriptional activator required for meiosis in yeast

Ndt80 is a transcriptional activator required for meiosis in the yeast Saccharomyces cerevisiae. Here, we report the crystal structure at 2.3 A resolution of the DNA-binding domain of Ndt80 experimentally phased by using the anomalous and isomorphous signal from a single ordered Se atom per molecule of 272-aa residues. The structure reveals a single approximately 32-kDa domain with a distinct fold comprising a beta-sandwich core elaborated with seven additional beta-sheets and three short alpha-helices. Inspired by the structure, we have performed a mutational analysis and defined a DNA-binding motif in this domain. The DNA-binding domain of Ndt80 is homologous to a number of proteins from higher eukaryotes, and the residues that we have shown are required for DNA binding by Ndt80 are highly conserved among this group of proteins. These results suggest that Ndt80 is the defining member of a previously uncharacterized family of transcription factors, including the human protein (C11orf9), which has been shown to be highly expressed in invasive or metastatic tumor cells.

Swapping functional specificity of a MADS box protein: residues required for Arg80 regulation of arginine metabolism

Arg80 and Mcm1, two members of the MADS box family of DNA-binding proteins, regulate the metabolism of arginine in association with Arg81, the arginine sensor. In spite of the high degree of sequence conservation between the MADS box domains of the Arg80 and Mcm1 proteins (56 of 81 amino acids), these domains are not interchangeable. To determine which amino acids define the specificity of Arg80, we swapped the amino acids in each secondary-structure element of the Arg80 MADS box domain with the corresponding amino acids of Mcm1 and assayed the ability of these chimeras to regulate arginine-metabolic genes in place of the wild-type Arg80. Also performed was the converse experiment in which each variant residue in the Mcm1 MADS box domain was swapped with the corresponding residue of Arg80 in the context of an Arg80-Mcm1 fusion protein. We show that multiple regions of Arg80 are important for its function. Interestingly, the residues which have important roles in determining the specificity of Arg80 are not those which could contact the DNA but are residues that are likely to be involved in protein interactions. Many of these residues are clustered on one side of the protein, which could serve as an interface for interaction with Arg81 or Mcm1. This interface is distinct from the region used by the Mcm1 and human serum response factor MADS box proteins to interact with their cofactors. It is possible that this alternative interface is used by other MADS box proteins to interact with their cofactors.

Interactions of the Mcm1 MADS box protein with cofactors that regulate mating in yeast

The yeast Mcm1 protein is a member of the MADS box family of transcriptional regulatory factors, a class of DNA-binding proteins that control numerous cellular and developmental processes in yeast, Drosophila melanogaster, plants, and mammals. Although these proteins bind DNA on their own, they often combine with different cofactors to bind with increased affinity and specificity to their target sites. To understand how this class of proteins functions, we have made a series of alanine substitutions in the MADS box domain of Mcm1 and examined the effects of these mutations in combination with its cofactors that regulate mating in yeast. Our results indicate which residues of Mcm1 are essential for viability and transcriptional regulation with its cofactors in vivo. Most of the mutations in Mcm1 that are lethal affect DNA-binding affinity. Interestingly, the lethality of many of these mutations can be suppressed if the MCM1 gene is expressed from a high-copy-number plasmid. Although many of the alanine substitutions affect the ability of Mcm1 to activate transcription alone or in combination with the alpha 1 and Ste12 cofactors, most mutations have little or no effect on Mcm1-mediated repression in combination with the alpha 2 cofactor. Even nonconservative amino acid substitutions of residues in Mcm1 that directly contact alpha 2 do not significantly affect repression. These results suggest that within the same region of the Mcm1 MADS box domain, there are different requirements for interaction with alpha 2 than for interaction with either alpha1 or Ste12. Our results suggest how a small domain, the MADS box, interacts with multiple cofactors to achieve specificity in transcriptional regulation and how subtle differences in the sequences of different MADS box proteins can influence the interactions with specific cofactors while not affecting the interactions with common cofactors.

Structural and thermodynamic characterization of the DNA binding properties of a triple alanine mutant of MATalpha2

Triply mutated MATalpha2 protein, alpha2-3A, in which all three major groove-contacting residues are mutated to alanine, is defective in binding DNA alone or in complex with Mcm1 yet binds with MATa1 with near wild-type affinity and specificity. To gain insight into this unexpected behavior, we determined the crystal structure of the a1/alpha2-3A/DNA complex. The structure shows that the triple mutation causes a collapse of the alpha2-3A/DNA interface that results in a reorganized set of alpha2-3A/DNA contacts, thereby enabling the mutant protein to recognize the wild-type DNA sequence. Isothermal titration calorimetry measurements reveal that a much more favorable entropic component stabilizes the a1/alpha2-3A/DNA complex than the alpha2-3A/DNA complex. The combined structural and thermodynamic studies provide an explanation of how partner proteins influence the sequence specificity of a DNA binding protein.

Engineered improvements in DNA-binding function of the MATa1 homeodomain reveal structural changes involved in combinatorial control

We have engineered enhanced DNA-binding function into the a1 homeodomain by making changes in a loop distant from the DNA-binding surface. Comparison of the free and bound a1 structures suggested a mechanism linking van der Waals stacking changes in this loop to the ordering of a final turn in the DNA-binding helix of a1. Inspection of the protein sequence revealed striking differences in amino acid identity at positions 24 and 25 compared to related homeodomain proteins. These positions lie in the loop connecting helix-1 and helix-2, which is involved in heterodimerization with the alpha 2 protein. A series of single and double amino acid substitutions (a1-Q24R, a1-S25Y, a1-S25F and a1-Q24R/S25Y) were engineered, expressed and purified for biochemical and biophysical study. Calorimetric measurements and HSQC NMR spectra confirm that the engineered variants are folded and are equally or more stable than the wild-type a1 homeodomain. NMR analysis of a1-Q24R/S25Y demonstrates that the DNA recognition helix (helix-3) is extended by at least one turn as a result of the changes in the loop connecting helix-1 and helix-2. As shown by EMSA, the engineered variants bind DNA with enhanced affinity (16-fold) in the absence of the alpha 2 cofactor and the variant alpha 2/a1 heterodimers bind cognate DNA with specificity and affinity reflective of the enhanced a1 binding affinity. Importantly, in vivo assays demonstrate that the a1-Q24R/S25Y protein binds with fivefold greater affinity than wild-type a1 and is able to partially suppress defects in repression by alpha 2 mutants. As a result of these studies, we show how subtle differences in residues at a surface distant from the functional site code for a conformational switch that allows the a1 homeodomain to become active in DNA binding in association with its cofactor alpha 2.

Altering the DNA-binding specificity of the yeast Matalpha 2 homeodomain protein

Homeodomain proteins are a highly conserved class of DNA-binding proteins that are found in virtually every eukaryotic organism. The conserved mechanism that these proteins use to bind DNA suggests that there may be at least a partial DNA recognition code for this class of proteins. To test this idea, we have investigated the sequence-specific requirements for DNA binding and repression by the yeast alpha2 homeodomain protein in association with its cofactors, Mcm1 and Mata1. We have determined the contribution for each residue in the alpha2 homeodomain that contacts the DNA in the co-crystal structures of the protein. We have also engineered mutants in the alpha2 homeodomain to alter the DNA-binding specificity of the protein. Although we were unable to change the specificity of alpha2 by making substitutions at residues 47, 54, and 55, we were able to alter the DNA-binding specificity by making substitutions at residue 50 in the homeodomain. Since other homeodomain proteins show similar changes in specificity with substitutions at residue 50, this suggests that there is at least a partial DNA recognition code at this position.

Localization and signaling of G(beta) subunit Ste4p are controlled by a-factor receptor and the a-specific protein Asg7p

Haploid yeast cells initiate pheromone signaling upon the binding of pheromone to its receptor and activation of the coupled G protein. A regulatory process termed receptor inhibition blocks pheromone signaling when the a-factor receptor is inappropriately expressed in MATa cells. Receptor inhibition blocks signaling by inhibiting the activity of the G protein beta subunit, Ste4p. To investigate how Ste4p activity is inhibited, its subcellular location was examined. In wild-type cells, alpha-factor treatment resulted in localization of Ste4p to the plasma membrane of mating projections. In cells expressing the a-factor receptor, alpha-factor treatment resulted in localization of Ste4p away from the plasma membrane to an internal compartment. An altered version of Ste4p that is largely insensitive to receptor inhibition retained its association with the membrane in cells expressing the a-factor receptor. The inhibitory function of the a-factor receptor required ASG7, an a-specific gene of previously unknown function. ASG7 RNA was induced by pheromone, consistent with increased inhibition as the pheromone response progresses. The a-factor receptor inhibited signaling in its liganded state, demonstrating that the receptor can block the signal that it initiates. ASG7 was required for the altered localization of Ste4p that occurs during receptor inhibition, and the subcellular location of Asg7p was consistent with its having a direct effect on Ste4p localization. These results demonstrate that Asg7p mediates a regulatory process that blocks signaling from a G protein beta subunit and causes its relocalization within the cell.

Transcriptional regulation of meiosis in yeast

The genes required for meiosis and sporulation in yeast are expressed at specific points in a highly regulated temporal pathway. Recent experiments using DNA microarrays to examine gene expression during meiosis and the identification of many regulatory factors have provided important advances in our understanding of how genes are regulated at the different stages of meiosis.

Scanning mutagenesis of Mcm1: residues required for DNA binding, DNA bending, and transcriptional activation by a MADS-box protein

MCM1 is an essential gene in the yeast Saccharomyces cerevisiae and is a member of the MADS-box family of transcriptional regulatory factors. To understand the nature of the protein-DNA interactions of this class of proteins, we have made a series of alanine substitutions in the DNA-binding domain of Mcm1 and examined the effects of these mutations in vivo and in vitro. Our results indicate which residues of Mcm1 are important for viability, transcriptional activation, and DNA binding and bending. Substitution of residues in Mcm1 which are highly conserved among the MADS-box proteins are lethal to the cell and abolish DNA binding in vitro. These positions have almost identical interactions with DNA in both the serum response factor-DNA and alpha2-Mcm1-DNA crystal structures, suggesting that these residues make up a conserved core of protein-DNA interactions responsible for docking MADS-box proteins to DNA. Substitution of residues which are not as well conserved among members of the MADS-box family play important roles in contributing to the specificity of DNA binding. These results suggest a general model of how MADS-box proteins recognize and bind DNA. We also provide evidence that the N-terminal extension of Mcm1 may have considerable conformational freedom, possibly to allow binding to different DNA sites. Finally, we have identified two mutants at positions which are critical for Mcm1-mediated DNA bending that have a slow-growth phenotype. This finding is consistent with our earlier results, indicating that DNA bending may have a role in Mcm1 function in the cell.

Sum1 and Hst1 repress middle sporulation-specific gene expression during mitosis in Saccharomyces cerevisiae

Meiotic development in yeast is characterized by the sequential induction of temporally distinct classes of genes. Genes that are induced at the middle stages of the pathway share a promoter element, termed the middle sporulation element (MSE), which interacts with the Ndt80 transcriptional activator. We have found that a subclass of MSEs are strong repressor sites during mitosis. SUM1 and HST1, genes previously associated with transcriptional silencing, are required for MSE-mediated repression. Sum1 binds specifically in vitro to MSEs that function as strong repressor sites in vivo. Repression by Sum1 is gene specific and does not extend to neighboring genes. These results suggest that mechanisms used to silence large regions of chromatin may also be used to regulate the expression of specific genes during development. NDT80 is regulated during mitosis by both the Sum1 and Ume6 repressors. These results suggest that progression through sporulation may be controlled by the regulated competition between the Sum1 repressor and Ndt80 activator at key MSEs.

Identification of target sites of the alpha2-Mcm1 repressor complex in the yeast genome

The alpha2 and Mcm1 proteins bind DNA as a heterotetramer to repress transcription of cell-type-specific genes in the yeast Saccharomyces cerevisiae. Based on the DNA sequence requirements for binding by the alpha2-Mcm1 complex, we have searched the yeast genome for all potential alpha2-Mcm1 binding sites. Genes adjacent to the sites were examined for expression in the different cell mating types. These sites were further analyzed by cloning the sequences into a heterologous promoter and assaying for alpha2-Mcm1-dependent repression in vivo and DNA-binding affinity in vitro. Fifty-nine potential binding sites were identified in the search. Thirty-seven sites are located within or downstream of coding region of the gene. None of the sites assayed from this group are functional repressor sites in vivo or bound by the alpha2-Mcm1 complex in vitro. Among the remaining 22 sites, six are in the promoters of known alpha-specific genes and two other sites have an alpha2-Mcm1-dependent role in determining the direction of mating type switching. Among the remaining sequences, we have identified a functional site located in the promoter region of a previously uncharacterized gene, SCYJL170C. This site functions to repress transcription of a heterologous promoter and the alpha2-Mcm1 complex binds to the site in vitro. SCYJL170C is repressed by alpha2-Mcm1 in vivo and therefore using this method we have identified a new a-specific gene, which we call ASG7.

The yeast a1 and alpha2 homeodomain proteins do not contribute equally to heterodimeric DNA binding

In diploid cells of the yeast Saccharomyces cerevisiae, the alpha2 and a1 homeodomain proteins bind cooperatively to sites in the promoters of haploid cell-type-specific genes (hsg) to repress their expression. Although both proteins bind to the DNA, in the alpha2 homeodomain substitutions of residues that are involved in contacting the DNA have little or no effect on repression in vivo or cooperative DNA binding with a1 protein in vitro. This result brings up the question of the contribution of each protein in the heterodimer complex to the DNA-binding affinity and specificity. To determine the requirements for the a1-alpha2 homeodomain DNA recognition, we systematically introduced single base-pair substitutions in an a1-alpha2 DNA-binding site and examined their effects on repression in vivo and DNA binding in vitro. Our results show that nearly all substitutions that significantly decrease repression and DNA-binding affinity are at positions which are specifically contacted by either the alpha2 or a1 protein. Interestingly, an alpha2 mutant lacking side chains that make base-specific contacts in the major groove is able to discriminate between the wild-type and mutant DNA sites with the same sequence specificity as the wild-type protein. These results suggest that the specificity of alpha2 DNA binding in complex with a1 does not rely solely on the residues that make base-specific contacts. We have also examined the contribution of the a1 homeodomain to the binding affinity and specificity of the complex. In contrast to the lack of a defective phenotype produced by mutations in the alpha2 homeodomain, many of the alanine substitutions of residues in the a1 homeodomain have large effects on a1-alpha2-mediated repression and DNA binding. This result shows that the two proteins do not make equal contributions to the DNA-binding affinity of the complex.

Crystal structure of the MATa1/MATalpha2 homeodomain heterodimer in complex with DNA containing an A-tract

The crystal structure of the heterodimer formed by the DNA binding domains of the yeast mating type transcription factors, MATa1 and MATalpha2, bound to a 21 bp DNA fragment has been determined at 2.5 A resolution. The DNA fragment in the present study differs at four central base pairs from the DNA sequence used in the previously studied ternary complex. These base pair changes give rise to a (dA5).(dT5) tract without changing the overall base composition of the DNA. The resulting A-tract occurs near the center of the overall 60 degrees bend in the DNA. Comparison of the two structures shows that the structural details of the DNA bend are maintained despite the DNA sequence changes. Analysis of the A5-tract DNA subfragment shows that it contains a bend toward the minor groove centered at one end of the A-tract. The observed bend is larger than that observed in the crystal structures of A-tracts embedded in uncomplexed DNA, which are straight and have been presumed to be quite rigid. Variation of the central DNA base sequence reverses the two AT base pairs contacted in the minor groove by Arg7 of the alpha2 N-terminal arm without significantly altering the DNA binding affinity of the a1/alpha2 heterodimer. The Arg7 side chain accommodates the sequence change by forming alternate H bond interactions, in agreement with the proposal that minor groove base pair recognition is insensitive to base pair reversal. Furthermore, the minor groove spine of hydration, which stabilizes the narrowed minor groove caused by DNA bending, is conserved in both structures. We also find that many of the water-mediated hydrogen bonds between the a1 and alpha2 homeodomains and the DNA are highly conserved, indicating an important role for water in stabilization of the a1/alpha2-DNA complex.

Transcriptional regulation of the SMK1 mitogen-activated protein kinase gene during meiotic development in Saccharomyces cerevisiae

Meiotic development (sporulation) in Saccharomyces cerevisiae is characterized by an ordered pattern of gene expression, with sporulation-specific genes classified as early, middle, mid-late, or late depending on when they are expressed. SMK1 encodes a mitogen-activated protein kinase required for spore morphogenesis that is expressed as a middle sporulation-specific gene. Here, we identify the cis-acting DNA elements that regulate SMK1 transcription and characterize the phenotypes of mutants with altered expression patterns. The SMK1 promoter contains an upstream activating sequence (UASS) that specifically interacts with the transcriptional activator Abf1p. The Abf1p-binding sites from the early HOP1 and the middle SMK1 promoters are functionally interchangeable, demonstrating that these elements do not play a direct role in their differential transcriptional timing. Timing of SMK1 expression is determined by another cis-acting DNA sequence termed MSE (for middle sporulation element). The MSE is required not only for activation of SMK1 transcription during middle sporulation but also for its repression during vegetative growth and early meiosis. In addition, the SMK1 MSE can repress vegetative expression in the context of the HOP1 promoter and convert HOP1 from an early to a middle gene. SMK1 function is not contingent on its tight transcriptional regulation as a middle sporulation-specific gene. However, promoter mutants with different quantitative defects in SMK1 transcript levels during middle sporulation show distinct sporulation phenotypes.

Homeodomain-DNA interactions of the Pho2 protein are promoter-dependent

The homeodomain (HD) is a conserved sequence-specific DNA-binding motif found in many eukaryotic transcriptional regulatory proteins. Despite the wealth of in vitro data on the mechanism HD proteins use to bind DNA, comparatively little is known about the roles of individual residues in these domains in vivo . The Saccharomyces cerevisiae Pho2 protein contains a HD that shares significant sequence identity with the Drosophila Engrailed protein. We have used the co-crystal structure of Engrailed as a model to predict how Pho2 might contact DNA and have examined how individual residues of the Pho2 HD contribute to transcriptional activation in vivo and to DNA binding in vitro. Our results demonstrate that Pho2 and Engrailed share many similar DNA-binding characteristics. However, our results also show that some highly conserved residues, which contact the DNA in many HD structures, make relatively small contributions to the DNA-binding affinity and in vivo activity of the Pho2 protein. We also show that the N-terminal arm of the Pho2 HD is a critical component in determining the DNA-binding specificity of the protein and that the requirements for residues in the N-terminal arm are promoter-dependent for Pho2 transcriptional activation and DNA binding.

Alpha2p controls donor preference during mating type interconversion in yeast by inactivating a recombinational enhancer of chromosome III

Homothallic strains of Saccharomyces cerevisiae can change mating type as often as every generation by replacing the allele at the MAT locus with a copy of mating type information present at one of two storage loci, HML and HMR, located on either end of chromosome III. Selection of the appropriate donor locus is dictated by a mating type-specific repressor protein, alpha2p: Cells containing alpha2p select HMR, whereas those lacking alpha2p select HML. As a repressor protein, alpha2p binds to DNA cooperatively with the transcriptional activator Mcm1p. Here we show that two alpha2p/Mcm1p-binding sites, DPS1 and DPS2, control donor selection. DPS1 and DPS2 are located approximately 30 kb from the left arm of chromosome III, well removed from HML, HMR, and MAT. Precise deletion of only DPS1 and DPS2 results in random selection of donor loci and in a cells without affecting selection in alpha cells. Reciprocally, deletion of only the alpha2p binding segments in each of these two sites results in selection of the wrong donor loci in alpha cells without affecting preference in a cells. These results suggest that Mcm1p, bound to these two sites in the absence of alpha2p, activates HML as donor. Binding of alpha2p blocks the ability of Mcm1p bound to DPS1 and DPS2 to activate HML, resulting in default selection of HMR as donor. DPS1 and DPS2 also regulate expression of several noncoding RNAs, although deletion of at least one of these RNA loci does not affect donor preference. This suggests that transcriptional activation, rather than transcription of a specific product, is the initiating event in activating the left arm of chromosome III for donor selection.

Analysis of a meiosis-specific URS1 site: sequence requirements and involvement of replication protein A

URS1 is a transcriptional repressor site found in the promoters of a wide variety of yeast genes that are induced under stress conditions. In the context of meiotic promoters, URS1 sites act as repressor sequences during mitosis and function as activator sites during meiosis. We have investigated the sequence requirements of the URS1 site of the meiosis-specific HOP1 gene (URS1H) and have found differences compared with a URS1 site from a nonmeiotic gene. We have also observed that the sequence specificity for meiotic activation at this site differs from that for mitotic repression. Base pairs flanking the conserved core sequence enhance meiotic induction but are not required for mitotic repression of HOP1. Electrophoretic mobility shift assays of mitotic and meiotic cell extracts show a complex pattern of DNA-protein complexes, suggesting that several different protein factors bind specifically to the site. We have determined that one of the complexes of URS1H is formed by replication protein A (RPA). Although RPA binds to the double-stranded URS1H site in vitro, it has much higher affinity for single-stranded than for double-stranded URS1H, and one-hybrid assays suggest that RPA does not bind to this site at detectable levels in vivo. In addition, conditional-lethal mutations in RPA were found to have no effect on URS1H-mediated repression. These results suggest that although RPA binds to URS1H in vitro, it does not appear to have a functional role in transcriptional repression through this site in vivo.

DNA-binding specificity of Mcm1: operator mutations that alter DNA-bending and transcriptional activities by a MADS box protein

The yeast Mcm1 protein is a member of the MADS box family of transcriptional regulatory factors, a class of DNA-binding proteins found in such diverse organisms as yeast, plants, flies, and humans. To explore the protein-DNA interactions of Mcm1 in vivo and in vitro, we have introduced an extensive series of base pair substitutions into an Mcm1 operator site and examined their effects on Mcm1-mediated transcriptional regulation and DNA-binding affinity. Our results show that Mcm1 uses a mechanism to contact the DNA that has some significant differences from the one used by the human serum response factor (SRF), a closely related MADS box protein in which the three-dimensional structure has been determined. One major difference is that 5-bromouracil-mediated photo-cross-linking experiments indicate that Mcm1 is in close proximity to functional groups in the major groove at the center of the recognition site whereas the SRF protein did not exhibit this characteristic. A more significant difference is that mutations at a position outside of the conserved CC(A/T)6GG site significantly reduce Mcm1-dependent DNA bending, while these substitutions have no effect on DNA bending by SRF. This result shows that the DNA bending by Mcm1 is sequence dependent and that the base-specific requirements for bending differ between Mcm1 and SRF. Interestingly, although these substitutions have a large effect on DNA bending and transcriptional activation by Mcm1, they have a relatively small effect on the DNA-binding affinity of the protein. This result suggests that the degree of DNA bending is important for transcriptional activation by Mcm1.

The yeast homeodomain protein MATalpha2 shows extended DNA binding specificity in complex with Mcm1

The MATalpha2 (alpha2) repressor interacts with the Mcm1 protein to turn off a-cell type-specific genes in the yeast Saccharomyces cerevisiae. We compared five natural alpha2-Mcm1 sites with an alpha2-Mcm1 symmetric consensus site (AMSC) for their relative strength of repression and found that the AMSC functions slightly better than any of the natural sites. To further investigate the DNA binding specificity of alpha2 in complex with Mcm1, symmetric substitutions at each position in the alpha2 half-sites of AMSC were constructed and assayed for their effect on repression in vivo and DNA binding affinity in vitro. As expected, substitutions at positions in which there are base-specific contacts decrease the level of repression. Interestingly, substitutions at other positions, in which there are no apparent base-specific contacts made by the protein in the alpha2-DNA co-crystal structure, also significantly decrease repression. As an alternative method to examining the DNA binding specificity of alpha2, we performed in vitro alpha2 binding site selection experiments in the presence and absence of Mcm1. In the presence of Mcm1, the consensus sequences obtained were extended and more closely related to the natural alpha2 sites than the consensus sequence obtained in the absence of Mcm1. These results demonstrate that in the presence of Mcm1 the sequence specificity of alpha2 is extended to these positions.

Protein interactions of homeodomain proteins

Homeodomain proteins are conserved DNA-binding factors that are involved in the transcriptional regulation of key developmental processes. Many homeodomain proteins require additional cofactors to bind with high affinity and specificity to their DNA sites. The recent structural determinations and biochemical analysis of several multimeric complexes have provided a better understanding of how protein interactions influence the DNA-binding activity of homeodomain proteins.

Participation of the yeast activator Abf1 in meiosis-specific expression of the HOP1 gene

The meiosis-specific gene HOP1, which encodes a component of the synaptonemal complex, is controlled through two regulatory elements, UASH and URS1H. Sites similar to URS1H have been identified in the promoter region of virtually every early meiosis-specific gene, as well as in many promoters of nonmeiotic genes, and it has been shown that the proteins that bind to this site function to regulate meiotic and nonmeiotic transcription. Sites similar to the UASH site have been found in a number of meiotic and nonmeiotic genes as well. Since it has been shown that UASH functions as an activator site in vegetative haploid cells, it seemed likely that the factors binding to this site regulate both meiotic and nonmeiotic transcription. We purified the factor binding to the UASH element of the HOP1 promoter. Sequence analysis identified the protein as Abf1 (autonomously replicating sequence-binding factor 1), a multifunctional protein involved in DNA replication, silencing, and transcriptional regulation. We show by mutational analysis of the UASH site, that positions outside of the proposed UASH consensus sequence (TNTGN[A/T]GT) are required for DNA binding in vitro and transcriptional activation in vivo. A new UASH consensus sequence derived from this mutational analysis closely matches a consensus Abf1 binding site. We also show that an Abf1 site from a nonmeiotic gene can replace the function of the UASH site in the HOP1 promoter. Taken together, these results show that Abf1 functions to regulate meiotic gene expression.

The yeast alpha2 and Mcm1 proteins interact through a region similar to a motif found in homeodomain proteins of higher eukaryotes

Homeodomain proteins are transcriptional regulatory factors that, in general, bind DNA with relatively low sequence specificity and affinity. One mechanism homeodomain proteins use to increase their biological specificity is through interactions with other DNA-binding proteins. We have examined how the yeast (Saccharomyces cerevisiae) homeodomain protein alpha2 specifically interacts with Mcm1, a MADS box protein, to bind DNA specifically and repress transcription. A patch of predominantly hydrophobic residues within a region preceding the homeodomain of alpha2 has been identified that specifies direct interaction with Mcm1 in the absence of DNA. This hydrophobic patch is required for cooperative DNA binding with Mcm1 in vitro and for transcriptional repression in vivo. We have also found that a conserved motif, termed YPWM, frequently found in homeodomain proteins of insects and mammals, partially functions in place of the patch in alpha2 to interact with Mcm1. These findings suggest that homeodomain proteins from diverse organisms may use analogous interaction motifs to associate with other proteins to achieve high levels of DNA binding affinity and specificity.

Altered DNA recognition and bending by insertions in the alpha 2 tail of the yeast a1/alpha 2 homeodomain heterodimer

The yeast MAT alpha 2 and MATa1 homeodomain proteins bind cooperatively as a heterodimer to sites upstream of haploid-specific genes, repressing their transcription. In the crystal structure of alpha 2 and a1 bound to DNA, each homeodomain makes independent base-specific contacts with the DNA and the two proteins contact each other through an extended tail region of alpha 2 that tethers the two homeodomains to one another. Because this extended region may be flexible, the ability of the heterodimer to discriminate among DNA sites with altered spacing between alpha 2 and a1 binding sites was examined. Spacing between the half sites was critical for specific DNA binding and transcriptional repression by the complex. However, amino acid insertions in the tail region of alpha 2 suppressed the effect of altering an a1/alpha 2 site by increasing the spacing between the half sites. Insertions in the tail also decreased DNA bending by a1/alpha 2. Thus tethering the two homeodomains contributes to DNA bending by a1/alpha 2, but the precise nature of the resulting bend is not essential for repression.

A homeo domain protein lacking specific side chains of helix 3 can still bind DNA and direct transcriptional repression

A series of mutations in the homeo domain of the yeast alpha 2 protein were constructed to test, both in vivo and in vitro, predictions based on the alpha 2-DNA cocrystal structure described by Wolberger et al. (1991). The effects of the mutations were observed in three different contexts using authentic target DNA sequences: alpha 2 binding alone to specific DNA, alpha 2 binding cooperatively with MCM1 to specific DNA, and alpha 2 binding cooperatively with a1 to specific DNA. As expected, changes in the amino acid residues that contact DNA in the X-ray structure severely compromised the ability of alpha 2 to bind DNA alone and to bind DNA cooperatively with MCM1. In contrast, many of these same mutations, including a triple change that altered all the "recognition" residues of helix 3, had little or no effect on the cooperative binding of alpha 2 and a1 to specific DNA, as determined both in vivo and in vitro. These results show that the ability of a homeo domain protein to correctly select and repress target genes does not necessarily depend on the residues commonly implicated in sequence-specific DNA binding.

A short, disordered protein region mediates interactions between the homeodomain of the yeast alpha 2 protein and the MCM1 protein

Homeodomains are folded into a characteristic three-dimensional structure capable of recognizing DNA in a sequence-specific manner. We show that correct target site selection by the yeast alpha 2 protein requires, as well as its homeodomain, an adjacent short and apparently unstructured region of the protein. This flexible homeodomain extension is responsible for specifying an interaction with a second regulatory protein, MCM1, which permits the cooperative binding of the two proteins to an operator. Two additional experiments suggest that this extension-homeodomain arrangement is likely to have some generality. First, when the extension of alpha 2 is grafted onto the Drosophila engrailed homeodomain, it yields a protein with the DNA binding specificity of engrailed and the ability to bind cooperatively to DNA with MCM1. Second, the alpha 2 extension specifies interaction not only with the yeast MCM1 protein, but also with the related human protein SRF.

Meiotic induction of the yeast HOP1 gene is controlled by positive and negative regulatory sites

The process of meiosis and sporulation in the yeast Saccharomyces cerevisiae is a highly regulated developmental pathway dependent on genetic as well as nutritional signals. The HOP1 gene, which encodes a component of meiotic chromosomes, is not expressed in mitotically growing cells, but its transcription is induced shortly after yeast cells enter the meiotic pathway. Through a series of deletions and mutations in the HOP1 promoter, we located two regulatory sites that are essential for proper regulation of HOP1. One site, called URS1H, brings about repression of HOP1 in mitotic cells and functions as an activator sequence in cells undergoing meiosis. The second site, which we designated UASH, acts as an activator sequence in meiotic cells and has similarity to the binding site of the mammalian CCAAT/enhancer binding protein (C/EBP). Both sites are required for full meiotic induction of the HOP1 promoter. We conclude that in mitotic yeast cells, the URS1H site maintains the repressed state of the HOP1 promoter, masking the effect of the UASH site. Upon entry into meiosis, repression is lifted, allowing the URS1H and UASH sites to activate high-level transcription.

Crystal structure of a MAT alpha 2 homeodomain-operator complex suggests a general model for homeodomain-DNA interactions

The MAT alpha 2 homeodomain regulates the expression of cell type-specific genes in yeast. We have determined the 2.7 A resolution crystal structure of the alpha 2 homeodomain bound to a biologically relevant DNA sequence. The DNA in this complex is contacted primarily by the third of three alpha-helices, with additional contacts coming from an N-terminal arm. Comparison of the yeast alpha 2 and the Drosophila engrailed homeodomain-DNA complexes shows that the protein fold is highly conserved, despite a 3-residue insertion in alpha 2 and only 27% sequence identity between the two homeodomains. Moreover, the orientation of the recognition helix on the DNA is also conserved. This docking arrangement is maintained by side chain contacts with the DNA--primarily the sugar-phosphate backbone--that are identical in alpha 2 and engrailed. Since these residues are conserved among all homeodomains, we propose that the contacts with the DNA are also conserved and suggest a general model for homeodomain-DNA interactions.

Secondary structure of the homeo domain of yeast alpha 2 repressor determined by NMR spectroscopy

The yeast alpha 2 protein is a regulator of cell type in Saccharomyces cerevisiae. It represses transcription of a set of target genes by binding to an operator located upstream of each of these genes. The alpha 2 protein shares weak sequence similarity with members of the homeo domain family; the homeo domain is a 60-amino-acid segment found in many eukaryotic transcriptional regulators. In this paper we address the question of whether alpha 2 is structurally related to prototypical members of the homeo domain family. We used solution 1H and 15N nuclear magnetic resonance [NMR] spectroscopy to determine the secondary structure of an 83-amino-acid residue fragment of alpha 2 that contains the homeo domain homology. We have obtained resonance assignments for the backbone protons and nitrogens of the entire 60-residue region of the putative homeo domain and for most of the remainder of the alpha 2 fragment. The secondary structure was determined by using NOE connectivities between backbone protons, 3JHN-H alpha coupling constants, and dynamical information from the hydrogen exchange kinetics of the backbone amides. Three helical segments exist in the alpha 2 fragment consisting of residues 11-23, 32-42, and 46-60 (corresponding to residues 138-150, 159-169, and 173-187 of the intact protein). The positions of these three helices correspond extremely well to those of the Drosophila Antennapedia (Antp) and engrailed (en) homeo domains, whose three-dimensional structures have recently been determined by NMR spectroscopy and X-ray crystallography, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization and preliminary X-ray diffraction studies of a MAT alpha 2-DNA complex

Crystals have been obtained of the DNA-binding domain of the yeast MAT alpha 2 repressor bound to a 21 base-pair DNA site. The crystals are grown from polyethylene glycol and CaCl2 and form in space group P2(1) with a = 60.1 A, b = 39.4 A, c = 68.7 A and beta = 98 degrees. They diffract to 2.9 A resolution and contain one protein-DNA complex in the crystallographic asymmetric unit.

NMR studies of Arc repressor mutants: proton assignments, secondary structure, and long-range contacts for the thermostable proline-8----leucine variant of Arc

Arc repressor is a 53-residue sequence-specific DNA binding protein. We report the assignment of the proton NMR spectrum and the secondary structure for the thermostable PL8 variant of Arc. This mutant, which differs from wild type by a Pro-8----Leu substitution, was chosen for study because its enhanced stability allows spectra to be acquired at elevated temperatures where spectral resolution is higher. The first five residues of the protein play important roles in DNA binding but appear to be disordered in solution. Residues 6-14 form the remaining part of the N-terminal DNA binding region of the protein and assume an antiparallel beta-conformation. This indicates that Arc is a member of a new class of DNA binding proteins. The observed interresidue nuclear Overhauser effects are consistent with a beta-strand, gamma-turn, beta-strand structure for the residue 6-14 region, although other structures are also consistent with the data. The remaining portion of the protein is predominantly alpha-helical. Residues 16-26 and 35-50 form amphipathic alpha-helices which may pack together in a four-helix bundle in the protein dimer.

The Arc and Mnt repressors. A new class of sequence-specific DNA-binding protein

Genetic, biochemical, and biophysical studies have begun to reveal details of the structures of Arc and Mnt and show that these repressors use residues at their N-terminal ends for operator recognition and binding. Some of the DNA contacts made by these residues have been identified, and this information together with NMR studies has permitted the construction of models of the DNA binding region. Although the accuracy of these models remains to be determined, it seems clear that Arc and Mnt are members of a new class of DNA-binding proteins.

Sequence-specific binding of arc repressor to DNA. Effects of operator mutations and modifications

A set of arc operators with transition and/or transversion mutations at each operator base pair has been constructed. By determining the ability of Arc to bind these variant operators, the importance of each base pair for Arc recognition has been assessed. Methylation protection experiments have also been used to probe points of close contact between Arc and most of the mutant operators. These data, together with phosphate interference results obtained previously for the wild type operator, provide information about the operator surface that is contacted when Arc binds.

Interaction of the bacteriophage P22 Arc repressor with operator DNA

Are repressor binds to a single, partially symmetric, 21 base-pair operator site that is centered between the -10 and -35 regions of the Pant promoter. Protection and interference experiments show that Arc makes contacts with the operator on one side of the DNA helix. Although Arc is a small protein (53 residues/subunit), it makes contacts that are farther from the center of the operator than those made by many larger repressors. These extended contacts include the phosphate groups at the ends of the 21 base-pair site. Under standard conditions (pH 7.5, 100 mM-KCl, 3 mM-MgCl2, 22 degrees C) half-maximal operator binding is observed at an Arc concentration of 2.5 X 10(-9) M and the protein-DNA complex is very stable (t1/2 approximately equal to 80 min).

Bacteriophage P22 Mnt repressor. DNA binding and effects on transcription in vitro

We have examined the binding of Mnt repressor to operator DNA in vitro and have determined how this binding affects the level of transcription from two nearby promoters, Pant and Pmnt. Mnt binds to a region of DNA that overlaps the startpoint of transcription of Pant and the -35 region of Pmnt. Mnt represses transcription in vitro from Pant and enhances transcription from Pmnt. Protection and interference experiments show that Mnt binds to a single, 17 base-pair operator site. The operator sequence and the protein-DNA contacts are symmetric. Mnt makes major groove contacts on both faces of the operator DNA. At pH 7.5, 200 mM-KCl, 22 degrees C, the Mnt tetramer binds operator with high affinity (Kd = 2.2 X 10(-11M) and the protein-DNA complex is quite stable (t1/2 = 48 min). Operator binding shows large dependencies on pH, salt concentration, and temperature.

Isolation and analysis of arc repressor mutants: evidence for an unusual mechanism of DNA binding

We have isolated 64 different missense mutations at 36 out of 53 residue positions in the Arc repressor of bacteriophage P22. Many of the mutant proteins with substitutions in the C-terminal 40 residues of Arc have reduced intracellular levels and probably have altered structures or stabilities. Mutations in the N-terminal ten residues of Arc cause large decreases in operator DNA binding affinity without affecting the ability of Arc to fold into a stable three-dimensional structure. We argue that these N-terminal residues are important for operator recognition but that they are not part of a conventional helix-turn-helix DNA binding structure. These results suggest that Arc may use a new mechanism for sequence specific DNA binding.

The bacteriophage P22 arc and mnt repressors. Overproduction, purification, and properties

The arc and mnt genes of bacteriophage P22 encode small repressor proteins. We have cloned these genes onto plasmids that overproduce Arc and Mnt to greater than 1% of the soluble cellular protein. Both proteins were purified to greater than 95% homogeneity, and N-terminal sequences and amino acid compositions were determined. These data, in combination with previously determined gene sequences, establish the complete protein sequences for Arc (53 residues) and Mnt (82 residues). Both proteins have melting temperatures between 45 and 55 degrees C and can be renatured to a fully active species. Arc is a dimer in solution and Mnt is a tetramer.