Indirect recognition in sequence-specific DNA binding by Escherichia coli integration host factor: the role of DNA deformation energy

Read-through transcription of tRNA underlies the cell cycle-dependent dissociation of IHF from the DnaA-inactivating sequence datA

Timely initiation of chromosomal DNA replication in Escherichia coli is achieved by cell cycle-coordinated regulation of the replication origin, oriC, and the replication initiator, ATP-DnaA. Cellular levels of ATP-DnaA increase and peak at the time for initiation at oriC, after which hydrolysis of DnaA-bound ATP causes those to fall, yielding initiation-inactive ADP-DnaA. This hydrolysis is facilitated by the chromosomal locus datA located downstream of the tRNA-Gly (glyV-X-Y) operon, which possesses a cluster of DnaA-binding sequences and a single binding site (IBS) for the DNA bending protein IHF (integration host factor). While IHF binding activates the datA function and is regulated to occur specifically at post-initiation time, the underlying regulatory mechanisms remain obscure. Here, we demonstrate that datA-IHF binding at pre-initiation time is down-regulated depending on the read-through transcription of datA IBS initiated at the glyV-X-Y promoter. During the cell cycle, the level of read-through transcription, but not promoter activity, fluctuated in a manner inversely related to datA-IHF binding. Transcription from the glyV-X-Y promoter was predominantly interrupted at datA IBS by IHF binding. The terminator/attenuator sequence of the glyV-X-Y operon, as well as DnaA binding within datA overall, contributed to attenuation of transcription upstream of datA IBS, preserving the timely fluctuation of read-through transcription. These findings provide a mechanistic insight of tRNA transcription-dependent datA-IHF regulation, in which an unidentified factor is additionally required for the timely datA-IHF dissociation, and support the significance of datA for controlling the cell cycle progression as a connecting hub of tRNA production and replication initiation.

IHF and Fis as Escherichia coli Cell Cycle Regulators: Activation of the Replication Origin oriC and the Regulatory Cycle of the DnaA Initiator

This review summarizes current knowledge about the mechanisms of timely binding and dissociation of two nucleoid proteins, IHF and Fis, which play fundamental roles in the initiation of chromosomal DNA replication in Escherichia coli. Replication is initiated from a unique replication origin called oriC and is tightly regulated so that it occurs only once per cell cycle. The timing of replication initiation at oriC is rigidly controlled by the timely binding of the initiator protein DnaA and IHF to oriC. The first part of this review presents up-to-date knowledge about the timely stabilization of oriC-IHF binding at oriC during replication initiation. Recent advances in our understanding of the genome-wide profile of cell cycle-coordinated IHF binding have revealed the oriC-specific stabilization of IHF binding by ATP-DnaA oligomers at oriC and by an initiation-specific IHF binding consensus sequence at oriC. The second part of this review summarizes the mechanism of the timely regulation of DnaA activity via the chromosomal loci DARS2 (DnaA-reactivating sequence 2) and datA. The timing of replication initiation at oriC is controlled predominantly by the phosphorylated form of the adenosine nucleotide bound to DnaA, i.e., ATP-DnaA, but not ADP-ADP, is competent for initiation. Before initiation, DARS2 increases the level of ATP-DnaA by stimulating the exchange of ADP for ATP on DnaA. This DARS2 function is activated by the site-specific and timely binding of both IHF and Fis within DARS2. After initiation, another chromosomal locus, datA, which inactivates ATP-DnaA by stimulating ATP hydrolysis, is activated by the timely binding of IHF. A recent study has shown that ATP-DnaA oligomers formed at DARS2-Fis binding sites competitively dissociate Fis via negative feedback, whereas IHF regulation at DARS2 and datA still remains to be investigated. This review summarizes the current knowledge about the specific role of IHF and Fis in the regulation of replication initiation and proposes a mechanism for the regulation of timely IHF binding and dissociation at DARS2 and datA.

Elucidation of Sequence-Function Relationships for an Improved Biobutanol In Vivo Biosensor in E. coli

Transcription factor (TF)–promoter pairs have been repurposed from native hosts to provide tools to measure intracellular biochemical production titer and dynamically control gene expression. Most often, native TF–promoter systems require rigorous screening to obtain desirable characteristics optimized for biotechnological applications. High-throughput techniques may provide a rational and less labor-intensive strategy to engineer user-defined TF–promoter pairs using fluorescence-activated cell sorting and deep sequencing methods (sort-seq). Based on the designed promoter library’s distribution characteristics, we elucidate sequence–function interactions between the TF and DNA. In this work, we use the sort-seq method to study the sequence–function relationship of a σ⁵⁴-dependent, butanol-responsive TF–promoter pair, BmoR-P_BMO derived from Thauera butanivorans, at the nucleotide level to improve biosensor characteristics, specifically an improved dynamic range. Activities of promoters from a mutagenized P_BMO library were sorted based on gfp expression and subsequently deep sequenced to correlate site-specific sequences with changes in dynamic range. We identified site-specific mutations that increase the sensor output. Double mutant and a single mutant, CA(129,130)TC and G(205)A, in P_BMO promoter increased dynamic ranges of 4-fold and 1.65-fold compared with the native system, respectively. In addition, sort-seq identified essential sites required for the proper function of the σ⁵⁴-dependent promoter biosensor in the context of the host. This work can enable high-throughput screening methods for strain development.

Negative feedback for DARS2-Fis complex by ATP-DnaA supports the cell cycle-coordinated regulation for chromosome replication

In Escherichia coli, the replication initiator DnaA oscillates between an ATP- and an ADP-bound state in a cell cycle-dependent manner, supporting regulation for chromosome replication. ATP-DnaA cooperatively assembles on the replication origin using clusters of low-affinity DnaA-binding sites. After initiation, DnaA-bound ATP is hydrolyzed, producing initiation-inactive ADP-DnaA. For the next round of initiation, ADP-DnaA binds to the chromosomal locus DARS2, which promotes the release of ADP, yielding the apo-DnaA to regain the initiation activity through ATP binding. This DnaA reactivation by DARS2 depends on site-specific binding of IHF (integration host factor) and Fis proteins and IHF binding to DARS2 occurs specifically during pre-initiation. Here, we reveal that Fis binds to an essential region in DARS2 specifically during pre-initiation. Further analyses demonstrate that ATP-DnaA, but not ADP-DnaA, oligomerizes on a cluster of low-affinity DnaA-binding sites overlapping the Fis-binding region, which competitively inhibits Fis binding and hence the DARS2 activity. DiaA (DnaA initiator-associating protein) stimulating ATP-DnaA assembly enhances the dissociation of Fis. These observations lead to a negative feedback model where the activity of DARS2 is repressed around the time of initiation by the elevated ATP-DnaA level and is stimulated following initiation when the ATP-DnaA level is reduced.

Whole-Genome Analysis Reveals That the Nucleoid Protein IHF Predominantly Binds to the Replication Origin oriC Specifically at the Time of Initiation

The structure and function of bacterial chromosomes are dynamically regulated by a wide variety of nucleoid-associated proteins (NAPs) and DNA superstructures, such as DNA supercoiling. In Escherichia coli, integration host factor (IHF), a NAP, binds to specific transcription promoters and regulatory DNA elements of DNA replication such as the replication origin oriC: binding to these elements depends on the cell cycle but underlying mechanisms are unknown. In this study, we combined GeF-seq (genome footprinting with high-throughput sequencing) with synchronization of the E. coli cell cycle to determine the genome-wide, cell cycle-dependent binding of IHF with base-pair resolution. The GeF-seq results in this study were qualified enough to analyze genomic IHF binding sites (e.g., oriC and the transcriptional promoters of ilvG and osmY) except some of the known sites. Unexpectedly, we found that before replication initiation, oriC was a predominant site for stable IHF binding, whereas all other loci exhibited reduced IHF binding. To reveal the specific mechanism of stable oriC–IHF binding, we inserted a truncated oriC sequence in the terC (replication terminus) locus of the genome. Before replication initiation, stable IHF binding was detected even at this additional oriC site, dependent on the specific DnaA-binding sequence DnaA box R1 within the site. DnaA oligomers formed on oriC might protect the oriC–IHF complex from IHF dissociation. After replication initiation, IHF rapidly dissociated from oriC, and IHF binding to other sites was sustained or stimulated. In addition, we identified a novel locus associated with cell cycle-dependent IHF binding. These findings provide mechanistic insight into IHF binding and dissociation in the genome.

Distribution and phasing of sequence motifs that facilitate CRISPR adaptation

CRISPR-associated proteins (Cas1 and Cas2) integrate foreign DNA at the "leader" end of CRISPR loci. Several CRISPR leader sequences are reported to contain a binding site for a DNA-bending protein called integration host factor (IHF). IHF-induced DNA bending kinks the leader of type I-E CRISPRs, recruiting an upstream sequence motif that helps dock Cas1-2 onto the first repeat of the CRISPR locus. To determine the prevalence of IHF-directed CRISPR adaptation, we analyzed 15,274 bacterial and archaeal CRISPR leaders. These experiments reveal multiple IHF binding sites and diverse upstream sequence motifs in a subset of the I-C, I-E, I-F, and II-C CRISPR leaders. We identify subtype-specific motifs and show that the phase of these motifs is critical for CRISPR adaptation. Collectively, this work clarifies the prevalence and mechanism(s) of IHF-dependent CRISPR adaptation and suggests that leader sequences and adaptation proteins may coevolve under the selective pressures of foreign genetic elements like plasmids or phages.

Unraveling the Complex Interplay of Fis and IHF Through Synthetic Promoter Engineering

Bacterial promoters are usually formed by multiple cis-regulatory elements recognized by a plethora of transcriptional factors (TFs). From those, global regulators are key elements since these TFs are responsible for the regulation of hundreds of genes in the bacterial genome. For instance, Fis and IHF are global regulators that play a major role in gene expression control in Escherichia coli, and usually, multiple cis-regulatory elements for these proteins are present at target promoters. Here, we investigated the relationship between the architecture of the cis-regulatory elements for Fis and IHF in E. coli. For this, we analyze 42 synthetic promoter variants harboring consensus cis-elements for Fis and IHF at different distances from the core -35/-10 region and in various numbers and combinations. We first demonstrated that although Fis preferentially recognizes its consensus cis-element, it can also recognize, to some extent, the consensus-binding site for IHF, and the same was true for IHF, which was also able to recognize Fis binding sites. However, changing the arrangement of the cis-elements (i.e., the position or number of sites) can completely abolish the non-specific binding of both TFs. More remarkably, we demonstrated that combining cis-elements for both TFs could result in Fis and IHF repressed or activated promoters depending on the final architecture of the promoters in an unpredictable way. Taken together, the data presented here demonstrate how small changes in the architecture of bacterial promoters could result in drastic changes in the final regulatory logic of the system, with important implications for the understanding of natural complex promoters in bacteria and their engineering for novel applications.

Physical and Functional Characterization of a Viral Genome Maturation Complex

Genome packaging is strongly conserved in the complex double-stranded DNA viruses, including the herpesviruses and many bacteriophages. In these cases, viral DNA is packaged into a procapsid shell by a terminase enzyme. The packaging substrate is typically a concatemer composed of multiple genomes linked in a head-to-tail fashion, and terminase enzymes perform two essential functions: 1) excision of a unit length genome from the concatemer (genome maturation) and 2) translocation of the duplex into a procapsid (genome packaging). While the packaging motors have been described in some detail, the maturation complexes remain ill characterized. Here we describe the assembly, physical characteristics, and catalytic activity of the λ-genome maturation complex. The λ-terminase protomer is composed of one large catalytic subunit tightly associated with two DNA recognition subunits. The isolated protomer binds DNA weakly and does not discriminate between nonspecific DNA and duplexes that contain the packaging initiation sequence, cos. The Escherichia coli integration host factor protein (IHF) is required for efficient λ-development in vivo and a specific IHF recognition sequence is found within cos. We show that IHF and the terminase protomer cooperatively assemble at the cos site and that the small terminase subunit plays the dominant role in complex assembly. Analytical ultracentrifugation analysis reveals that the maturation complex is composed of four protomers and one IHF heterodimer bound at the cos site. Tetramer assembly activates the cos-cleavage nuclease activity of the enzyme, which matures the genome end in preparation for packaging. The stoichiometry and catalytic activity of the complex is reminiscent of the type IIE and IIF restriction endonucleases and the two systems may share mechanistic features. This study, to our knowledge, provides our first detailed glimpse into the structural and functional features of a viral genome maturation complex, an essential intermediate in the development of complex dsDNA viruses.

Cooperative DnaA Binding to the Negatively Supercoiled datA Locus Stimulates DnaA-ATP Hydrolysis

Timely initiation of replication in Escherichia coli requires functional regulation of the replication initiator, ATP-DnaA. The cellular level of ATP-DnaA increases just before initiation, after which its level decreases through hydrolysis of DnaA-bound ATP, yielding initiation-inactive ADP-DnaA. Previously, we reported a novel DnaA-ATP hydrolysis system involving the chromosomal locus datA and named it datA-dependent DnaA-ATP hydrolysis (DDAH). The datA locus contains a binding site for a nucleoid-associating factor integration host factor (IHF) and a cluster of three known DnaA-binding sites, which are important for DDAH. However, the mechanisms underlying the formation and regulation of the datA-IHF·DnaA complex remain unclear. We now demonstrate that a novel DnaA box within datA is essential for ATP-DnaA complex formation and DnaA-ATP hydrolysis. Specific DnaA residues, which are important for interaction with bound ATP and for head-to-tail inter-DnaA interaction, were also required for ATP-DnaA-specific oligomer formation on datA Furthermore, we show that negative DNA supercoiling of datA stabilizes ATP-DnaA oligomers, and stimulates datA-IHF interaction and DnaA-ATP hydrolysis. Relaxation of DNA supercoiling by the addition of novobiocin, a DNA gyrase inhibitor, inhibits datA function in cells. On the basis of these results, we propose a mechanistic model of datA-IHF·DnaA complex formation and DNA supercoiling-dependent regulation for DDAH.

Insights into the Structure of Sulfolobus Nucleoid Using Engineered Sac7d Dimers with a Defined Orientation

The structure of Archaeal chromatin or nucleoid is believed to have characteristics similar to that found in both eukaryotes and bacteria. Recent comparative studies have suggested that DNA compaction in Archaea requires a bridging protein (e.g., Alba) along with either a wrapping protein (e.g., a histone) or a bending protein such as Sac7d. While X-ray crystal structures demonstrate that Sac7d binds as a monomer to create a significant kink in duplex DNA, the structure of a multiprotein-DNA complex has not been established. Using cross-linked dimers of Sac7d with a defined orientation, we present evidence that indicates that Sac7d is able to largely coat duplex DNA in vivo by binding in alternating head-to-head and tail-to-tail orientations. Although each Sac7d monomer promotes a significant kink of nearly 70°, coated DNA is expected to be largely extended because of compensation of repetitive kinks with helical symmetry.

Analytical Ultracentrifugation as a Tool to Study Nonspecific Protein-DNA Interactions

Analytical ultracentrifugation (AUC) is a powerful tool that can provide thermodynamic information on associating systems. Here, we discuss how to use the two fundamental AUC applications, sedimentation velocity (SV), and sedimentation equilibrium (SE), to study nonspecific protein-nucleic acid interactions, with a special emphasis on how to analyze the experimental data to extract thermodynamic information. We discuss three specific applications of this approach: (i) determination of nonspecific binding stoichiometry of E. coli integration host factor protein to dsDNA, (ii) characterization of nonspecific binding properties of Adenoviral IVa2 protein to dsDNA using SE-AUC, and (iii) analysis of the competition between specific and nonspecific DNA-binding interactions observed for E. coli integration host factor protein assembly on dsDNA. These approaches provide powerful tools that allow thermodynamic interrogation and thus a mechanistic understanding of how proteins bind nucleic acids by both specific and nonspecific interactions.

Nucleoid-associated proteins encoded on plasmids: Occurrence and mode of function

Nucleoid-associated proteins (NAPs) play a role in changing the shape of microbial DNA, making it more compact and affecting the regulation of transcriptional networks in host cells. Genes that encode NAPs include H-NS family proteins (H-NS, Ler, MvaT, BpH3, Bv3F, HvrA, and Lsr2), FIS, HU, IHF, Lrp, and NdpA, and are found in both microbial chromosomes and plasmid DNA. In the present study, NAP genes were distributed among 442 plasmids out of 4602 plasmid sequences, and many H-NS family proteins, and HU, IHF, Lrp, and NdpA were found in plasmids of Alpha-, Beta-, and Gammaproteobacteria, while HvrA, Lsr2, HU, and Lrp were found in other classes including Actinobacteria and Bacilli. Larger plasmids frequently carried multiple NAP genes. In addition, NAP genes were more frequently found in conjugative plasmids than non-transmissible plasmids. Several host cells carried the same types of H-NS family proteins on both their plasmids and chromosome(s), while this was not observed for other NAPs. Recent studies have shown that NAP genes on plasmids and chromosomes play important roles in the physical and regulatory integration of plasmids into the host cell.

Integration host factor assembly at the cohesive end site of the bacteriophage lambda genome: implications for viral DNA packaging and bacterial gene regulation

Integration host factor (IHF) is an Escherichia coli protein involved in (i) condensation of the bacterial nucleoid and (ii) regulation of a variety of cellular functions. In its regulatory role, IHF binds to a specific sequence to introduce a strong bend into the DNA; this provides a duplex architecture conducive to the assembly of site-specific nucleoprotein complexes. Alternatively, the protein can bind in a sequence-independent manner that weakly bends and wraps the duplex to promote nucleoid formation. IHF is also required for the development of several viruses, including bacteriophage lambda, where it promotes site-specific assembly of a genome packaging motor required for lytic development. Multiple IHF consensus sequences have been identified within the packaging initiation site (cos), and we here interrogate IHF–cos binding interactions using complementary electrophoretic mobility shift (EMS) and analytical ultracentrifugation (AUC) approaches. IHF recognizes a single consensus sequence within cos (I1) to afford a strongly bent nucleoprotein complex. In contrast, IHF binds weakly but with positive cooperativity to nonspecific DNA to afford an ensemble of complexes with increasing masses and levels of condensation. Global analysis of the EMS and AUC data provides constrained thermodynamic binding constants and nearest neighbor cooperativity factors for binding of IHF to I1 and to nonspecific DNA substrates. At elevated IHF concentrations, the nucleoprotein complexes undergo a transition from a condensed to an extended rodlike conformation; specific binding of IHF to I1 imparts a significant energy barrier to the transition. The results provide insight into how IHF can assemble specific regulatory complexes in the background of extensive nonspecific DNA condensation.

Timely binding of IHF and Fis to DARS2 regulates ATP-DnaA production and replication initiation

In Escherichia coli, the ATP-bound form of DnaA (ATP-DnaA) promotes replication initiation. During replication, the bound ATP is hydrolyzed to ADP to yield the ADP-bound form (ADP-DnaA), which is inactive for initiation. The chromosomal site DARS2 facilitates the regeneration of ATP-DnaA by catalyzing nucleotide exchange between free ATP and ADP bound to DnaA. However, the regulatory mechanisms governing this exchange reaction are unclear. Here, using in vitro reconstituted experiments, we show that two nucleoid-associated proteins, IHF and Fis, bind site-specifically to DARS2 to activate coordinately the exchange reaction. The regenerated ATP-DnaA was fully active in replication initiation and underwent DnaA-ATP hydrolysis. ADP-DnaA formed heteromultimeric complexes with IHF and Fis on DARS2, and underwent nucleotide dissociation more efficiently than ATP-DnaA. Consistently, mutant analyses demonstrated that specific binding of IHF and Fis to DARS2 stimulates the formation of ATP-DnaA production, thereby promoting timely initiation. Moreover, we show that IHF-DARS2 binding is temporally regulated during the cell cycle, whereas Fis only binds to DARS2 in exponentially growing cells. These results elucidate the regulation of ATP-DnaA and replication initiation in coordination with the cell cycle and growth phase.

Molecular dissection of Mycobacterium tuberculosis integration host factor reveals novel insights into the mode of DNA binding and nucleoid compaction

The annotated whole-genome sequence of Mycobacterium tuberculosis revealed that Rv1388 (Mtihf) is likely to encode for a putative 20-kDa integration host factor (mIHF). However, very little is known about the functional properties of mIHF or the organization of the mycobacterial nucleoid. Molecular modeling of the mIHF three-dimensional structure, based on the cocrystal structure of Streptomyces coelicolor IHF duplex DNA, a bona fide relative of mIHF, revealed the presence of Arg-170, Arg-171, and Arg-173, which might be involved in DNA binding, and a conserved proline (Pro-150) in the tight turn. The phenotypic sensitivity of Escherichia coli ΔihfA and ΔihfB strains to UV and methyl methanesulfonate could be complemented with the wild-type Mtihf but not its alleles bearing mutations in the DNA-binding residues. Protein-DNA interaction assays revealed that wild-type mIHF, but not its DNA-binding variants, binds with high affinity to fragments containing attB and attP sites and curved DNA. Strikingly, the functionally important amino acid residues of mIHF and the mechanism(s) underlying its binding to DNA, DNA bending, and site-specific recombination are fundamentally different from that of E. coli IHFαβ. Furthermore, we reveal novel insights into IHF-mediated DNA compaction depending on the placement of its preferred binding sites; mIHF promotes DNA compaction into nucleoid-like or higher order filamentous structures. We therefore propose that mIHF is a distinct member of a subfamily of proteins that serve as essential cofactors in site-specific recombination and nucleoid organization and that these findings represent a significant advance in our understanding of the role(s) of nucleoid-associated proteins.

Sequence-specific transitions of the torsion angle gamma change the polar-hydrophobic profile of the DNA grooves: implication for indirect protein-DNA recognition

Variations of the shape and polarity of the DNA grooves caused by changes of the DNA conformation play an important role in the DNA readout. Despite the fact that non-canonical trans and gauche- conformations of the DNA backbone angle γ (O5'-C5'-C4'-C3') are frequently found in the DNA crystal structures, their possible role in the DNA recognition has not been studied systematically. In order to fill in this gap, we analyze the available high-resolution crystal structures of the naked and complexed DNA. The analysis shows that the non-canonical γ angle conformations are present both in the naked and bound DNA, more often in the bound vs. naked DNA, and in the nucleotides with the A-like vs. the B-like sugar pucker. The alternative angle γ torsions are more frequently observed in the purines with the A-like sugar pucker and in the pyrimidines with the B-like sugar conformation. The minor groove of the nucleotides with non-canonical γ angle conformation is more polar, while the major groove is more hydrophobic than in the nucleotides with the classical γ torsions due to variations in exposure of the polar and hydrophobic groups of the DNA backbone. The propensity of the nucleotides with different γ angle conformations to participate in the protein-nucleic acid contacts in the minor and major grooves is connected with their sugar pucker and sequence-specific. Our findings imply that the angle γ transitions contribute to the process of the protein-DNA recognition due to modification of the polar/hydrophobic profile of the DNA grooves.

An A-tract at the AtzR binding site assists DNA binding, inducer-dependent repositioning and transcriptional activation of the PatzDEF promoter

The LysR-type regulator AtzR activates the Pseudomonas sp. ADP atzDEF operon in response to nitrogen limitation and cyanuric acid. Activation involves repositioning of the AtzR tetramer on the PatzDEF promoter and relaxation of an AtzR-induced DNA bend. Here we examine the in vivo and in vitro contribution of an A5 -tract present at the PatzDEF promoter region to AtzR binding and transcriptional activation. Substitution of the A-tract for the sequence ACTCA prevented PatzDEF activation and high-affinity AtzR binding, impaired AtzR contacts with the activator binding site and shifted the position of the AtzR-induced DNA bend. Analysis of a collection of mutants bearing different alterations in the A-tract sequence showed that the extent of AtzR-dependent activation does not correlate with the magnitude or orientation of the spontaneous DNA bend generated at this site. Our results support the notion that indirect readout of the A-tract-associated narrow minor groove is essential for the AtzR-DNA complex to achieve a conformation competent for activation of the PatzDEF promoter. Conservation of this motif in several binding sites of LysR-type regulators suggests that this mechanism may be shared by other proteins in this family.

Indirect read-out of the promoter DNA by RNA polymerase in the closed complex

Transcription is initiated when RNA polymerase recognizes the duplex promoter DNA in the closed complex. Due to its transient nature, the closed complex has not been well characterized. How the initial promoter recognition occurs may offer important clues to regulation of transcription initiation. In this article, we have carried out single-base pair substitution experiments on two Escherichia coli promoters belonging to two different classes, the -35 and the extended -10, under conditions which stabilize the closed complex. Single-base pair substitution experiments indicate modest base-specific effects on the stability of the closed complex of both promoters. Mutations of base pairs in the -10 region affect the closed complexes of two promoters differently, suggesting different modes of interaction of the RNA polymerase and the promoter in the two closed complexes. Two residues on σ(70) which have been suggested to play important role in promoter recognition, Q437 and R436, were mutated and found to have different effects on the closed-complex stability. DNA circular dichroism (CD) and FRET suggest that the promoter DNA in the closed complex is distorted. Modeling suggests two different orientations of the recognition helix of the RNA polymerase in the closed complex. We propose that the RNA polymerase recognizes the sequence dependent conformation of the promoter DNA in the closed complex.

Nucleosome accessibility governed by the dimer/tetramer interface

Nucleosomes are multi-component macromolecular assemblies which present a formidable obstacle to enzymatic activities that require access to the DNA, e.g. DNA and RNA polymerases. The mechanism and pathway(s) by which nucleosomes disassemble to allow DNA access are not well understood. Here we present evidence from single molecule FRET experiments for a previously uncharacterized intermediate structural state before H2A–H2B dimer release, which is characterized by an increased distance between H2B and the nucleosomal dyad. This suggests that the first step in nucleosome disassembly is the opening of the (H3–H4)₂ tetramer/(H2A–H2B) dimer interface, followed by H2A–H2B dimer release from the DNA and, lastly, (H3–H4)₂ tetramer removal. We estimate that the open intermediate state is populated at 0.2–3% under physiological conditions. This finding could have significant in vivo implications for factor-mediated histone removal and exchange, as well as for regulating DNA accessibility to the transcription and replication machinery.

In vitro recombination and inverted terminal repeat binding activities of the Mcmar1 transposase

The Mcmar1 mariner element (MLE) presents some intriguing features with two large, perfectly conserved, 355 bp inverted terminal repeats (ITRs) containing two 28 bp direct repeats (DRs). The presence of a complete ORF in Mcmar1 makes it possible to explore the transposition of this unusual MLE. Mcmar1 transposase (MCMAR1) was purified, and in vitro transposition assays showed that it is able to promote ITR-dependent DNA cleavages and recombination events, which correspond to plasmid fusions and transpositions with imprecise ends. Further analyses indicated that MCMAR1 is able to interact with the 355 bp ITR through two DRs: the EDR (external DR) is a high-affinity binding site for MCMAR1, whereas the IDR (internal DR) is a low-affinity binding site. The main complex detected within the EDR contained a transposase dimer and only one DNA molecule. We hypothesize that the inability of MCMAR1 to promote precise in vitro transposition events could be due to mutations in its ORF sequence or to the specific features of transposase binding to the ITR. Indeed, the ITR region spanning from EDR to IDR resembles a MITE and could be bent by specific host factors. This suggests that the assembly of the transposition complex is more complex than that of those involved in the mobility of the Mos1 and Himar1 mariner elements.

An all-atom knowledge-based energy function for protein-DNA threading, docking decoy discrimination, and prediction of transcription-factor binding profiles

How to make an accurate representation of protein-DNA interaction by an energy function is a long-standing unsolved problem in structural biology. Here, we modified a statistical potential based on the distance-scaled, finite ideal-gas reference state so that it is optimized for protein-DNA interactions. The changes include a volume-fraction correction to account for unmixable atom types in proteins and DNA in addition to the usage of a low-count correction, residue/base-specific atom types, and a shorter cutoff distance for protein-DNA interactions. The new statistical energy functions are tested in threading and docking decoy discriminations and prediction of protein-DNA binding affinities and transcription-factor binding profiles. The results indicate that new proposed energy functions are among the best in existing energy functions for protein-DNA interactions. The new energy functions are available as a web-server called DDNA 2.0 at http://sparks.informatics.iupui.edu. The server version was trained by the entire 212 protein-DNA complexes.

IHF-binding sites inhibit DNA loop formation and transcription initiation

Transcriptional activation of enhancer and σ⁵⁴-dependent promoters requires efficient interactions between enhancer-binding proteins (EBP) and promoter bound σ⁵⁴-RNA polymerase (Eσ⁵⁴) achieved by DNA looping, which is usually facilitated by the integration host factor (IHF). Since the lengths of the intervening region supporting DNA-loop formation are similar among IHF-dependent and IHF-independent promoters, the precise reason(s) why IHF is selectively important for the frequency of transcription initiation remain unclear. Here, using kinetic cyclization and in vitro transcription assays we show that, in the absence of IHF protein, the DNA fragments containing an IHF-binding site have much less looping-formation ability than those that lack an IHF-binding site. Furthermore, when an IHF consensus-binding site was introduced into the intervening region between promoter and enhancer of the target DNA fragments, loop formation and DNA-loop-dependent transcriptional activation are significantly reduced in a position-independent manner. DNA-looping-independent transcriptional activation was unaffected. The binding of IHF to its consensus site in the target promoters clearly restored efficient DNA looping formation and looping-dependent transcriptional activation. Our data provide evidence that one function for the IHF protein is to release a communication block set by intrinsic properties of the IHF DNA-binding site.

Formation of a wrapped DNA-protein interface: experimental characterization and analysis of the large contributions of ions and water to the thermodynamics of binding IHF to H' DNA

To characterize driving forces and driven processes in formation of a large-interface, wrapped protein-DNA complex analogous to the nucleosome, we have investigated the thermodynamics of binding the 34-base pair (bp) H' DNA sequence to the Escherichia coli DNA-remodeling protein integration host factor (IHF). Isothermal titration calorimetry and fluorescence resonance energy transfer are applied to determine effects of salt concentration [KCl, KF, K glutamate (KGlu)] and of the excluded solute glycine betaine (GB) on the binding thermodynamics at 20 degrees C. Both the binding constant K(obs) and enthalpy Delta H degrees (obs) depend strongly on [salt] and anion identity. Formation of the wrapped complex is enthalpy driven, especially at low [salt] (e.g., Delta H(o)(obs)=-20.2 kcal x mol(-1) in 0.04 M KCl). Delta H degrees (obs) increases linearly with [salt] with a slope (d Delta H degrees (obs)/d[salt]), which is much larger in KCl (38+/-3 kcal x mol(-1) M(-1)) than in KF or KGlu (11+/-2 kcal x mol(-1) M(-1)). At 0.33 M [salt], K(obs) is approximately 30-fold larger in KGlu or KF than in KCl, and the [salt] derivative SK(obs)=dlnK(obs)/dln[salt] is almost twice as large in magnitude in KCl (-8.8+/-0.7) as in KF or KGlu (-4.7+/-0.6). A novel analysis of the large effects of anion identity on K(obs), SK(obs) and on Delta H degrees (obs) dissects coulombic, Hofmeister, and osmotic contributions to these quantities. This analysis attributes anion-specific differences in K(obs), SK(obs), and Delta H degrees (obs) to (i) displacement of a large number of water molecules of hydration [estimated to be 1.0(+/-0.2)x10(3)] from the 5340 A(2) of IHF and H' DNA surface buried in complex formation, and (ii) significant local exclusion of F(-) and Glu(-) from this hydration water, relative to the situation with Cl(-), which we propose is randomly distributed. To quantify net water release from anionic surface (22% of the surface buried in complexation, mostly from DNA phosphates), we determined the stabilizing effect of GB on K(obs): dlnK(obs)/d[GB]=2.7+/-0.4 at constant KCl activity, indicating the net release of ca. 150 H(2)O molecules from anionic surface.

Fine-tuning of intrinsic N-Oct-3 POU domain allostery by regulatory DNA targets

The 'POU' (acronym of Pit-1, Oct-1, Unc-86) family of transcription factors share a common DNA-binding domain of approximately 160 residues, comprising so-called 'POUs' and 'POUh' sub-domains connected by a flexible linker. The importance of POU proteins as developmental regulators and tumor-promoting agents is due to linker flexibility, which allows them to adapt to a considerable variety of DNA targets. However, because of this flexibility, it has not been possible to determine the Oct-1/Pit-1 linker structure in crystallographic POU/DNA complexes. We have previously shown that the neuronal POU protein N-Oct-3 linker contains a structured region. Here, we have used a combination of hydrodynamic methods, DNA footprinting experiments, molecular modeling and small angle X-ray scattering to (i) structurally interpret the N-Oct-3-binding site within the HLA DRalpha gene promoter and deduce from this a novel POU domain allosteric conformation and (ii) analyze the molecular mechanisms involved in conformational transitions. We conclude that there might exist a continuum running from free to 'pre-bound' N-Oct-3 POU conformations and that regulatory DNA regions likely select pre-existing conformers, in addition to molding the appropriate DBD structure. Finally, we suggest that a specific pair of glycine residues in the linker might act as a major conformational switch.

Shaping the Borrelia burgdorferi genome: crystal structure and binding properties of the DNA-bending protein Hbb

The genome of the Lyme disease-causing spirochete Borrelia burgdorferi encodes only a single polypeptide from the integration host factor (IHF)/HU or 'DNABII' family of nucleoid-associated proteins - Hbb. DNABII proteins induce large bends in DNA and serve as architectural factors in a variety of prokaryotic cellular processes. We have solved the crystal structure of an Hbb-DNA complex in which the DNA is bent by over 180 degrees . We find that like IHF, Hbb relies exclusively on indirect readout to recognize its cognate site. Additional binding studies show that the sequence preferences of Hbb are related to, yet distinct from those of IHF. Defining these binding characteristics may help to uncover additional roles for Hbb in Borrelia DNA metabolism as well as further our understanding of the mechanism of indirect readout.

Pressure dissociation of integration host factor-DNA complexes reveals flexibility-dependent structural variation at the protein-DNA interface

E. coli Integration host factor (IHF) condenses the bacterial nucleoid by wrapping DNA. Previously, we showed that DNA flexibility compensates for structural characteristics of the four consensus recognition elements associated with specific binding (Aeling et al., J. Biol. Chem. 281, 39236-39248, 2006). If elements are missing, high-affinity binding occurs only if DNA deformation energy is low. In contrast, if all elements are present, net binding energy is unaffected by deformation energy. We tested two hypotheses for this observation: in complexes containing all elements, (1) stiff DNA sequences are less bent upon binding IHF than flexible ones; or (2) DNA sequences with differing flexibility have interactions with IHF that compensate for unfavorable deformation energy. Time-resolved Förster resonance energy transfer (FRET) shows that global topologies are indistinguishable for three complexes with oligonucleotides of different flexibility. However, pressure perturbation shows that the volume change upon binding is smaller with increasing flexibility. We interpret these results in the context of Record and coworker's model for IHF binding (J. Mol. Biol. 310, 379-401, 2001). We propose that the volume changes reflect differences in hydration that arise from structural variation at IHF-DNA interfaces while the resulting energetic compensation maintains the same net binding energy.