HU binding to DNA: evidence for multiple complex formation and DNA bending

Integration Host Factor Binds DNA Holliday Junctions

Integration host factor (IHF) is a nucleoid-associated protein involved in DNA packaging, integration of viral DNA and recombination. IHF binds with nanomolar affinity to duplex DNA containing a 13 bp consensus sequence, inducing a bend of ~160° upon binding. We determined that IHF binds to DNA Four-way or Holliday junctions (HJ) with high affinity regardless of the presence of the consensus sequence, signifying a structure-based mechanism of recognition. Junctions, important intermediates in DNA repair and homologous recombination, are dynamic and can adopt either an open or stacked conformation, where the open conformation facilitates branch migration and strand exchange. Using ensemble and single molecule Förster resonance energy transfer (FRET) methods, we investigated IHF-induced changes in the population distribution of junction conformations and determined that IHF binding shifts the population to the open conformation. Further analysis of smFRET dynamics revealed that even in the presence of protein, the junctions remain dynamic as fast transitions are observed for the protein-bound open state. Protein binding alters junction conformational dynamics, as cross correlation analyses reveal the protein slows the transition rate at 1 mM Mg²⁺ but accelerates the transition rate at 10 mM Mg²⁺. Stopped flow kinetic experiments provide evidence for two binding steps, a rapid, initial binding step followed by a slower step potentially associated with a conformational change. These measurements also confirm that the protein remains bound to the junction during the conformer transitions and further suggest that the protein forms a partially dissociated state that allows junction arms to be dynamic. These findings, which demonstrate that IHF binds HJs with high affinity and stabilizes junctions in the open conformation, suggest that IHF may play multiple roles in the processes of integration and recombination in addition to stabilizing bacterial biofilms.

The Dynamics of Mycoplasma gallisepticum Nucleoid Structure at the Exponential and Stationary Growth Phases

The structure and dynamics of bacterial nucleoids play important roles in regulating gene expression. Bacteria of class Mollicutes and, in particular, mycoplasmas feature extremely reduced genomes. They lack multiple structural proteins of the nucleoid, as well as regulators of gene expression. We studied the organization of Mycoplasma gallisepticum nucleoids in the stationary and exponential growth phases at the structural and protein levels. The growth phase transition results in the structural reorganization of M. gallisepticum nucleoid. In particular, it undergoes condensation and changes in the protein content. The observed changes corroborate with the previously identified global rearrangement of the transcriptional landscape in this bacterium during the growth phase transition. In addition, we identified that the glycolytic enzyme enolase functions as a nucleoid structural protein in this bacterium. It is capable of non-specific DNA binding and can form fibril-like complexes with DNA.

Fundamental photophysics of isomorphic and expanded fluorescent nucleoside analogues

Fluorescent nucleoside analogues (FNAs) are structurally diverse mimics of the natural essentially non-fluorescent nucleosides which have found numerous applications in probing the structure and dynamics of nucleic acids as well as their interactions with various biomolecules. In order to minimize disturbance in the labelled nucleic acid sequences, the FNA chromophoric groups should resemble the natural nucleobases in size and hydrogen-bonding patterns. Isomorphic and expanded FNAs are the two groups that best meet the criteria of non-perturbing fluorescent labels for DNA and RNA. Significant progress has been made over the past decades in understanding the fundamental photophysics that governs the spectroscopic and environmentally sensitive properties of these FNAs. Herein, we review recent advances in the spectroscopic and computational studies of selected isomorphic and expanded FNAs. We also show how this information can be used as a rational basis to design new FNAs, select appropriate sequences for optimal spectroscopic response and interpret fluorescence data in FNA applications.

Two-step interrogation then recognition of DNA binding site by Integration Host Factor: an architectural DNA-bending protein

The dynamics and mechanism of how site-specific DNA-bending proteins initially interrogate potential binding sites prior to recognition have remained elusive for most systems. Here we present these dynamics for Integration Host factor (IHF), a nucleoid-associated architectural protein, using a μs-resolved T-jump approach. Our studies show two distinct DNA-bending steps during site recognition by IHF. While the faster (∼100 μs) step is unaffected by changes in DNA or protein sequence that alter affinity by >100-fold, the slower (1–10 ms) step is accelerated ∼5-fold when mismatches are introduced at DNA sites that are sharply kinked in the specific complex. The amplitudes of the fast phase increase when the specific complex is destabilized and decrease with increasing [salt], which increases specificity. Taken together, these results indicate that the fast phase is non-specific DNA bending while the slow phase, which responds only to changes in DNA flexibility at the kink sites, is specific DNA kinking during site recognition. Notably, the timescales for the fast phase overlap with one-dimensional diffusion times measured for several proteins on DNA, suggesting that these dynamics reflect partial DNA bending during interrogation of potential binding sites by IHF as it scans DNA.

MutSγ-Induced DNA Conformational Changes Provide Insights into Its Role in Meiotic Recombination

In many organisms, MutSγ plays a role in meiotic recombination, facilitating crossover formation between homologous chromosomes. Failure to form crossovers leads to improper segregation of chromosomes and aneuploidy, which in humans result in infertility and birth defects. To improve current understanding of MutSγ function, this study investigates the binding affinities and structures of MutSγ in complex with DNA substrates that model homologous recombination intermediates. For these studies, we overexpressed and isolated from Escherichia coli the yeast MutSγ protein Saccharomyces cerevisiae (Sc) Msh4-Msh5. Sc Msh4-Msh5 binds Holliday junction (HJ)-like substrates, 3' overhangs, single-stranded (ss) forks, and the displacement loop with nanomolar affinity. The weakest binding affinities are detected for an intact duplex and open-junction construct. Similar to the human protein, Sc Msh4-Msh5 exhibits the highest affinity for the HJ with a K_d < 0.4 nM in solution. Energy-transfer experiments further demonstrate that DNA structure is modulated by the binding interaction with the largest changes associated with substrates containing an ss end. Upon binding, Sc Msh4-Msh5 displaces the ss away from the duplex in most of the ss-containing intermediates, potentially enabling the binding of RPA and other proteins. In the case of the junction-like intermediates, Msh4-Msh5 binding either stabilizes the existing stacked structure or induces formation of the stacked X conformation. Significantly, we find that upon binding, Msh4-Msh5 stacks an open-junction construct to the same extent as the standard junction. Stabilization of the junction in the stacked conformation is generally refractory to branch migration, which is consistent with a potential role for MutSγ to stabilize HJs and prevent branch migration until resolution by MutLγ. The different binding modalities observed suggest that Msh4-Msh5 not only binds to and stabilizes stacked junctions but also participates in meiotic recombination before junction formation through the stabilization of single-end invasion intermediates.

Effects of the histone-like protein HU on cellulose degradation and biofilm formation of Cytophaga hutchinsonii

Cytophaga hutchinsonii, belonging to Bacteroidetes, is speculated to use a novel cell-contact mode to digest cellulose. In this study, we identified a histone-like protein HU, CHU_2750, in C. hutchinsonii, whose transcription could be induced by crystalline but not amorphous cellulose. We constructed a CHU_2750-deleted mutant and expressed CHU_2750 in Escherichia coli to study the gene's functions. Our results showed that although the deletion of CHU_2750 was not lethal to C. hutchinsonii, the mutant displayed an abnormal filamentous morphology, loose nucleoid, and obvious defects in the degradation of crystalline cellulose and cell motility. Further study indicated that the mutant displayed significantly decreased cell surface and intracellular endoglucanase activities but with β-glucosidase activities similar to the wild-type strain. Analyses by real-time quantitative PCR revealed that the transcription levels of many genes involved in cellulose degradation and/or cell motility were significantly downregulated in the mutant. In addition, we found that CHU_2750 was important for biofilm formation of C. hutchinsonii. The main extracellular components of the biofilm were analyzed, and the results showed that the mutant yielded significantly less exopolysaccharide but more extracellular DNA and protein than the wild-type strain. Collectively, our findings demonstrated that CHU_2750 is important for cellulose degradation, cell motility, and biofilm formation of C. hutchinsonii by modulating transcription of certain related genes, and it is the first identified transcriptional regulator in these processes of C. hutchinsonii. Our study shed more light on the mechanisms of cellulose degradation, cell motility, and biofilm formation by C. hutchinsonii.

Fluorescent nucleobases as tools for studying DNA and RNA

Understanding the diversity of dynamic structures and functions of DNA and RNA in biology requires tools that can selectively and intimately probe these biomolecules. Synthetic fluorescent nucleobases that can be incorporated into nucleic acids alongside their natural counterparts have emerged as a powerful class of molecular reporters of location and environment. They are enabling new basic insights into DNA and RNA, and are facilitating a broad range of new technologies with chemical, biological and biomedical applications. In this Review, we will present a brief history of the development of fluorescent nucleobases and explore their utility as tools for addressing questions in biophysics, biochemistry and biology of nucleic acids. We provide chemical insights into the two main classes of these compounds: canonical and non-canonical nucleobases. A point-by-point discussion of the advantages and disadvantages of both types of fluorescent nucleobases is made, along with a perspective into the future challenges and outlook for this burgeoning field.

Structural and Biochemical Characterization of Poly-ADP-ribose Polymerase from Trypanosoma brucei

Trypanosoma brucei is a unicellular parasite responsible for African trypanosomiasis or sleeping sickness. It contains a single PARP enzyme opposed to many higher eukaryotes, which have numerous PARPs. PARPs are responsible for a post-translational modification, ADP-ribosylation, regulating a multitude of cellular events. T. brucei PARP, like human PARPs-1-3, is activated by DNA binding and it potentially functions in DNA repair processes. Here we characterized activation requirements, structure and subcellular localization of T. brucei PARP. T. brucei PARP was found to be selectively activated by 5′ phosphorylated and 3′ phosphorylated DNA breaks. Importantly, the N-terminal region is responsible for high-affinity DNA-binding and required for DNA-dependent enzymatic activation. This module is also required for nuclear localization of the protein in response to oxidative stress. Solution structures of activating and non-activating PARP-DNA complexes were determined with small-angle X-ray scattering revealing distinct differences in their DNA-binding modes.

Photophysical Characterization of Enhanced 6-Methylisoxanthopterin Fluorescence in Duplex DNA

The structure and dynamic motions of bases in DNA duplexes and other constructs are important for understanding mechanisms of selectivity and recognition of DNA-binding proteins. The fluorescent guanine analogue, 6-methylisoxanthopterin 6-MI, is well suited to this purpose as it exhibits an unexpected 3- to 4-fold increase in relative quantum yield upon duplex formation when incorporated into the following sequences: ATFAA, AAFTA, or ATFTA (where F represents 6-MI). To better understand some of the factors leading to the 6-MI fluorescence increase upon duplex formation, we characterized the effect of local sequence and structural perturbations on 6-MI photophysics through temperature melts, quantum yield measurements, fluorescence quenching assays, and fluorescence lifetime measurements. By examining 21 sequences we have determined that the duplex-enhanced fluorescence (DEF) depends on the composition of bases adjacent to 6-MI and the presence of adenines at locations n ± 2 from the probe. Investigation of duplex stability and local solvent accessibility measurements support a model in which the DEF arises from a constrained geometry of 6-MI in the duplex, which remains H-bonded to cytosine, stacked with adjacent bases and inaccessible to quenchers. Perturbation of DNA structure through the introduction of an unpaired base 3' to 6-MI or a mismatched basepair increases 6-MI dynamic motion leading to fluorescence quenching and a reduction in quantum yield. Molecular dynamics simulations suggest the enhanced fluorescence results from a greater degree of twist at the X-F step relative to the quenched duplexes examined. These results point to a model where adenine residues located at n ± 2 from 6-MI induce a structural geometry with greater twist in the duplex that hinders local motion reducing dynamic quenching and producing an increase in 6-MI fluorescence.

DNA melting properties of the dityrosine cross-linked dimer of Ribonuclease A

Several DNA binding proteins exist in dimeric form when bound with DNA to be able to exhibit various biological processes such as DNA repair, DNA replication and gene expression. Various dimeric forms of Ribonuclease A (RNase A) and other members of the ribonuclease A superfamily are endowed with a multitude of biological activities such as antitumor and antiviral activity. In the present study, we have compared the DNA binding properties between the RNase A monomer and the dityrosine (DT) cross-linked RNase A dimer, and checked the inhibitory effect of DNA on the ribonucleolytic activity of the dimeric protein. An agarose gel based assay shows that like the monomer, the dimer also binds with DNA. The number of nucleotides bound per monomer unit of the dimer is higher than the number of nucleotides that bind with the each monomer. From fluorescence measurements, the association constant (Ka) values for complexation of the monomer and the dimer with ct-DNA are (4.95±0.45)×10(4)M(-1) and (1.29±0.05)×10(6)M(-1) respectively. Binding constant (Kb) values for the binding of the monomer and the dimer with ct-DNA were determined using UV-vis spectroscopy and were found to be (4.96±1.67)×10(4)M(-1) and (4.32±0.31)×10(5)M(-1) respectively. Circular dichroism studies shows that the dimer possesses significant effect on DNA conformation. The melting profile for the ct-DNA-dimer indicated that the melting temperature (Tm) for the ct-DNA-dimer complex is lower compared to the ct-DNA-monomer complex. The ribonucleolytic activity of the dimer, like the monomer, diminishes upon binding with DNA.

The Bacteroides thetaiotaomicron protein Bacteroides host factor A participates in integration of the integrative conjugative element CTnDOT into the chromosome

CTnDOT is a conjugative transposon found in Bacteroides species. It encodes multiple antibiotic resistances and is stimulated to transfer by exposure to tetracycline. CTnDOT integration into the host chromosome requires IntDOT and a previously unknown host factor. We have identified a protein, designated BHFa (Bacteroides host factor A), that participates in integrative recombination. BHFa is the first host factor identified for a site-specific recombination reaction in the CTnDOT family of integrative and conjugative elements. Based on the amino acid sequence of BHFa, the ability to bind specifically to 4 sites in the attDOT DNA, and its activity in the integration reaction, BHFa is a member of the IHF/HU family of nucleoid-associated proteins. Other DNA bending proteins that bind DNA nonspecifically can substitute for BHFa in the integration reaction.

IMPORTANCE

Bacteroides species are normal members of the human colonic microbiota. These species can harbor and spread self-transmissible genetic elements (integrative conjugative elements [ICEs]) that contain antibiotic resistance genes. This work describes the role of a protein, BHFa, and its importance in the integration reaction required for the element CTnDOT to persist in Bacteroides host cells.

β-Arm flexibility of HU fromStaphylococcus aureusdictates the DNA-binding and recognition mechanism

HU, one of the major nucleoid-associated proteins, interacts with the minor groove of DNA in a nonspecific manner to induce DNA bending or to stabilize bent DNA. In this study, crystal structures are reported for both free HU fromStaphylococcus aureusMu50 (SHU) and SHU bound to 21-mer dsDNA. The structures, in combination with electrophoretic mobility shift assays (EMSAs), isothermal titration calorimetry (ITC) measurements and molecular-dynamics (MD) simulations, elucidate the overall and residue-specific changes in SHU upon recognizing and binding to DNA. Firstly, structural comparison showed the flexible nature of the β-sheets of the DNA-binding domain and that the β-arms bend inwards upon complex formation, whereas the other portions are nearly unaltered. Secondly, it was found that the disruption and formation of salt bridges accompanies DNA binding. Thirdly, residue-specific free-energy analyses using the MM-PBSA method with MD simulation data suggested that the successive basic residues in the β-arms play a central role in recognizing and binding to DNA, which was confirmed by the EMSA and ITC analyses. Moreover, residue Arg55 resides in the hinge region of the flexible β-arms, exhibiting a remarkable role in their flexible nature. Fourthly, EMSAs with various DNAs revealed that SHU prefers deformable DNA. Taken together, these data suggest residue-specific roles in local shape and base readouts, which are primarily mediated by the flexible β-arms consisting of residues 50–80.

Single-molecule unzipping force analysis of HU-DNA complexes

The genome of bacteria is organized and compacted by the action of nucleoid-associated proteins. These proteins are often present in tens of thousands of copies and bind with low specificity along the genome. DNA-bound proteins thus potentially act as roadblocks to the progression of machinery that moves along the DNA. In this study, we have investigated the effect of histone-like protein from strain U93 (HU), one of the key proteins involved in shaping the bacterial nucleoid, on DNA helix stability by mechanically unzipping single dsDNA molecules. Our study demonstrates that individually bound HU proteins have no observable effect on DNA helix stability, whereas HU proteins bound side-by-side within filaments increase DNA helix stability. As the stabilizing effect is small compared to the power of DNA-based motor enzymes, our results suggest that HU alone does not provide substantial hindrance to the motor's progression in vivo.

Time-dependent bending rigidity and helical twist of DNA by rearrangement of bound HU protein

HU is a protein that plays a role in various bacterial processes including compaction, transcription and replication of the genome. Here, we use atomic force microscopy to study the effect of HU on the stiffness and supercoiling of double-stranded DNA. First, we measured the persistence length, height profile, contour length and bending angle distribution of the DNA-HU complex after different incubation times of HU with linear DNA. We found that the persistence and contour length depend on the incubation time. At high concentrations of HU, DNA molecules first become stiff with a larger value of the persistence length. The persistence length then decreases over time and the molecules regain the flexibility of bare DNA after ∼2 h. Concurrently, the contour length shows a slight increase. Second, we measured the change in topology of closed circular relaxed DNA following binding of HU. Here, we observed that HU induces supercoiling over a similar time span as the measured change in persistence length. Our observations can be rationalized in terms of the formation of a nucleoprotein filament followed by a structural rearrangement of the bound HU on DNA. The rearrangement results in a change in topology, an increase in bending flexibility and an increase in contour length through a decrease in helical pitch of the duplex.

Applying 6-methylisoxanthopterin-enhanced fluorescence to examine protein-DNA interactions in the picomolar range

Incorporation of fluorescent nucleoside analogues into duplex DNA usually leads to a reduction in quantum yield, which significantly limits their potential use and application. We have identified two pentamer DNA sequences containing 6-methylisoxanthopterin (6-MI) (ATFAA and AAFTA, where F is 6-MI) that exhibit significant enhancement of fluorescence upon formation of duplex DNA with quantum yields close to that of monomeric 6-MI. The enhanced fluorescence dramatically increases the utility and sensitivity of the probe and is used to study protein-DNA interactions of nanomolar specificity in this work. The increased sensitivity of 6-MI allows anisotropy binding measurements to be performed at DNA concentrations of 1 nM and fluorescence intensity measurements at 50 pM DNA. The ATFAA sequence was incorporated into DNA constructs to measure the binding affinity of four different protein-DNA interactions that exhibit sequence-specific and non-sequence-specific recognition. In all cases, the K(d) values obtained were consistent with previously reported values measured by other methods. Time-resolved and steady-state fluorescence measurements demonstrate that 6-MI fluorescence is very sensitive to local distortion and reports on different degrees of protein-induced perturbations with single-base resolution, where the largest changes occur at the site of protein binding.

COMPACTION AND SUPERCOILING OF SINGLE, LONG DNA MOLECULES BY HU PROTEIN

The bacterial cell contains the highly conserved protein HU in abundance. To characterize its architectural role, we studied the elastic behavior of single, supercoilable DNA molecules (tens of kilobases long) in solution with HU from B. stearothermophilus (BstHU) by a micromanipulation assay. We point out quantitative yet notable differences to the behavior of HU from E. coli (EcoHU) observed by others. Our main contribution here, however, is to characterize the interaction of BstHU with single molecules of DNA in arbitrary states of supercoiling. BstHU clearly distinguishes under- and overwound substrates, breaking the characteristic symmetry in the elastic response of bare DNA. We demonstrate that BstHU shifts the preferred linking number of the complex, consistent with a model in which bound proteins untwist the double helix. The model qualitatively explains various features, such as overall compaction and weaker dependence on supercoiling, by a softening of the DNA to twist and bending. Previously reported reversal of binding effects at protein concentrations above a threshold also extends to supercoiling. All observed effects are highly sensitive to salt concentrations. Their range and magnitude lend HU great versatility in dynamically altering the physical properties and organization of the nucleoid.

Analysis of PKR-RNA interactions by sedimentation velocity

PKR is an interferon-induced kinase that plays a pivotal role in the innate immunity pathway for defense against viral infection. PKR is activated to undergo autophosphorylation upon binding to RNAs that contain duplex regions. Some highly structured viral RNAs do not activate and function as PKR inhibitors. In order to define the mechanisms of activation and inhibition of PKR by RNA, it is necessary to characterize the stoichiometries, affinities, and free energy couplings governing the assembly of the relevant complexes. We have found sedimentation velocity analytical ultracentrifugation to be particularly useful in the study of PKR-RNA interactions. Here, we describe protocols for designing and analyzing sedimentation velocity experiments that are generally applicable to studies of protein-nucleic acid interactions. Initially, velocity data obtained at multiple protein:RNA ratios are analyzed using the dc/dt method's to define the association model and to test whether the system is kinetically limited. The sedimentation velocity data obtained at multiple loading concentrations are then globally fitted to this model to determine the relevant association constants. The frictional ratios of the complexes are calculated using the fitted sedimentation coefficients to determine whether the hydrodynamic properties are physically reasonable. We demonstrate the utility of this approach using examples from our studies of PKR interactions with simple dsRNAs, the HIV TAR RNA, and the VAI RNA from adenovirus.

Nonspecific DNA binding and bending by HUαβ: interfaces of the three binding modes characterized by salt-dependent thermodynamics

Previous isothermal titration calorimetry (ITC) and Förster resonance energy transfer studies demonstrated that Escherichia coli HU(αβ) binds nonspecifically to duplex DNA in three different binding modes: a tighter-binding 34-bp mode that interacts with DNA in large (>34 bp) gaps between bound proteins, reversibly bending it by 140(o) and thereby increasing its flexibility, and two weaker, modestly cooperative small site-size modes (10 bp and 6 bp) that are useful for filling gaps between bound proteins shorter than 34 bp. Here we use ITC to determine the thermodynamics of these binding modes as a function of salt concentration, and we deduce that DNA in the 34-bp mode is bent around-but not wrapped on-the body of HU, in contrast to specific binding of integration host factor. Analyses of binding isotherms (8-bp, 15-bp, and 34-bp DNA) and initial binding heats (34-bp, 38-bp, and 160-bp DNA) reveal that all three modes have similar log-log salt concentration derivatives of the binding constants (Sk(i)) even though their binding site sizes differ greatly; the most probable values of Sk(i) on 34-bp DNA or larger DNA are -7.5±0.5. From the similarity of Sk(i) values, we conclude that the binding interfaces of all three modes involve the same region of the arms and saddle of HU. All modes are entropy-driven, as expected for nonspecific binding driven by the polyelectrolyte effect. The bent DNA 34-bp mode is most endothermic, presumably because of the cost of HU-induced DNA bending, while the 6-bp mode is modestly exothermic at all salt concentrations examined. Structural models consistent with the observed Sk(i) values are proposed.

Understanding apparent DNA flexibility enhancement by HU and HMGB architectural proteins

Understanding and predicting the mechanical properties of protein/DNA complexes are challenging problems in biophysics. Certain architectural proteins bind DNA without sequence specificity and strongly distort the double helix. These proteins rapidly bind and unbind, seemingly enhancing the flexibility of DNA as measured by cyclization kinetics. The ability of architectural proteins to overcome DNA stiffness has important biological consequences, but the detailed mechanism of apparent DNA flexibility enhancement by these proteins has not been clear. Here, we apply a novel Monte Carlo approach that incorporates the precise effects of protein on DNA structure to interpret new experimental data for the bacterial histone-like HU protein and two eukaryotic high-mobility group class B (HMGB) proteins binding to ∼200-bp DNA molecules. These data (experimental measurement of protein-induced increase in DNA cyclization) are compared with simulated cyclization propensities to deduce the global structure and binding characteristics of the closed protein/DNA assemblies. The simulations account for all observed (chain length and concentration dependent) effects of protein on DNA behavior, including how the experimental cyclization maxima, observed at DNA lengths that are not an integral helical repeat, reflect the deformation of DNA by the architectural proteins and how random DNA binding by different proteins enhances DNA cyclization to different levels. This combination of experiment and simulation provides a powerful new approach to resolve a long-standing problem in the biophysics of protein/DNA interactions.

Force-driven unbinding of proteins HU and Fis from DNA quantified using a thermodynamic Maxwell relation

Determining numbers of proteins bound to large DNAs is important for understanding their chromosomal functions. Protein numbers may be affected by physical factors such as mechanical forces generated in DNA, e.g. by transcription or replication. We performed single-DNA stretching experiments with bacterial nucleoid proteins HU and Fis, verifying that the force-extension measurements were in thermodynamic equilibrium. We, therefore, could use a thermodynamic Maxwell relation to deduce the change of protein number on a single DNA due to varied force. For the binding of both HU and Fis under conditions studied, numbers of bound proteins decreased as force was increased. Our experiments showed that most of the bound HU proteins were driven off the DNA at 6.3 pN for HU concentrations lower than 150 nM; our HU data were fit well by a statistical-mechanical model of protein-induced bending of DNA. In contrast, a significant amount of Fis proteins could not be forced off the DNA at forces up to 12 pN and Fis concentrations up to 20 nM. This thermodynamic approach may be applied to measure changes in numbers of a wide variety of molecules bound to DNA or other polymers. Force-dependent DNA binding by proteins suggests mechano-chemical mechanisms for gene regulation.

HU binding to a DNA four-way junction probed by Förster resonance energy transfer

The Escherichia coli protein HU is a non-sequence-specific DNA-binding protein that interacts with DNA primarily through electrostatic interactions. In addition to nonspecific binding to linear DNA, HU has been shown to bind with nanomolar affinity to discontinuous DNA substrates, such as repair and recombination intermediates. This work specifically examines the HU-four-way junction (4WJ) interaction using fluorescence spectroscopic methods. The conformation of the junction in the presence of different counterions was investigated by Förster resonance energy transfer (FRET) measurements, which revealed an ion-type conformational dependence, where Na(+) yields the most stacked conformation followed by K(+) and Mg(2+). HU binding induces a greater degree of stacking in the Na(+)-stabilized and Mg(2+)-stabilized junctions but not the K(+)-stabilized junction, which is attributed to differences in the size of the ionic radii and potential differences in ion binding sites. Interestingly, junction conformation modulates binding affinity, where HU exhibits the lowest affinity for the Mg(2+)-stabilized form (24 μM(-1)), which is the least stacked conformation. Protein binding to a mixed population of open and stacked forms of the junction leads to nearly complete formation of a protein-stabilized stacked-X junction. These results strongly support a model in which HU binds to and stabilizes the stacked-X conformation.

Photophysical and structural properties of the fluorescent nucleobase analogues of the tricyclic cytosine (tC) family

Fundamental insight into the unique fluorescence and nucleobase-mimicking properties of the fluorescent nucleobase analogues of the tC family is not only vital in explaining the behaviour of these probes in nucleic acid environments, but will also be profitable in the development of new and improved fluorescent base analogues. Here, temperature-dependent fluorescence quantum yield measurements are used to successfully separate and quantify the temperature-dependent and temperature-independent non-radiative excited-state decay processes of the three nucleobase analogues tC, tC(O) and tC(nitro); all of which are derivatives of a phenothiazine or phenoxazine tricyclic framework. These results strongly suggest that the non-radiative decay process dominating the fast deactivation of tC(nitro) is an internal conversion of a different origin than the decay pathways of tC and tC(O). tC(nitro) is reported to be fluorescent only in less dipolar solvents at room temperature, which is explained by an increase in excited-state dipole moment along the main non-radiative decay pathway, a suggestion that applies in the photophysical discussion of large polycyclic nitroaromatics in general. New insight into the ground and excited-state potential energy surfaces of the isolated tC bases is obtained by means of high level DFT and TDDFT calculations. The S(0) potential energy surfaces of tC and tC(nitro) possess two global minima corresponding to geometries folded along the middle sulfur-nitrogen axis separated by an energy barrier of 0.05 eV as calculated at the B3LYP/6-311+G(2d,p) level. The ground-state potential energy surface of tC(O) is also predicted to be shallow along the bending coordinate but with an equilibrium geometry corresponding to the planar conformation of the tricyclic framework, which may explain some of the dissimilar properties of tC and tC(O) in various confined (biological) environments. The S(1) equilibrium geometries of all three base analogues are predicted to be planar. These results are discussed in the context of the tC bases positioned in double-stranded DNA scenarios.

Modulation of HU-DNA interactions by salt concentration and applied force

HU is one of the most abundant proteins in bacterial chromosomes and participates in nucleoid compaction and gene regulation. We report experiments using DNA stretching that study the dependence of DNA condensation by HU on force, salt and HU concentration. Previous experiments at sub-physiological salt levels revealed that low concentrations of HU could compact DNA, whereas larger HU concentrations formed a DNA-stiffening complex. Here we report that this bimodal binding behavior depends sensitively on salt concentration. Only the compaction mode was observed for 150 mM and higher NaCl levels, i.e. for physiological salt concentrations. Similar results were obtained for the more physiological salt K-glutamate. Real-time studies of dissociation kinetics revealed that HU unbound slowly (minutes to hours under the conditions studied) but completely for salt concentrations at or above 100 mM NaCl; the lifetime of HU complexes was observed to increase with the HU concentration at which the complexes were formed, and to decrease with salt concentration. Higher salt levels of 300 mM NaCl completely eliminated observable HU binding to DNA. Finally, we observed that the dissociation kinetics depend on force applied to the DNA: increased applied force in the sub-piconewton range accelerates dissociation, suggesting a mechanism for DNA tension to regulate chromosome structure and gene expression.

Fluorescent nucleic acid base analogues

The use of fluorescent nucleic acid base analogues is becoming increasingly important in the fields of biology, biochemistry and biophysical chemistry as well as in the field of DNA nanotechnology. The advantage of being able to incorporate a fluorescent probe molecule close to the site of examination in the nucleic acid-containing system of interest with merely a minimal perturbation to the natural structure makes fluorescent base analogues highly attractive. In recent years, there has been a growing interest in developing novel candidates in this group of fluorophores for utilization in various investigations. This review describes the different classes of fluorophores that can be used for studying nucleic acid-containing systems, with an emphasis on choosing the right kind of probe for the system under investigation. It describes the characteristics of the large group of base analogues that has an emission that is sensitive to the surrounding microenvironment and gives examples of investigations in which this group of molecules has been used so far. Furthermore, the characterization and use of fluorescent base analogues that are virtually insensitive to changes in their microenvironment are described in detail. This group of base analogues can be used in several fluorescence investigations of nucleic acids, especially in fluorescence anisotropy and fluorescence resonance energy transfer (FRET) measurements. Finally, the development and characterization of the first nucleic base analogue FRET pair, tC(O)-tC(nitro), and its possible future uses are discussed.

Two distinct overstretched DNA states

The DNA double helix undergoes an ‘overstretching’ transition in a narrow force range near 65 pN. Despite numerous studies the basic question of whether the strands are separated or not remains controversial. Here we show that overstretching in fact involves two distinct types of double-helix reorganization: slow hysteretic ‘unpeeling’ of one strand off the other; and a fast, non-hysteretic transition to an elongated double-stranded form. We demonstrate that the relative fraction of these two overstretched forms is sensitive to factors that affect DNA base pair stability, including DNA sequence, salt concentration and temperature. The balance between the two forms shifts near physiological solution conditions. This result, in addition to establishing the existence of an overstretched double-stranded state, also shows that double helix physical properties are tuned so that either unpeeling or overextension can be selected via small changes in molecule environment.

Characterization of nucleobase analogue FRET acceptor tCnitro

The fluorescent nucleobase analogues of the tricyclic cytosine (tC) family, tC and tC(O), possess high fluorescence quantum yields and single fluorescence lifetimes, even after incorporation into double-stranded DNA, which make these base analogues particularly useful as fluorescence resonance energy transfer (FRET) probes. Recently, we reported the first all-nucleobase FRET pair consisting of tC(O) as the donor and the novel tC(nitro) as the acceptor. The rigid and well-defined position of this FRET pair inside the DNA double helix, and consequently excellent control of the orientation factor in the FRET efficiency, are very promising features for future studies of nucleic acid structures. Here, we provide the necessary spectroscopic and photophysical characterization of tC(nitro) needed in order to utilize this probe as a FRET acceptor in nucleic acids. The lowest energy absorption band from 375 to 525 nm is shown to be the result of a single in-plane polarized electronic transition oriented approximately 27 degrees from the molecular long axis. This band overlaps the emission bands of both tC and tC(O), and the Forster characteristics of these donor-acceptor pairs are calculated for double-stranded DNA scenarios. In addition, the UV-vis absorption of tC(nitro) is monitored in a broad pH range and the neutral form is found to be totally predominant under physiological conditions with a pK(a) of 11.1. The structure and electronic spectrum of tC(nitro) is further characterized by density functional theory calculations.

Fluorescence properties of pyrimidopyrimidoindole nucleoside dC(PPI) incorporated into oligodeoxynucleotides

A series of oligodeoxynucleotides labeled by a pyrimidopyrimidoindole deoxynucleoside (1a: dC(PPI)) and its derivatives 2a and 3a substituted with electron-donating and -withdrawing groups, respectively, were synthesized according to the phosphoramidite approach. The photophysical properties and quenching efficiencies of oligonucleotides incorporating dC(PPI) derivatives were studied in detail. The thermal denaturation experiments and molecular dynamics simulation of DNA duplexes incorporating dC(PPI) suggested that a modified base of dC(PPI) could form base pairs with guanine and adenine in canonical Watson-Crick and reverse-wobble geometries, respectively. The fluorescence of oligonucleotides incorporating dC(PPI) derivatives increased upon binding to the counter strands, except when dC(PPI) and guanine formed a base pair. It was revealed that dGMP quenched the fluorescence of the cyano derivative 3a most effectively, whereas it affected that of the methoxy derivative 2a least effectively. The involvement of the electron transfer from guanine to the dC(PPI) derivatives in the fluorescence quenching was supported by energy considerations.

The multifaceted proteins MvaT and MvaU, members of the H-NS family, control arginine metabolism, pyocyanin synthesis, and prophage activation in Pseudomonas aeruginosa PAO1

The MvaT and MvaU proteins belonging to the H-NS family were identified as DNA-binding proteins that interact with the regulatory region of the aotJQMOP-argR operon for arginine uptake and regulation. Recombinant MvaT and MvaU proteins were purified, and binding of these purified proteins to the aotJ regulatory region was demonstrated using electromobility shift assays. Polyclonal antibodies against purified MvaT and MvaU were prepared and employed in supershift assays to support these observations. Knockout mutations resulting in a single lesion in mvaT or mvaU, as well as knockout mutations resulting in double lesions, were constructed using biparental conjugation, and the absence of MvaT and MvaU in the resulting mutants was confirmed by immunoblot analysis. Using measurements of the beta-galactosidase activities from aotJ::lacZ fusions in the mutants and the parental strain, it was found that MvaT and MvaU serve as repressors in control of aotJ expression. The effects of MvaT and MvaU on pyocyanin synthesis and CupA fimbrial expression in these mutants were also analyzed. Pyocyanin synthesis was induced in the single mutants but was completely abolished in the double mutant, suggesting that there is a complicated regulatory scheme in which MvaT and MvaU are essential elements. In comparison, MvaT had a more profound role than MvaU as a repressor of cupA expression; however, a combination of MvaT depletion and MvaU depletion had a strong synergistic effect on cupA. Moreover, prophage Pf4 integrated into the chromosome of Pseudomonas aeruginosa PAO1 was activated in an mvaT mvaU double mutant but not in a single mutant. These results were supported by purification and nucleotide sequencing of replicative-form DNA and by the release of phage particles in plaque assays. In summary, the mvaT mvaU double mutant was viable, and depletion of MvaT and MvaU had serious effects on a variety of physiological functions in P. aeruginosa.

Sequence-specific binding to a subset of IscR-regulated promoters does not require IscR Fe-S cluster ligation

IscR is an Fe-S protein that functions as a transcriptional regulator of Fe-S biogenesis and other Fe-S protein-encoding genes in Escherichia coli. In this study, we investigated the requirement for the ligation of the [2Fe-2S] cluster of IscR to regulate a subset of IscR target promoters (P(hyaA), P(ydiU), P(napF), and P(hybO)) and defined the requirements for sequence-specific binding to the IscR target site in the hyaA promoter region. In contrast to previous results with the iscR promoter, we found that the Fe-S cluster is dispensable for IscR regulation of P(hyaA), P(ydiU), P(napF), and P(hybO), since IscR mutants containing alanine substitutions of the cysteine Fe-S ligands retained IscR-dependent regulation of these promoters in vivo. In vitro assays showed that both [2Fe-2S]-IscR and an IscR mutant lacking the cluster (IscR-C92A/C98A/C104A) bound the hya site with similar affinity, explaining why the mutant protein retained its ability to repress P(hyaA) in vivo. Characterization of the oligomeric state of IscR showed that both apo-IscR and [2Fe-2S]-IscR were dimers in solution, and four protomers of either form bound to the hya site. Also, binding of either apo- or [2Fe-2S]-IscR to the hya site showed cooperativity, suggesting that both forms interact similarly with the target site. Analysis of mutations in the hya site using DNA competition assays showed that apo-IscR most likely recognizes an imperfect palindrome within the hya promoter. Furthermore, the strength of apo-IscR binding to P(sufA), P(ydiU), P(napF), and P(hybO) IscR sites correlated with the number of matches to the hya site bases shown to be important in the competition assay. Thus, our data indicated that, unexpectedly, apo-IscR is a site-specific DNA-binding protein, and the role of apo-IscR needs to be considered in developing models for how IscR globally regulates transcription.

DNA binding mode transitions of Escherichia coli HU(alphabeta): evidence for formation of a bent DNA--protein complex on intact, linear duplex DNA

Escherichia coli HU(alphabeta), a major nucleoid-associated protein, organizes chromosomal DNA and facilitates numerous DNA transactions. Using isothermal titration calorimetry, fluorescence resonance energy transfer and a series of DNA lengths (8 bp, 15 bp, 34 bp, 38 bp and 160 bp) we established that HU(alphabeta) interacts with duplex DNA using three different nonspecific binding modes. Both the HU to DNA molar ratio ([HU]/[DNA]) and DNA length dictate the dominant HU binding mode. On sufficiently long DNA (> or =34 bp), at low [HU]/[DNA], HU populates a noncooperative 34 bp binding mode with a binding constant of 2.1+/-0.4x10(6) M(-1), and a binding enthalpy of +7.7+/-0.6 kcal/mol at 15 degrees C and 0.15 M Na(+). With increasing [HU]/[DNA], HU bound in the noncooperative 34 bp mode progressively converts to two cooperative (omega approximately 20) modes with site sizes of 10 bp and 6 bp. These latter modes exhibit smaller binding constants (1.1+/-0.2x10(5) M(-1) for the 10 bp mode, 3.5+/-1.4x10(4) M(-1) for the 6 bp mode) and binding enthalpies (4.2+/-0.3 kcal/mol for the 10 bp mode, -1.6+/-0.3 kcal/mol for the 6 bp mode). As DNA length increases to 34 bp or more at low [HU]/[DNA], the small modes are replaced by the 34 bp binding mode. Fluorescence resonance energy transfer data demonstrate that the 34 bp mode bends DNA by 143+/-6 degrees whereas the 6 bp and 10 bp modes do not. The model proposed in this study provides a novel quantitative and comprehensive framework for reconciling previous structural and solution studies of HU, including single molecule (force extension measurement), fluorescence, and electrophoretic gel mobility-shift assays. In particular, it explains how HU condenses or extends DNA depending on the relative concentrations of HU and DNA.

The essentiality and involvement of Streptococcus intermedius histone-like DNA-binding protein in bacterial viability and normal growth

Streptococcus intermedius histone-like DNA-binding protein (Si-HLP) is a homodimeric protein and, conserved with Escherichia coli HU, a well-documented nucleoid-associated protein (NAP). In E. coli, HU plays important roles as both structural and regulatory factors, but it is not essential for E. coli viability. Streptococcal HLP has been found to bind host cells and induce cytokine production, but its physiological role remains poorly defined. In the present study, using gene insertion knockout and tetracycline-regulated antisense RNA expression techniques, we determined whether Si-HLP is essential for bacterial viability and normal growth in S. intermedius. The Si-HLP-downregulated S. intermedius strain showed alterations in its morphology and surface properties. Downregulation of Si-HLP led to an expanded nucleoid to fill the intracellular space. Transcription levels of several genes, including virulence-associated factors, were found to be activated or repressed in the antisense Si-hlp RNA-expressing strain by real-time PCR and reverse-transcription PCR. Collectively, these data suggest that Si-HLP serves as an essential NAP governing the nucleoid architecture and controlling the gene transcription profile in S. intermedius.

Fluorescent pteridine probes for nucleic acid analysis

This chapter is focused on the fluorescent pteridine guanine analogs, 3MI and 6MI and on the pteridine adenine analog, 6MAP. A brief overview of commonly used methods to fluorescently label oligonucleotides reveals the role the pteridines play in the extensive variety of available probes. We describe the fluorescence characteristics of the pteridine probes as monomers and incorporated into DNA and review a variety of applications including changes in fluorescence intensity, anisotropies, time resolved studies, two photon excitation and single molecule detection.

Characterization and use of an unprecedentedly bright and structurally non-perturbing fluorescent DNA base analogue

This article presents the first evidence that the DNA base analogue 1,3-diaza-2-oxophenoxazine, tC(O), is highly fluorescent, both as free nucleoside and incorporated in an arbitrary DNA structure. tC(O) is thoroughly characterized with respect to its photophysical properties and structural performance in single- and double-stranded oligonucleotides. The lowest energy absorption band at 360 nm (epsilon = 9000 M(-1) cm(-1)) is dominated by a single in-plane polarized electronic transition and the fluorescence, centred at 465 nm, has a quantum yield of 0.3. When incorporated into double-stranded DNA, tC(O) shows only minor variations in fluorescence intensity and lifetime with neighbouring bases, and the average quantum yield is 0.22. These features make tC(O), on average, the brightest DNA-incorporated base analogue so far reported. Furthermore, it base pairs exclusively with guanine and causes minimal perturbations to the native structure of DNA. These properties make tC(O) a promising base analogue that is perfectly suited for e.g. photophysical studies of DNA interacting with macromolecules (proteins) or for determining size and shape of DNA tertiary structures using techniques such as fluorescence anisotropy and fluorescence resonance energy transfer (FRET).

Synthesis, purification and sample experiment for fluorescent pteridine-containing DNA: tools for studying DNA interactive systems

Fluorescent nucleoside analogs provide a means to study DNA interactive systems through direct measurement of fluorescence properties. As integrated parts of DNA, these probes provide opportunities for monitoring subtle changes in DNA structure as it meets and reacts with other molecules. This protocol describes modifications to standard DNA synthesis to efficiently use smaller volumes of the probe phosphoramidite, purification of pteridine-containing sequences and a deprotection procedure specific for 6MI-containing sequences. Yields for probe incorporation in DNA synthesis are comparable to those for standard phosphoramidites. Examples of the fluorescence signals one can expect are described. Automated synthesis, which is dependent on the length of the sequence, takes about 4-5 h for a 20-mer. The deprotection of 6MI-containing sequences takes approximately 6-7 h before the standard ammonium hydroxide overnight incubation. Purification through polyacrylamide gels, electroelution and ethanol precipitation can be accomplished in 6-8 h.

Bacterial protein HU dictates the morphology of DNA condensates produced by crowding agents and polyamines

Controlling the size and shape of DNA condensates is important in vivo and for the improvement of nonviral gene delivery. Here, we demonstrate that the morphology of DNA condensates, formed under a variety of conditions, is shifted completely from toroids to rods if the bacterial protein HU is present during condensation. HU is a non-sequence-specific DNA binding protein that sharply bends DNA, but alone does not condense DNA into densely packed particles. Less than one HU dimer per 225 bp of DNA is sufficient to completely control condensate morphology when DNA is condensed by spermidine. We propose that rods are favored in the presence of HU because rods contain sharply bent DNA, whereas toroids contain only smoothly bent DNA. The results presented illustrate the utility of naturally derived proteins for controlling the shape of DNA condensates formed in vitro. HU is a highly conserved protein in bacteria that is implicated in the compaction and shaping of nucleoid structure. However, the exact role of HU in chromosome compaction is not well understood. Our demonstration that HU governs DNA condensation in vitro also suggests a mechanism by which HU could act as an architectural protein for bacterial chromosome compaction and organization in vivo.

A gamut of loops: meandering DNA

Nucleoprotein complexes comprising short DNA loops (150 base pairs or less) are involved in a wide variety of DNA transactions (e.g. transcription regulation, replication and recombination) in both prokaryotes and eukaryotes, and also can be useful in designing nanostructures. In these higher-order nucleoprotein complexes, proteins bound to spatially separated sites on a DNA interact with each other by looping out the relatively stiff intervening DNA. Recent technological developments have enabled determination of DNA trajectories in a few DNA-loop-containing regulatory complexes. Results show that, in a given system, a specific DNA trajectory is preferred over others.

Effects of kinks on DNA elasticity

We study the elastic response of a wormlike polymer chain with reversible kinklike structural defects. This is a generic model for (a) the double-stranded DNA with sharp bends induced by binding of certain proteins, and (b) effects of trans-gauche rotations in the backbone of the single-stranded DNA. The problem is solved both analytically and numerically by generalizing the well-known analogy to the quantum rotator. In the small stretching force regime, we find that the persistence length is renormalized due to the presence of the kinks. In the opposite regime, the response to the strong stretching is determined solely by the bare persistence length with exponential corrections due to the "ideal gas of kinks." This high-force behavior changes significantly in the limit of high bending rigidity of the chain. In that case, the leading corrections to the mechanical response are likely to be due to the formation of multikink structures, such as kink pairs.

The HU-DNA binding interaction probed with UV resonance Raman spectroscopy: structural elements of specificity

The Escherichia coli protein HU functions as an architectural DNA-binding protein by facilitating DNA looping or bending to form multiprotein complexes. Although HU does not recognize a specific DNA sequence, site-specific binding to a number of discontinuous, looped, or bent DNA substrates has been observed. In this study UV resonance Raman (UVRR) spectroscopy is used to identify structural elements associated with low- and high-affinity binding by examining three different HU-DNA complexes. UVRR spectra obtained with an excitation wavelength of 210 nm, which preferentially enhances protein backbone amide vibrations, indicate that HU secondary structure content increases and the protein structure becomes more rigid upon binding to DNA. The increase in alpha-helical content is attributed to the C-terminal helix, which interacts with the DNA and may play a role in binding affinity and specificity. UVRR spectra obtained with a 215 nm excitation wavelength demonstrate that Pro mode intensity at 1455 cm(-1) decreases upon complex formation. This intensity decrease is attributed to the intercalation of Pro residues between DNA base pairs to induce a bend in the DNA, as has been observed previously in the IHF-DNA and HU-DNA cocrystal structures. DNA vibrational modes are also indicative of significant base unstacking and opening of the minor groove upon protein binding, consistent with bending and distortion of the DNA. In all three complexes, A-DNA conformational features are indicated by deoxyribose-phosphate backbone modes. These and other results suggest that protein-induced bending plays an important role in HU site-specific binding and supports a model of a mutually induced fit.

Substrate specificity of Helicobacter pylori histone-like HU protein is determined by insufficient stabilization of DNA flexure points

The histone-like HU protein is ubiquitous in the eubacteria. A role for Escherichia coli HU in compaction of the bacterial genome has been reported, along with regulatory roles in DNA replication, transposition, repair and transcription. We show here that HU from the human pathogen Helicobacter pylori, which has been implicated in the development of ulcers and gastric cancer, exhibits enhanced thermal stability and distinct DNA substrate specificity. Thermal denaturation of HpyHU (H. pylori HU) measured by CD spectroscopy yields a melting temperature (T(m)) of 56.4+/-0.1 degrees C. HpyHU binds linear duplex DNA with a site size of approximately 19 bp and with low affinity, but in striking contrast to E. coli HU, HpyHU has only modest preference for DNA with mismatches, nicks or gaps. Instead, HpyHU binds stably to four-way DNA junctions with half-maximal saturation of 5 nM. Substitution of two residues adjacent to the DNA-intercalating prolines attenuates both the preference for flexible DNA and the ability to bend and supercoil DNA. These observations suggest that proline intercalation generates hinges that must be stabilized by adjacent residues; insufficient stabilization leads to reduced bending and a failure to bind preferably to DNA with flexure points, such as gaps and mismatches.

DNA mismatch correction in Haemophilus influenzae: characterization of MutL, MutH and their interaction

Haemophilus influenzae DNA mismatch repair proteins, MutS, MutL and MutH, are functionally characterized in this study. Introduction of mutS, mutL and mutH genes of H. influenzae resulted in complementation of the mismatch repair activity of the respective mutant strains of Escherichia coli to varying levels. DNA binding studies using H. influenzae MutH have shown that the protein is capable of binding to any DNA sequence non-specifically in a co-operative and metal independent manner. Presence of MutL and ATP in the binding reaction resulted in the formation of a more specific complex, which indicates that MutH is conferred specificity for binding hemi-methylated DNA through structural alterations mediated by its interaction with MutL. To study the role of conserved amino acids Ile213 and Leu214 in the helix at the C-terminus of MutH, they were mutated to alanine. The mutant proteins showed considerably reduced DNA binding and nicking, as well as MutL-mediated activation. MutH failed to nick HU bound DNA whereas MboI and Sau3AI, which have the same recognition sequence as MutH, efficiently cleaved the substrate. MutS ATPase activity was found to be reduced two-fold in presence of covalently closed circular duplex containing a mismatched base pair whereas, the activity was regained upon linearization of the circular duplex. This observation possibly suggests that the MutS clamps are trapped in the closed DNA heteroduplex. These studies, therefore, serve as the basis for a detailed investigation of the structure-function relationship among the protein partners of the mismatch repair pathway of H. influenzae.

IHF and HU: flexible architects of bent DNA

The energetic cost of bending short segments of DNA is very high. This bending is critical for the packaging of DNA and is exploited to regulate many cellular processes. In prokaryotes, IHF and HU are key architectural proteins present at high concentrations. New protein-DNA co-crystal structures, and the adaptation of advanced biophysical and biochemical techniques have led to an improved understanding of how these proteins interact with DNA. These techniques include time-resolved synchrotron X-ray footprinting, differential scanning calorimetry, isothermal titration calorimetry and single-molecule experiments.

Effects of HU binding on the equilibrium cyclization of mismatched, curved, and normal DNA

The effects of HU, the histone-like protein from Escherichia coli, on the equilibrium cyclization of duplex DNAs have been observed as a function of protein concentration and DNA sequence. The results indicate that the presence of HU significantly enhances the extent of cyclization and increases the melting temperature, T(m), of the cyclized form of the DNA by >10 K. The stabilization of equilibrium cyclization by HU binding is at least -1.2 kcal/mol. The results are consistent with two HU homotypic dimers binding to each of the three 29-mer duplexes studied. One of the 29-mer duplexes contains a central dA tract, one contains mismatched sites, and one a conventional sequence. Stepwise or microscopic association constants, determined from the fluorescence data, range from 1.5 to 0.6 micro M(-1). The binding affinity of the HU dimer is strongest for the mismatched duplex and lowest for the dA tract, consistent with HU dimers having a preference for flexible DNA substrates. These results demonstrate the utility of the equilibrium cyclization approach to monitor DNA-protein interactions. These results have been considered along with those previously obtained to refine a model for the interaction of HU with duplex DNA.

Global analysis of non-specific protein-nucleic interactions by sedimentation equilibrium

Protein-nucleic acid interactions govern a variety of processes, including replication, transcription, recombination and repair. These interactions take place in both sequence-specific and non-specific modes, and the latter occur in many biologically significant contexts. Analytical ultracentrifugation is a useful method for the detailed characterization of the stoichiometry and affinity of macromolecular interactions in free solution. There has been a resurgence of interest in the application of sedimentation equilibrium methods to protein-nucleic acid interactions. However, these studies have been generally focused on sequence-specific interactions. Here we describe an approach to analyze non-specific interactions using sedimentation equilibrium. We have adapted an existing model for non-specific interaction of proteins with finite, one-dimensional nucleic acid lattices for global fitting of multiwavelength sedimentation equilibrium data. The model is extended to accommodate protein binding to multiple faces of the nucleic acid, resulting in overlap of consecutive ligands along the sequence of the RNA or DNA. The approach is illustrated in a sedimentation equilibrium analysis of the interaction of the double-stranded RNA binding motif of protein kinase R with a 20-basepair RNA construct.

Modulation of DNA conformations through the formation of alternative high-order HU-DNA complexes

HU is an abundant, highly conserved protein associated with the bacterial chromosome. It belongs to a small class of proteins that includes the eukaryotic proteins TBP, SRY, HMG-I and LEF-I, which bind to DNA non-specifically at the minor groove. HU plays important roles as an accessory architectural factor in a variety of bacterial cellular processes such as DNA compaction, replication, transposition, recombination and gene regulation. In an attempt to unravel the role this protein plays in shaping nucleoid structure, we have carried out fluorescence resonance energy transfer measurements of HU-DNA oligonucleotide complexes, both at the ensemble and single-pair levels. Our results provide direct experimental evidence for concerted DNA bending by HU, and the abrogation of this effect at HU to DNA ratios above about one HU dimer per 10-12 bp. These findings support a model in which a number of HU molecules form an ordered helical scaffold with DNA lying in the periphery. The abrogation of these nucleosome-like structures for high HU to DNA ratios suggests a unique role for HU in the dynamic modulation of bacterial nucleoid structure.