Protein-DNA conformational changes in the crystal structure of a lambda Cro-operator complex

TetR and OmpR family regulators in natural product biosynthesis and resistance

This article provides a comprehensive review and sequence-structure analysis of transcription regulator (TR) families, TetR and OmpR/PhoB, involved in specialized secondary metabolite (SSM) biosynthesis and resistance. Transcription regulation is a fundamental process, playing a crucial role in orchestrating gene expression to confer a survival advantage in response to frequent environmental stress conditions. This process, coupled with signal sensing, enables bacteria to respond to a diverse range of intra and extracellular signals. Thus, major bacterial signaling systems use a receptor domain to sense chemical stimuli along with an output domain responsible for transcription regulation through DNA-binding. Sensory and output domains on a single polypeptide chain (one component system, OCS) allow response to stimuli by allostery, that is, DNA-binding affinity modulation upon signal presence/absence. On the other hand, two component systems (TCSs) allow cross-talk between the sensory and output domains as they are disjoint and transmit information by phosphorelay to mount a response. In both cases, however, TRs play a central role. Biosynthesis of SSMs, which includes antibiotics, is heavily regulated by TRs as it diverts the cell's resources towards the production of these expendable compounds, which also have clinical applications. These TRs have evolved to relay information across specific signals and target genes, thus providing a rich source of unique mechanisms to explore towards addressing the rapid escalation in antimicrobial resistance (AMR). Here, we focus on the TetR and OmpR family TRs, which belong to OCS and TCS, respectively. These TR families are well-known examples of regulators in secondary metabolism and are ubiquitous across different bacteria, as they also participate in a myriad of cellular processes apart from SSM biosynthesis and resistance. As a result, these families exhibit higher sequence divergence, which is also evident from our bioinformatic analysis of 158 389 and 77 437 sequences from TetR and OmpR family TRs, respectively. The analysis of both sequence and structure allowed us to identify novel motifs in addition to the known motifs responsible for TR function and its structural integrity. Understanding the diverse mechanisms employed by these TRs is essential for unraveling the biosynthesis of SSMs. This can also help exploit their regulatory role in biosynthesis for significant pharmaceutical, agricultural, and industrial applications.

How structural biology transformed studies of transcription regulation

The past 4 decades have seen remarkable advances in our understanding of the structural basis of gene regulation. Technological advances in protein expression, nucleic acid synthesis, and structural biology made it possible to study the proteins that regulate transcription in the context of ever larger complexes containing proteins bound to DNA. This review, written on the occasion of the 50th anniversary of the founding of the Protein Data Bank focuses on the insights gained from structural studies of protein-DNA complexes and the role the PDB has played in driving this research. I cover highlights in the field, beginning with X-ray crystal structures of the first DNA-binding domains to be studied, through recent cryo-EM structures of transcription factor binding to nucleosomal DNA.

A thermophilic phage uses a small terminase protein with a fixed helix-turn-helix geometry

Tailed bacteriophages use a DNA-packaging motor to encapsulate their genome during viral particle assembly. The small terminase (TerS) component of this DNA-packaging machinery acts as a molecular matchmaker that recognizes both the viral genome and the main motor component, the large terminase (TerL). However, how TerS binds DNA and the TerL protein remains unclear. Here we identified gp83 of the thermophilic bacteriophage P74-26 as the TerS protein. We found that TerS^P76-26 oligomerizes into a nonamer that binds DNA, stimulates TerL ATPase activity, and inhibits TerL nuclease activity. A cryo-EM structure of TerS^P76-26 revealed that it forms a ring with a wide central pore and radially arrayed helix-turn-helix domains. The structure further showed that these helix-turn-helix domains, which are thought to bind DNA by wrapping the double helix around the ring, are rigidly held in an orientation distinct from that seen in other TerS proteins. This rigid arrangement of the putative DNA-binding domain imposed strong constraints on how TerS^P76-26 can bind DNA. Finally, the TerS^P76-26 structure lacked the conserved C-terminal β-barrel domain used by other TerS proteins for binding TerL. This suggests that a well-ordered C-terminal β-barrel domain is not required for TerS^P76-26 to carry out its matchmaking function. Our work highlights a thermophilic system for studying the role of small terminase proteins in viral maturation and presents the structure of TerS^P76-26, revealing key differences between this thermophilic phage and its mesophilic counterparts.

Studying protein-DNA interactions using atomic force microscopy

Atomic force microscopy (AFM) has made significant contributions to the study of protein-DNA interactions by making it possible to topographically image biological samples. A single protein-DNA binding reaction imaged by AFM can reveal protein binding specificity and affinity, protein-induced DNA bending, and protein binding stoichiometry. Changes in DNA structure, complex conformation, and cooperativity, can also be analyzed. In this review we highlight some important examples in the literature and discuss the advantages and limitations of these measurements. We also discuss important advances in technology that will facilitate the progress of AFM in the future.

Ultrafast differential flexibility of Cro-protein binding domains of two operator DNAs with different sequences

The crucial ultrafast domain fluctuation of the operator DNA O_R3 over O_R2 upon complexation with the repressor Cro-protein dimer has been investigated.

Engineering bacterial transcription regulation to create a synthetic in vitro two-hybrid system for protein interaction assays

Transcriptional activation of σ(54)-RNA polymerase holoenzyme (σ(54)-RNAP) in bacteria is dependent on a cis-acting DNA element (bacterial enhancer), which recruits the bacterial enhancer-binding protein to contact the holoenzyme via DNA looping. Using a constructive synthetic biology approach, we recapitulated such process of transcriptional activation by recruitment in a reconstituted cell-free system, assembled entirely from a defined number of purified components. We further engineered the bacterial enhancer-binding protein PspF to create an in vitro two-hybrid system (IVT2H), capable of carrying out gene regulation in response to expressed protein interactions. Compared with genetic systems and other in vitro methods, IVT2H not only allows detection of different types of protein interactions in just a few hours without involving cells but also provides a general correlation of the relative binding strength of the protein interaction with the IVT2H signal. Due to its reconstituted nature, IVT2H provides a biochemical assay platform with a clean and defined background. We demonstrated the proof-of-concept of using IVT2H as an alternative assay for high throughput screening of small-molecule inhibitors of protein-protein interaction.

Local conformational changes in the DNA interfaces of proteins

When a protein binds to DNA, a conformational change is often induced so that the protein will fit into the DNA structure. Therefore, quantitative analyses were conducted to understand the conformational changes in proteins. The results showed that conformational changes in DNA interfaces are more frequent than in non-interfaces, and DNA interfaces have more conformational variations in the DNA-free form. As expected, the former indicates that interaction with DNA has some influence on protein structure. The latter suggests that the intrinsic conformational flexibility of DNA interfaces is important for adjusting their conformation for DNA. The amino acid propensities of the conformationally changed regions in DNA interfaces indicate that hydrophilic residues are preferred over the amino acids that appear in the conformationally unchanged regions. This trend is true for disordered regions, suggesting again that intrinsic flexibility is of importance not only for DNA binding but also for interactions with other molecules. These results demonstrate that fragments destined to be DNA interfaces have an intrinsic flexibility and are composed of amino acids with the capability of binding to DNA. This information suggests that the prediction of DNA binding sites may be improved by the integration of amino acid preference for DNA and one for disordered regions.

A synthetic peptide mimic of λ-Cro shows sequence-specific binding in vitro and in vivo

Development of small synthetic transcription factors is important for future cellular engineering and therapeutics. This article describes the chemical synthesis of α-amino-isobutyric acid (Aib) substituted, conformationally constrained, helical peptide mimics of Cro protein from bacteriophage λ that encompasses the DNA recognition elements. The Aib substituted constrained helical peptide monomer shows a moderately reduced dissociation constant compared to the corresponding unsubstituted wild type peptide. A suitably cross-linked dimeric version of the peptide, mimicking the dimeric protein, recapitulates some of the important features of Cro. It binds to the operator site O(R)3, a high affinity Cro binding site in the λ genome, with good affinity and single base-pair discrimination specificity. A dimeric version of an even shorter peptide mimic spanning only the recognition helix of the helix-turn-helix motif of the Cro protein was created following the same design principles. This dimeric peptide binds to O(R)3 with affinity greater than that of the longer version. Chemical shift perturbation experiments show that the binding mode of this peptide dimer to the cognate operator site sequence is similar to the wild type Cro protein. A Green Fluorescent Protein based reporter assay in vivo reveals that the peptide dimer binds the operator site sequences with considerable selectivity and inhibits gene expression. Peptide mimics designed in this way may provide a future framework for creating effective synthetic transcription factors.

Recognition of dual symmetry by the controller protein C.Esp1396I based on the structure of the transcriptional activation complex

The controller protein C.Esp1396I regulates the timing of gene expression of the restriction–modification (RM) genes of the RM system Esp1396I. The molecular recognition of promoter sequences by such transcriptional regulators is poorly understood, in part because the DNA sequence motifs do not conform to a well-defined symmetry. We report here the crystal structure of the controller protein bound to a DNA operator site. The structure reveals how two different symmetries within the operator are simultaneously recognized by the homo-dimeric protein, underpinned by a conformational change in one of the protein subunits. The recognition of two different DNA symmetries through movement of a flexible loop in one of the protein subunits may represent a general mechanism for the recognition of pseudo-symmetric DNA sequences.

[Regulation of gene expression in type II restriction-modification system]

Type II restriction-modification systems are comprised of a restriction endonuclease and methyltransferase. The enzymes are coded by individual genes and recognize the same DNA sequence. Endonuclease makes a double-stranded break in the recognition site, and methyltransferase covalently modifies the DNA bases within the recognition site, thereby down-regulating endonuclease activity. Coordinated action of these enzymes plays a role of primitive immune system and protects bacterial host cell from the invasion of foreign (for example, viral) DNA. However, uncontrolled expression of the restriction-modification system genes can result in the death of bacterial host cell because of the endonuclease cleavage of host DNA. In the present review, the data on the expression regulation of the type II restriction-modification enzymes are discussed.

Evaluating conformational changes in protein structures binding RNA

Many protein-RNA recognition events are known to exhibit conformational changes from qualitative observations of individual complexes. However, a quantitative estimation of conformational changes is required if protein-RNA docking and template-based methods for RNA binding site prediction are to be developed. This study presents the first quantitative evaluation of conformational changes that occur when proteins bind RNA. The analysis of twelve RNA-binding proteins in the bound and unbound states using error-scaled difference distance matrices is presented. The binding site residues are mapped to each structure, and the conformational changes that affect these residues are evaluated. Of the twelve proteins four exhibit greater movements in nonbinding site residues, and a further four show the greatest movements in binding site residues. The remaining four proteins display no significant conformational change. When interface residues are found to be in conformationally variable regions of the protein they are typically seen to move less than 2 A between the bound and unbound conformations. The current data indicate that conformational changes in the binding site residues of RNA binding proteins may not be as significant as previously suggested, but a larger data set is required before wider conclusions may be drawn. The implications of the observed conformational changes for protein function prediction are discussed.

Two structures of a lambda Cro variant highlight dimer flexibility but disfavor major dimer distortions upon specific binding of cognate DNA

Previously reported crystal structures of free and DNA-bound dimers of lambda Cro differ strongly (about 4 A backbone rmsd), suggesting both flexibility of the dimer interface and induced-fit protein structure changes caused by sequence-specific DNA binding. Here, we present two crystal structures, in space groups P3(2)21 and C2 at 1.35 and 1.40 A resolution, respectively, of a variant of lambda Cro with three mutations in its recognition helix (Q27P/A29S/K32Q, or PSQ for short). One dimer structure (P3(2)21; PSQ form 1) resembles the DNA-bound wild-type Cro dimer (1.0 A backbone rmsd), while the other (C2; PSQ form 2) resembles neither unbound (3.6 A) nor bound (2.4 A) wild-type Cro. Both PSQ form 2 and unbound wild-type dimer crystals have a similar interdimer beta-sheet interaction between the beta1 strands at the edges of the dimer. In the former, an infinite, open beta-structure along one crystal axis results, while in the latter, a closed tetrameric barrel is formed. Neither the DNA-bound wild-type structure nor PSQ form 1 contains these interdimer interactions. We propose that beta-sheet superstructures resulting from crystal contact interactions distort Cro dimers from their preferred solution conformation, which actually resembles the DNA-bound structure. These results highlight the remarkable flexibility of lambda Cro but also suggest that sequence-specific DNA binding may not induce large changes in the protein structure.

Energetics of the protein-DNA-water interaction

Background

To understand the energetics of the interaction between protein and DNA we analyzed 39 crystallographically characterized complexes with the HINT (Hydropathic INTeractions) computational model. HINT is an empirical free energy force field based on solvent partitioning of small molecules between water and 1-octanol. Our previous studies on protein-ligand complexes demonstrated that free energy predictions were significantly improved by taking into account the energetic contribution of water molecules that form at least one hydrogen bond with each interacting species.

Results

An initial correlation between the calculated HINT scores and the experimentally determined binding free energies in the protein-DNA system exhibited a relatively poor r² of 0.21 and standard error of ± 1.71 kcal mol^-1. However, the inclusion of 261 waters that bridge protein and DNA improved the HINT score-free energy correlation to an r² of 0.56 and standard error of ± 1.28 kcal mol^-1. Analysis of the water role and energy contributions indicate that 46% of the bridging waters act as linkers between amino acids and nucleotide bases at the protein-DNA interface, while the remaining 54% are largely involved in screening unfavorable electrostatic contacts.

Conclusion

This study quantifies the key energetic role of bridging waters in protein-DNA associations. In addition, the relevant role of hydrophobic interactions and entropy in driving protein-DNA association is indicated by analyses of interaction character showing that, together, the favorable polar and unfavorable polar/hydrophobic-polar interactions (i.e., desolvation) mostly cancel.

Characterization of the lytic–lysogenic switch of the lactococcal bacteriophage Tuc2009

Tuc2009 is a temperate bacteriophage of Lactococcus lactis subsp. cremoris UC509 which encodes a CI- and Cro-type lysogenic-lytic switch region. A helix-swap of the alpha3 helices of the closely related CI-type proteins from the lactococcal phages r1t and Tuc2009 revealed the crucial elements involved in DNA recognition while also pointing to conserved functional properties of phage CI proteins infecting different hosts. CI-type proteins have been shown to bind to specific sequences located in the intergenic switch region, but to date, no detailed binding studies have been performed on lactococcal Cro analogues. Experiments shown here demonstrate alternative binding sites for these two proteins of Tuc2009. CI2009 binds to three inverted repeats, two within the intergenic region and one within the cro2009 gene. This DNA-binding pattern appears to be conserved among repressors of lactococcal and streptococcal phages. The Cro2009 protein appears to bind to three direct repeats within the intergenic region causing distortion of the bound DNA.

Members of the IclR family of bacterial transcriptional regulators function as activators and/or repressors

Members of the IclR family of regulators are proteins with around 250 residues. The IclR family is best defined by a profile covering the effector binding domain. This is supported by structural data and by a number of mutants showing that effector specificity lies within a pocket in the C-terminal domain. These regulators have a helix-turn-helix DNA binding motif in the N-terminal domain and bind target promoters as dimers or as a dimer of dimers. This family comprises regulators acting as repressors, activators and proteins with a dual role. Members of the IclR family control genes whose products are involved in the glyoxylate shunt in Enterobacteriaceae, multidrug resistance, degradation of aromatics, inactivation of quorum-sensing signals, determinants of plant pathogenicity and sporulation. No clear consensus exists on the architecture of DNA binding sites for IclR activators: the MhpR binding site is formed by a 15-bp palindrome, but the binding sites of PcaU and PobR are three perfect 10-bp sequence repetitions forming an inverted and a direct repeat. IclR-type positive regulators bind their promoter DNA in the absence of effector. The mechanism of repression differs among IclR-type regulators. In most of them the binding sites of RNA polymerase and the repressor overlap, so that the repressor occludes RNA polymerase binding. In other cases the repressor binding site is distal to the RNA polymerase, so that the repressor destabilizes the open complex.

The TetR family of transcriptional repressors

We have developed a general profile for the proteins of the TetR family of repressors. The stretch that best defines the profile of this family is made up of 47 amino acid residues that correspond to the helix-turn-helix DNA binding motif and adjacent regions in the three-dimensional structures of TetR, QacR, CprB, and EthR, four family members for which the function and three-dimensional structure are known. We have detected a set of 2,353 nonredundant proteins belonging to this family by screening genome and protein databases with the TetR profile. Proteins of the TetR family have been found in 115 genera of gram-positive, alpha-, beta-, and gamma-proteobacteria, cyanobacteria, and archaea. The set of genes they regulate is known for 85 out of the 2,353 members of the family. These proteins are involved in the transcriptional control of multidrug efflux pumps, pathways for the biosynthesis of antibiotics, response to osmotic stress and toxic chemicals, control of catabolic pathways, differentiation processes, and pathogenicity. The regulatory network in which the family member is involved can be simple, as in TetR (i.e., TetR bound to the target operator represses tetA transcription and is released in the presence of tetracycline), or more complex, involving a series of regulatory cascades in which either the expression of the TetR family member is modulated by another regulator or the TetR family member triggers a cell response to react to environmental insults. Based on what has been learned from the cocrystals of TetR and QacR with their target operators and from their three-dimensional structures in the absence and in the presence of ligands, and based on multialignment analyses of the conserved stretch of 47 amino acids in the 2,353 TetR family members, two groups of residues have been identified. One group includes highly conserved positions involved in the proper orientation of the helix-turn-helix motif and hence seems to play a structural role. The other set of less conserved residues are involved in establishing contacts with the phosphate backbone and target bases in the operator. Information related to the TetR family of regulators has been updated in a database that can be accessed at www.bactregulators.org.

Atomic force microscopy reveals kinks in the p53 response element DNA

p53 is a 53 kDa nuclear phosphoprotein. Its function as a tumor suppressor critically lies in its ability to recognize its target DNA response elements as a tetramer. Here, we report the structural theme intrinsic to the response element DNA that governs this recognition phenomenon. The intrinsic flexibility or dynamic bending between two distinctly different, but naturally occurring p53 response elements has been compared by ring closure. Results show that DNA binding sites containing helically phased d(CATG.CATG) tetra-nucleotide sequences at the centers of quasi-dyad symmetry in each half-response site are more intrinsically flexible (i.e. preferentially bent under axial stress) than their d(CTTG.CTTG) counterparts. Intriguingly, p53 binding sites containing these more flexible d(CATG.CATG) sequence elements also exhibit a stronger tendency for tetrameric binding of the p53 DNA binding domain peptide. Examination of the shapes of DNA microcircles obtained by circularization of oligomers constructed from such flexible p53 target DNA sequences in tandem using MacMode atomic force microscopy directly revealed sequence-specific kinks in solution. The tetra-nucleotide sequence d(CATG.CATG) is highly conserved in most functional p53 response elements. Consequently, we propose that the sequence-specific kinks originating from d(CATG.CATG) sequences could be a common structural theme in p53 response elements and as evident from the results reported here, could be a determinant of binding site recognition by the p53 protein and the subsequent stability of the p53-DNA complex.

A verotoxin 1 B subunit-lambda CRO chimeric protein specifically binds both DNA and globotriaosylceramide (Gb(3)) to effect nuclear targeting of exogenous DNA in Gb(3) positive cells

Inefficient nuclear incorporation of foreign DNA remains a critical roadblock in the development of effective nonviral gene delivery systems. DNA delivered by traditional protocols remains within endosomal/lysosomal vesicles, or is rapidly degraded in the cytoplasm. Verotoxin I (VT), an AB(5) subunit toxin produced by enterohaemorrhagic Escherichia coli, binds to the cell surface glycolipid, globotriaosylceramide (Gb(3)) and is internalized into preendosomes. VT is then retrograde transported to the Golgi, endoplasmic reticulum (ER), and nucleus of highly VT-sensitive cells. We have utilized this nuclear targeting of VT to design a unique delivery system which transports exogenous DNA via vesicular traffic to the nucleus. The nontoxic VT binding subunit (VTB) was fused to the lambda Cro DNA-binding repressor, generating a 14-kDa VTB-Cro chimera. VTB-Cro binds specifically via the Cro domain to a 25-bp DNA fragment containing the consensus Cro operator. VTB-Cro demonstrates simultaneous specific binding to Gb(3). Treatment of Vero cells with fluorescent-labeled Cro operator DNA in the presence of VTB-Cro, results in DNA internalization to the Golgi, ER, and nucleus, whereas fluorescent DNA alone is incorporated poorly and randomly within the cytoplasm. VTB-Cro mediated nuclear DNA transport is prevented by brefeldin A, consistent with Golgi/ER intracellular routing. Pretreatment with filipin had no effect, indicating that caveoli are not involved. This novel VTB-Cro shuttle protein may find practical applications in the fields of intracellular targeting, gene delivery, and gene therapy.

Coupled energetics of lambda cro repressor self-assembly and site-specific DNA operator binding II: cooperative interactions of cro dimers

The bacteriophage lambda relies on interactions of the cI and cro repressors which self assemble and bind the two operators (O(R) and O(L)) of the phage genome to control the lysogenic to lytic switch. While the self assembly and O(R) binding of cI have been investigated in detail, a more complete understanding of gene regulation by phage lambda also requires detailed knowledge of the role of cro repressor as it dimerizes and binds at O(R) sites. Since dimerization and operator binding are coupled processes, a full elucidation of the regulatory energetics in this system requires that the equilibrium constants for dimerization and cooperative binding be determined. The dimerization constant for cro has been measured as a prelude to these binding studies. Here, the energetics of cro binding to O(R) are evaluated using quantitative DNaseI footprint titration techniques. Binding data for wild-type and modified O(R) site combinations have been simultaneously analyzed in concert with the dimerization energetics to obtain both the intrinsic and cooperative DNA binding energies for cro with the three O(R) sites. Binding of cro dimers is strongest to O(R)3, then O(R)1 and lastly, O(R)2. Adjacently bound repressors exhibit positive cooperativity ranging from -0.6 to -1.0 kcal/mol. Implications of these, newly resolved, energetics are discussed in the framework of a dynamic model for gene regulation. This characterization of the DNA-binding properties of cro repressor establishes the foundation on which the system can be explored for other, more complex, regulatory elements such as cI-cro cooperativity.

Conformational changes and cleavage by the homing endonuclease I-PpoI: a critical role for a leucine residue in the active site

The homing endonuclease I-PpoI severely bends its DNA target, resulting in significant deformations of the minor and major groove near the scissile phosphate groups. To study the role of conformational changes within the protein catalyst and the DNA substrate, we have determined the structure of the enzyme in the absence of bound DNA, performed gel retardation analyses of DNA binding and bending, and have mutagenized a leucine residue that contacts an adenine nucleotide at the site of cleavage. The structure of the L116A/DNA complex has been determined and the effects of the mutation on affinity and catalysis have been measured. The wild-type protein displays a rigid-body rotation of its individual subunits upon DNA binding. Homing site DNA is not detectably bent in the absence of protein, but is sharply bent in both the wild-type and L116A complexes. These results indicate that binding involves a large distortion of the DNA and a smaller change in protein conformation. Leucine 116 is critical for binding and catalysis: it appears to be important for forming a well-ordered protein-DNA complex at the cleavage site, for maximal deformation of the DNA, and for desolvation of the nucleotide bases that are partially unstacked in the enzyme complex.

The structural basis for enhanced stability and reduced DNA binding seen in engineered second-generation Cro monomers and dimers

It was previously shown that the Cro repressor from phage lambda, which is a dimer, can be converted into a stable monomer by a five-amino acid insertion. Phe58 is the key residue involved in this transition, switching from interactions which stabilize the dimer to those which stabilize the monomer. Structural studies, however, suggested that Phe58 did not penetrate into the core of the monomer as well as it did into the native dimer. This was strongly supported by the finding that certain core-repacking mutations, including in particular, Phe58-->Trp, increased the stability of the monomer. Unexpectedly, the same substitution also increased the stability of the native dimer. At the same time it decreased the affinity of the dimer for operator DNA. Here we describe the crystal structures of the Cro F58W mutant, both as the monomer and as the dimer. The F58W monomer crystallized in a form different from that of the original monomer. In contrast to that structure, which resembled the DNA-bound form of Cro, the F58W monomer is closer in structure to wild-type (i.e. non-bound) Cro. The F58W dimer also crystallizes in a form different from the native dimer but has a remarkably similar overall structure which tends to confirm the large changes in conformation of Cro on binding DNA. Introduction of Trp58 perturbs the position occupied by the side-chain of Arg38, a DNA-contact residue, providing a structural explanation for the reduction in DNA-binding affinity. The improved thermal stability is seen to be due to the enhanced solvent transfer free energy of Trp58 relative to Phe58, supplemented in the dimer structure, although not the monomer, by a reduction in volume of internal cavities.

Crystal structure of ribosomal protein L4 shows RNA-binding sites for ribosome incorporation and feedback control of the S10 operon

Ribosomal protein L4 resides near the peptidyl transferase center of the bacterial ribosome and may, together with rRNA and proteins L2 and L3, actively participate in the catalysis of peptide bond formation. Escherichia coli L4 is also an autogenous feedback regulator of transcription and translation of the 11 gene S10 operon. The crystal structure of L4 from Thermotoga maritima at 1.7 A resolution shows the protein with an alternating alpha/beta fold and a large disordered loop region. Two separate binding sites for RNA are discernible. The N-terminal site, responsible for binding to rRNA, consists of the disordered loop with flanking alpha-helices. The C-terminal site, a prime candidate for the interaction with the leader sequence of the S10 mRNA, involves two non-consecutive alpha-helices. The structure also suggests a C-terminal protein-binding interface, through which L4 could be interacting with protein components of the transcriptional and/or translational machineries.

DNA-induced conformational changes in bacteriophage 434 repressor

Although bacteriophage 434 repressor binds to its specific DNA sites only as a dimer, formation of the dimers in solution occurs at concentrations three orders of magnitude higher than those needed to bind the 434 operator DNA. Our results suggest that both specific and non-specific DNA induce conformational changes in repressor that lead to formation of repressor dimers. The repressor conformational changes induced by DNA occur at concentrations much lower than those needed for binding of repressor, suggesting that the alternative conformations of repressor persist even if the protein is not in direct contact with DNA. Hence, DNA acts in a "catalytic" fashion to induce a steady-state amount of an alternative repressor conformation that has an enhanced affinity for its specific binding site. These findings suggest that the repressor conformer induced by non-specific DNA is the form of the repressor that is optimized for searching for DNA binding sites along non-specific DNA. Upon finding a binding site, the repressor protein undergoes an additional conformational change that allows it to "lock-on" to its specific site.

A2 cro, the lysogenic cycle repressor, specifically binds to the genetic switch region of Lactobacillus casei bacteriophage A2

Lysogenic induction of temperate bacteriophage A2 of Lactobacillus casei is controlled by the action of its cI and cro products at the phage operator region. Three 20-bp inverted repeated DNA segments (subsites O1, O2, and O3) and the two divergent (PL and PR) promoters were mapped within the 153-bp operator region. The A2-encoded Cro product is shown to be the functional homolog of lambda Cro. The binding of Cro to the three operator subsites is noncooperative and yields two discrete protein-DNA complexes of retarded migration in mobility shift assays. The Kapp value for the Cro-PL-PR DNA complex was estimated to be 6 nM. Cro shows a slightly higher affinity for O3 than for O1 and O2 subsites. The O3 subsite overlaps the -35 hexamer of the PL promoter, which directs cI expression. A Cro mutant protein, devoid of the last 12 residues (Cro*), allowed the assignment of the DNA-binding domain to the NH2 end of Cro. The C end enhances its affinity for the DNA and probably stabilizes bending induced by Cro.

Specific contacts between residues in the DNA-binding domain of the TyrR protein and bases in the operator of the tyrP gene of Escherichia coli

In the presence of tyrosine, the TyrR protein of Escherichia coli represses the expression of the tyrP gene by binding to the double TyrR boxes which overlap the promoter. Previously, we have carried out methylation, uracil, and ethylation interference experiments and have identified both guanine and thymine bases and phosphates within the TyrR box sequences that are contacted by the TyrR protein (J. S. Hwang, J. Yang, and A. J. Pittard, J. Bacteriol. 179:1051-1058, 1997). In this study, we have used missing contact probing to test the involvement of all of the bases within the tyrP operator in the binding of TyrR. Our results indicate that nearly all the bases within the palindromic arms of the strong and weak boxes are important for the binding of the TyrR protein. Two alanine-substituted mutant TyrR proteins, HA494 and TA495, were purified, and their binding affinities for the tyrP operator were measured by a gel shift assay. HA494 was shown to be completely defective in binding to the tyrP operator in vitro, while, in comparison with wild-Type TyrR, TA495 had only a small reduction in DNA binding. Missing contact probing was performed by using the purified TA495 protein, and the results suggest that T495 makes specific contacts with adenine and thymine bases at the +/-5 positions in the TyrR boxes.

Intermolecular contacts between the lambda-Cro repressor and the operator DNA characterized by nuclear magnetic resonance spectroscopy

The specific interaction between lambda phage Cro repressor and the DNA fragment bearing the consensus sequence of operators has been studied using nuclear magnetic resonance (NMR). Using both 15N- and 13C/15N- labeled lambda-Cro in complex with unlabeled DNA, chemical shift assignments of the lambda-Cro-DNA complex were obtained using heteronuclear NMR experiments. Inter-molecular contacts between the protein and DNA were identified using heteronuclear filtered NOESY experiments. The inter-molecular contacts were supplemented with intra-protein and intra-DNA NOE constraints to dock lambda-Cro to the bent B-form DNA using restrained molecular dynamics. The structure of one of the subunits of lambda-Cro in the complex is essentially the same as that of the unbound form. In the complex, inter-molecular NOEs were observed between the "helix-turn-helix" region comprising the alpha2 and alpha3 helices of the lambda-Cro protein and the major groove of the DNA. The methyl group of Thr17 forms a hydrophobic contact with the methyl group of the thymine at base pair 1 in the DNA, and Val25 and Ala29 make hydrophobic contacts with the methyl group of the thymine at base pair 5. The presence and the absence of these contacts can explain the difference in the affinity of lambda-Cro to several variants of the operator sequence.

Probing site-specific interactions in protein-DNA complexes using heteronuclear NMR spectroscopy and molecular modeling: binding of Cro repressor to OR3

In this paper, a general method is developed to study site-specific interactions in DNA-protein complexes using heteronuclear NMR spectroscopy and molecular modeling. This method involves two steps: (a) homonuclear 1H NMR and molecular modeling are used to develop a low resolution model and (b) 15N7-guanosine containing oligonucleotides are employed to probe the specific intermolecular interactions predicted in (a). This method is applied to Cro-operator complex due to its small size and extensive prior characterization. Non-exchangeable and exchangeable base protons have been assigned by nuclear Overhauser effect spectroscopy (NOESY) and chemical shift correlation spectroscopy. Extensive line-broadening has been observed in the 1H NMR spectra of the operator DNA in the presence of protein. Differential line-broadening observed in the imino proton region and the comparison of NOESY spectra in the presence and absence of Cro protein show that guanosine-12 and guanosine-14 are involved in the Cro-DNA interaction, while the three A.T base-pairs at the 3'- and 5'-termini play only a minor role in the binding. A model of the Cro-operator DNA complex has been constructed by docking helix-3 of the Cro protein in the major groove and it predicted specific hydrogen bonds between N7 of guanosines-12 and -14 and the side-chain of Lys-32 and Ser-28, respectively. The appearance of a new resonance in the temperature dependent proton detected heteronuclear multiple quantum coherence (HMQC) spectra of the Cro-DNA complex also demonstrates a specific interaction of Cro with guanosine-14 of the operator DNA.

LSF and NTF-1 share a conserved DNA recognition motif yet require different oligomerization states to form a stable protein-DNA complex

The mammalian transcription factor LSF (also known as CP2 and LBP-1c) binds as a homo-oligomer to directly repeated elements in viral and cellular promoters. LSF and the Drosophila transcription factor NTF-1 (also known as Elf-1 and Grainyhead) share a similar DNA binding region, which is unlike any established DNA binding motifs. However, we demonstrate that dimeric NTF-1 can bind an LSF half-site, whereas LSF cannot. To characterize further the DNA binding and oligomerization characteristics of LSF, truncation mutants were used to demonstrate that between 234 and 320 amino acids of LSF are required for high affinity DNA binding. Mixing of a truncation mutant with full-length LSF in a DNA binding assay established that the form of LSF that binds DNA is larger than a dimer. Unexpectedly, one C-terminal deletion derivative, partially defective in oligomerization properties, could occupy odd numbers of adjacent, tandem LSF half-sites, unlike full-length LSF. The numbers of DNA-protein complexes formed on multiple half-sites with this mutant indicated that LSF binds DNA as a tetramer, although cross-linking experiments confirmed a previous report concluding that LSF is primarily dimeric in solution. The DNA binding and oligomerization properties of LSF support models depicting novel mechanisms to prevent continual, adjacent binding by a protein that recognizes directly repeated DNA sequences.

Refined structure of Cro repressor protein from bacteriophage lambda suggests both flexibility and plasticity

The structure of the Cro repressor protein from phage lambda has been refined to a crystallographic R-value of 19.3% at 2.3 A resolution. The re fined model supports the structure as originally described in 1981 and provides a basis for comparison with the Cro-operator complex described in the accompanying paper. Changes in structure seen in different crystal forms and modifications of Cro suggest that the individual subunits are somewhat plastic in nature. In addition, the dimer of Cro suggests a high degree of flexibility, which may be important in forming the Cro-DNA complex. The structure of the Cro subunit as determined by NMR agrees reasonably well with that in the crystals (root-mean-square discrepancy of about 2 A for all atoms). There are, however, only a limited number of intersubunit distance constraints and, presumably for this reason, the different NMR models for the dimer vary substantially among themselves (discrepancies of 1.3 to 5.5 A). Because of this variation it is not possible to say whether the range of discrepancies between the X-ray and NMR Cro dimers (2.9 to 7.5 A) represent a significant difference between the X-ray and solution structures. It has previously been proposed that substitutions of Tyr26 in Cro increase thermal stability by the "reverse hydrophobic effect", i.e. by exposing 40% more hydrophobic surface to solvent in the folded form than in the unfolded state. The refined structure, however, suggests that Tyr26 is equally solvent exposed in the folded and unfolded states. The most stabilizing substitution is Tyr26-->Asp and in this case it appears that interaction with an alpha-helix dipole is at least partly responsible for the enhanced stability.

Crystal structure of lambda-Cro bound to a consensus operator at 3.0 A resolution

The structure of the Cro protein from bacteriophage lambda in complex with a 19 base-pair DNA duplex that includes the 17 base-pair consensus operator has been determined at 3.0 A resolution. The structure confirms the large changes in the protein and DNA seen previously in a crystallographically distinct low-resolution structure of the complex and, for the first time, reveals the detailed interactions between the side-chains of the protein and the base-pairs of the operator. Relative to the crystal structure of the free protein, the subunits of Cro rotate 53 degrees with respect to each other on binding DNA. At the same time the DNA is bent by 40 degrees through the 19 base-pairs. The intersubunit connection includes a region within the protein core that is structurally reminiscent of the "ball and socket" motif seen in the immunoglobulins and T-cell receptors. The crystal structure of the Cro complex is consistent with virtually all available biochemical and related data. Some of the interactions between Cro and DNA proposed on the basis of model-building are now seen to be correct, but many are different. Tests of the original model by mutagenesis and biochemical analysis corrected some but not all of the errors. Within the limitations of the crystallographic resolution it appears that operator recognition is achieved almost entirely by direct hydrogen-bonding and van der Waals contacts between the protein and the exposed bases within the major groove of the DNA. The discrimination of Cro between the operators OR3 and OR1, which differ in sequence at just three positions, is inferred to result from a combination of small differences, both favorable and unfavorable. A van der Waals contact at one of the positions is of primary importance, while the other two provide smaller, indirect effects. Direct hydrogen bonding is not utilized in this distinction.

Crystal structure of an engineered Cro monomer bound nonspecifically to DNA: possible implications for nonspecific binding by the wild-type protein

The structure has been determined at 3.0 A resolution of a complex of engineered monomeric Cro repressor with a seven-base pair DNA fragment. Although the sequence of the DNA corresponds to the consensus half-operator that is recognized by each subunit of the wild-type Cro dimer, the complex that is formed in the crystals by the isolated monomer appears to correspond to a sequence-independent mode of association. The overall orientation of the protein relative to the DNA is markedly different from that observed for Cro dimer bound to a consensus operator. The recognition helix is rotated 48 degrees further out of the major groove, while the turn region of the helix-turn-helix remains in contact with the DNA backbone. All of the direct base-specific interactions seen in the wild-type Cro-operator complex are lost. Virtually all of the ionic interactions with the DNA backbone, however, are maintained, as is the subset of contacts between the DNA backbone and a channel on the protein surface. Overall, 25% less surface area is buried at the protein DNA interface than for half of the wild-type Cro-operator complex, and the contacts are more ionic in character due to a reduction of hydrogen bonding and van der Waals interactions. Based on this crystal structure, model building was used to develop a possible model for the sequence-nonspecific interaction of the wild-type Cro dimer with DNA. In the sequence-specific complex, the DNA is bent, the protein dimer undergoes a large hinge-bending motion relative to the uncomplexed form, and the complex is twofold symmetric. In contrast, in the proposed nonspecific complex the DNA is straight, the protein retains a conformation similar to the apo form, and the complex lacks twofold symmetry. The model is consistent with thermodynamic, chemical, and mutagenic studies, and suggests that hinge bending of the Cro dimer may be critical in permitting the transition from the binding of protein at generic sites on the DNA to binding at high affinity operator sites.

DNA-methyltransferase SsoII interaction with own promoter region binding site

The investigation of Sso II DNA-methyltransferase (M.Sso II) interaction with the intergenic region of Sso II restriction-modification system was carried out. Seven guanine residues protected by M. Sso II from methylation with dimethylsulfate and thus probably involved in enzyme-DNA recognition were identified. Six of them are located symmetrically within the 15 bp inverted repeat inside the Sso II promoter region. The crosslinking of Sso II methyltransferase with DNA duplexes containing 5-bromo-2'-deoxyuridine (br5dU) instead of thymidine was performed. The crosslinked products were obtained in all cases, thus proving that tested thymines were in proximity with enzyme. The ability to produce the crosslinked products in one case was 2-5-fold higher than in other ones. This allowed us to imply that thymine residue in this position of the inverted repeat could be in contact with M. Sso II. Based on the experimental data, two symmetrical 4 bp clusters (GGAC), which could be involved in the interaction with M. Sso II in the DNA-protein complex, were identified. The model of M. Sso II interaction with its own promoter region was proposed.

Single-chain lambda Cro repressors confirm high intrinsic dimer-DNA affinity

The overall affinity of the bacteriophage lambda Cro repressor for its operator DNA site is limited by dimer dissociation at submicromolar concentrations. Since Cro dimer-operator complexes form at nanomolar concentrations of Cro subunits where free dimers are rare, these dimers must bind with compensating high affinities. Previous studies of the covalent dimer Cro V55C suggest little change in DNA binding affinity even though the dimeric species is quantitatively populated; this is an apparent contradiction to the expectation of high intrinsic dimer-DNA affinity. In contrast to the disulfide linkage at the center of the dimer interface in Cro V55C, polypeptide linkers that join the two subunits allow single-chain Cro repressors to bind operator DNA with picomolar affinities. A series of five single-chain Cro repressors have been expressed from fused tandem cro genes. Each contains a peptide linker of 8-16 hydrophilic residues that connects the C-terminus of one subunit to the N-terminus of the next. All bind to operator DNA with at least 100-fold higher affinity than Cro V55C. Proteins containing the longest and shortest linkers have been purified and characterized in detail. Both exhibit similar CD spectra to wild-type Cro and enhanced thermal stability. Sedimentation equilibrium experiments show that single-chain Cro repressors do not associate at concentrations up to 30 microM. The rate of dissociation of Cro-DNA complexes is almost unchanged by covalent linkage. Biophysical characterization of Cro variants such as these, where DNA binding is uncoupled from subunit assembly, is necessary for a quantitative understanding of the structural and energetic determinants of DNA recognition in this simple model system.

Solution structure and dynamics of a designed monomeric variant of the lambda Cro repressor

The solution structure of a monomeric variant of the lambda Cro repressor has been determined by multidimensional NMR. Cro K56[DGEVK] differs from wild-type Cro by the insertion of five amino acids at the center of the dimer interface. 1H and 15N resonances for 70 of the 71 residues have been assigned. Thirty-two structures were calculated by hybrid distance geometry/simulated annealing methods using 463 NOE-distance restraints, 26 hydrogen-bond, and 39 dihedral-angle restraints. The root-mean-square deviation (RMSD) from the average structure for atoms in residues 3-60 is 1.03 +/- 0.44 A for the peptide backbone and 1.6 +/- 0.73 A for all nonhydrogen atoms. The overall structure conforms very well to the original design. Although the five inserted residues form a beta hairpin as expected, this engineered turn as well as other turns in the structure are not well defined by the NMR data. Dynamics studies of backbone amides reveal T1/T2 ratios of residues in the alpha2-alpha3, beta2-beta3, and engineered turn that are reflective of chemical exchange or internal motion. The solution structure and dynamics are discussed in light of the conformational variation that has been observed in other Cro structures, and the importance of flexibility in DNA recognition.

How Cro and lambda-repressor distinguish between operators: the structural basis underlying a genetic switch

Knowledge of the three-dimensional structures of the lambda-Cro and lambda-repressor proteins in complex with DNA has made it possible to evaluate how these proteins discriminate between different operators in phage lambda. As anticipated in previous studies, the helix-turn-helix units of the respective proteins bind in very different alignments. In Cro the recognition helices are 29 A apart and are tilted by 55 degrees with respect to each other, but bind parallel to the major groove of the DNA. In lambda-repressor [Beamer, L. J. & Pabo, C. O. (1992) J. Mol. Biol. 227, 177-196] the helices are 34 A apart and are essentially parallel to each other, but are inclined to the major grooves. The DNA is much more bent when bound by Cro than in the case with lambda-repressor. The first two amino acids of the recognition helices of the two proteins, Gln-27 and Ser-28 in Cro, and Gln-44 and Ser-45 in lambda-repressor, make very similar interactions with the invariant bps 2 and 4. There are also analogous contacts between the thymine of bp 5 and, respectively, the backbone of Ala-29 of Cro and the backbone of Gly-46 of lambda-repressor. Otherwise, however, unrelated parts of the two proteins are used in sequence-specific recognition. It appears that similar contacts to the invariant or almost invariant bps (especially 2 and 4) are used by both Cro and lambda-repressor to differentiate the operator sites as a group from other sites on the DNA. The discrimination of Cro and lambda-repressor between their different operators is more subtle and seems to be achieved primarily through differences in van der Waals contacts at bp 3', together with weaker, less direct effects at bps 5' and 8', all in the nonconsensus half of the operators. The results provide further support for the idea that there is no simple code for DNA-protein recognition.

A folded monomeric intermediate in the formation of lambda Cro dimer-DNA complexes

The folding, dimerization and DNA binding equilibria of the bacteriophage lambda Cro repressor have been characterized. Comparison with four engineered variants shows that a folded monomeric species is substantially populated under conditions used for the formation of dimer-DNA complexes. Although Cro dimers are the only DNA-bound species observed in electrophoretic mobility shift assays, cooperativity in Cro-DNA binding isotherms shows that the predominant free protein species is monomeric at nanomolar concentrations. Micromolar dissociation constants for Cro dimers have been measured in the absence of DNA by sedimentation equilibrium and gel filtration chromatography. Denaturation of Cro dimers in the 10 to 100 micromolar concentration range by guanidine hydrochloride (GdnHCl) is well modeled as a two-state process, with folded dimers and unfolded monomers as the only significantly populated species. However, linear extrapolation of this composite unfolding and dimer dissociation free energy predicts a nanomolar dissociation constant in the absence of denaturant. This extrapolation is clearly inconsistent with the DNA binding and hydrodynamic measurements. Our interpretation of these results is that the monomeric species detected in DNA binding and hydrodynamic experiments is predominantly folded. The stability of the folded monomeric species can be calculated as the difference between the dimerization free energy determined from hydrodynamic measurements and the folding free energy extrapolated from GdnHCl denaturation. The calculated stability of the Cro F58W monomer is greater than that of the wild-type Cro monomer. Thus, residue 58, which makes critical intermolecular contacts across the dimer interface, is also involved in intramolecular stabilization of the monomeric intermediate.

A nucleoprotein activation complex between the leucine-responsive regulatory protein and DNA upstream of the gltBDF operon in Escherichia coli

The global regulator Lrp (leucine-responsive regulatory protein), in some cases modulated by its co-regulator leucine, has been shown to regulate more than 40 genes and operons in Escherichia coli. Leucine modulates Lrp regulation of leucine-responsive operons. The level of sensitivity of these operons to leucine varies greatly, but the basis for this variation is only partially understood. One operon controlled by Lrp that is relatively insensitive to leucine is gltBDF, which includes genes specifying the large (GltB) and small (GltD) subunits of glutamate synthase. Earlier gel mobility shift assays have demonstrated that Lrp binds to a fragment of DNA containing the gltBDF promoter region. To further define the nature of this Lrp-gltBDF interaction, DNase I footprinting experiments were performed. The results indicate that Lrp binds cooperatively to three sites quite far upstream, spanning the region from -140 to -260 base-pairs relative to the start of transcription. Phased hypersensitivity is observed throughout the entire binding region, suggesting that Lrp bends the DNA. To determine the relative importance of these three sites for the transcriptional activation of gltBDF, a series of site-directed mutations was generated. The effects of these mutations on Lrp binding were determined both by DNase I footprinting and by quantitative mobility shift assays, while their effects on transcription in vivo were examined by measuring beta-galactosidase activity levels of chromosomal gltB::lacZ operon fusions. Our results indicate that all three sites are required for maximal gene expression, as is the proper phasing of the sites with one another and with the start of transcription. Our results suggest that Lrp binds a central palindromic site, interacting predominantly with the major groove of its DNA target, and that additional dimers bind to flanking sites to form a nucleoprotein activation complex.

The transcription activation domains of Fos and Jun induce DNA bending through electrostatic interactions

Transcription factor-induced DNA bending is essential for the assembly of active transcription complexes at many promoters. However, most eukaryotic transcription regulatory proteins have modular DNA-binding and activation domains, which appeared to exclude DNA bending as a mechanism of transcription activation by these proteins. We show that the transcription activation domains of Fos and Jun induce DNA bending. In chimeric proteins, the transcription activation domains induce DNA bending independent of the DNA-binding domains. DNA bending by the chimeric proteins is directed diametrically away from the transcription activation domains. Therefore, the opposite directions of DNA bending by Fos and Jun are caused, in part, by the opposite locations of the transcription activation domains relative to the DNA-binding domains in these proteins. DNA bending is reduced in the presence of multivalent cations, indicating that electrostatic interactions contribute to DNA bending by Fos and Jun. Consequently, regions outside the minimal DNA-binding domain can influence DNA structure, and may thereby contribute to the architectural reorganization of the promoter region required for gene activation.

Recombinant human O6-alkylguanine-DNA alkyltransferase induces conformational change in bound DNA

Circular dichroism, and steady-state and time-resolved fluorescence spectroscopy were used to compare the native recombinant human DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) with AGT bound to ds-DNA. Contrary to fluorescence, analysis of the far-UV CD spectra indicated a conformational change of AGT upon binding to DNA: its alpha-helical content is increased by approximately 12%. Analysis of near-UV CD spectra revealed that DNA was also affected, probably being separated into single strands locally.

Lac repressor-operator complex

For many years the lac operon of Escherichia coli has been the paradigm for gene regulation. Recently, the structures of the lac repressor core bound to isopropyl-beta-D-1-thiogalactoside (IPTG), the intact apo lac repressor, the intact lac repressor complexes with IPTG and a 21-base-pair symmetric operator, and the refined headpiece of the repressor have been determined. These structures have provided a framework for understanding a wealth of biochemical and genetic information. An analysis of these structures, as well as a description of their function and a comparison to homologous proteins, is now possible.

Mutants in position 69 of the Trp repressor of Escherichia coli K12 with altered DNA-binding specificity

Structural analysis by X-ray crystallography has indicated that direct contact occurs between Arg69, the second residue of the first helix of the helix-turnhelix (HTH) motif of the Trp repressor, and guanine in position 9 of the alpha-centred consensus trp operator. We therefore replaced residue 69 of the Trp repressor with Gly, Ile, Leu or Gln and tested the resultant repressor mutants for their binding to synthetic symmetrical alpha- or beta-centred trp operator variants, in vivo and in vitro. We present genetic and biochemical evidence that Ile in position 69 of the Trp repressor interacts specifically with thymine in position 9 of the alpha-centred trp operator. There are also interactions with other bases in positions 8 and 9 of the alpha-centred trp operator. In vitro, the Trp repressor of mutant RI69 binds to the consensus alpha-centred trp operator and a similar trp operator variant that carries a T in position 9. In vivo analysis of the interactions of Trp repressor mutant RI69 with symmetrical variants of the beta-centred trp operator shows a change in the specificity of binding to a beta-centred symmetrical trp operator variant with a gua-nine to thymine substitution in position 5, which corresponds to position 9 of the alpha-centred trp operator.

The Hin dimer interface is critical for Fis-mediated activation of the catalytic steps of site-specific DNA inversion

BACKGROUND

Hin is a member of an extended family of site-specific recombinases--the DNA invertase/resolvase family--that catalyze inversion or deletion of DNA. DNA inversion by Hin occurs between two recombination sites and requires the regulatory protein Fis, which associates with a cis-acting recombinational enhancer sequence. Hin recombinase dimers bind to the two recombination sites and assemble onto the Fis-bound enhancer to generate an invertasome structure, at which time they become competent to catalyze DNA cleavage and strand exchange. In this report, we investigate the role of the Hin dimer interface in the activation of its catalytic functions.

RESULTS

We show that the Hin dimer is formed at an interface that contains putative amphipathic alpha-helices in a manner that is very similar to gamma delta resolvase. Certain detergents weakened cooperative interactions between the subunits of the Hin dimer and dramatically increased the rate of the first chemical step of the reaction--double-strand cleavage events at the center of the recombination sites. Amino-acid substitutions within the dimer interface led to profound changes in the catalytic properties of the recombinase. Nearly all mutations strongly affected the ability of the dimer to cleave DNA and most abolished DNA strand exchange in vitro. Some amino-acid substitutions altered the concerted nature of the DNA cleavage events within both recombination sites, and two mutations resulted in cleavage activity that was independent of Fis activation in vitro. Disulfide-linked Hin dimers were catalytically inactive; however, subsequent to the addition of the Fis-bound enhancer sequence, catalytic activity was no longer affected by the presence of oxidizing agents.

CONCLUSIONS

The combined results demonstrate that the Hin dimer interface is of critical importance for the activation of catalysis and imply that interactions with the Fis-bound enhancer may trigger a conformational adjustment within the region that is important for concerted DNA cleavage within both recombination sites, and possibly for the subsequent exchange of DNA strands.

Conformational characteristics of high affinity Sp1 binding enhancer elements of HIV-LTR by high resolution 2D-NMR

The understanding of early events in the expression of genes has vastly improved in recent years with the identification of a variety of gene- and sequence-specific DNA binding transcription factors. One such protein, Sp1, has been implicated in activating transcription of various cellular and viral genes including those of HIV, and SIV types of retroviruses. The basic recognition site for Sp1 has been identified as variants of a 10 base-pairs long GC-rich DNA, often containing a hexanucleotide segment 5'-GGGCGG (termed GC-box). However, variations in both the relative protein-DNA binding affinity and the nature of binding sequences have been noted. Two-dimensional 1H-NMR experiments (500 MHz) were employed for conformational studies of two decadeoxyribonucleotide duplexes, d(GAGGCGTGGC).d(GCCACGCCTC), termed Sp1-III, and d(GGGAGTGGCG).d(CGCCACTCCC), termed Sp1-I. These are two of the highest affinity Sp1 binding sites and consist of diverse positioning of the tri- and tetranucleotide segments GAG, GTG, GCG, GGCG, GTGG and GGAG, that occur frequently in other Sp1 binding sites as well, and may form specific contacts with the protein. Phase-sensitive nuclear Overhauser enhancement (2D-NOESY and MINSY) and correlation (COSY) spectra were obtained for the assignment of the exchangeable and nonexchangeable protons in a sequence-specific fashion. As a prelude to determination of the detailed solution structures of the selected sequences, numerous structural constraints were obtained from angle-dependent coupling constants and relative intensities of distance-dependent intra- and internucleotide NOEs. Overall, each duplex adopts a structure similar to B-DNA with predominantly C2'-endo/S-type sugar conformation and anti-glycosidic torsion angles. A selective disruption of sequential NOE connectivities at the GAG.CAC and GTG.CAC steps, irrespective of the flanking sequence, suggests that conformational changes at these sites may act as unique determinants of sequence specific recognition/binding of Sp1. Implications for a specific inhibition of Sp1-mediated transcription by minor groove binding class of drugs, designed to recognize GC-rich sequences, are also briefly discussed.

Binding geometry of alpha-helices that recognize DNA

Many transcription factors have an alpha-helix that binds to DNA bases in a specific fashion. The DNA-binding geometry of these recognition helices varies substantially. We define a set of parameters to describe the binding geometry of recognition helices and analyze specific stereochemical elements that determine particular geometries. Because the convex surface of the helix must fit into the concave surface of the DNA major groove, the number of degrees of freedom of the recognition helix is reduced from a possible six to a single angle, which we call alpha. The chemically interacting DNA bases and amino acid residues must lie along a common line and have the same spacing along it. This pairing of base positions with residue positions seems to restrict the binding geometry further to a set of discrete values for alpha.