Multiple specific CytR binding sites at the Escherichia coli deoP2 promoter mediate both cooperative and competitive interactions between CytR and cAMP receptor protein

Flexible Target Recognition of the Intrinsically Disordered DNA-Binding Domain of CytR Monitored by Single-Molecule Fluorescence Spectroscopy

The intrinsically disordered DNA-binding domain of cytidine repressor (CytR-DBD) folds in the presence of target DNA and regulates the expression of multiple genes in E. coli. To explore the conformational rearrangements in the unbound state and the target recognition mechanisms of CytR-DBD, we carried out single-molecule Förster resonance energy transfer (smFRET) measurements. The smFRET data of CytR-DBD in the absence of DNA show one major and one minor population assignable to an expanded unfolded state and a compact folded state, respectively. The population of the folded state increases and decreases upon titration with salt and denaturant, respectively, in an apparent two-state manner. The peak FRET efficiencies of both the unfolded and folded states change continuously with denaturant concentration, demonstrating the intrinsic flexibility of the DNA-binding domain and the deviation from a strict two-state transition. Remarkably, the CytR-DBD exhibits a compact structure when bound to both the specific and nonspecific DNA; however, the peak FRET efficiencies of the two structures are slightly but consistently different. The observed conformational heterogeneity highlights the potential structural changes required for CytR to bind variably spaced operator sequences.

The Regulator OmpR in Yersinia enterocolitica Participates in Iron Homeostasis by Modulating Fur Level and Affecting the Expression of Genes Involved in Iron Uptake

In this study, we found that the loss of OmpR, the response regulator of the two-component EnvZ/OmpR system, increases the cellular level of Fur, the master regulator of iron homeostasis in Y. enterocolitica. Furthermore, we demonstrated that transcription of the fur gene from the YePfur promoter is subject to negative OmpR-dependent regulation. Four putative OmpR-binding sites (OBSs) were indicated by in silico analysis of the fur promoter region, and their removal affected OmpR-dependent fur expression. Moreover, OmpR binds specifically to the predicted OBSs which exhibit a distinct hierarchy of binding affinity. Finally, the data demonstrate that OmpR, by direct binding to the promoters of the fecA, fepA and feoA genes, involved in the iron transport and being under Fur repressor activity, modulates their expression. It seems that the negative effect of OmpR on fecA and fepA transcription is sufficient to counteract the indirect, positive effect of OmpR resulting from decreasing the Fur repressor level. The expression of feoA was positively regulated by OmpR and this mode of action seems to be direct and indirect. Together, the expression of fecA, fepA and feoA in Y. enterocolitica has been proposed to be under a complex mode of regulation involving OmpR and Fur regulators.

The cooperative binding energetics of CytR and cAMP receptor protein support a quantitative model of differential activation and repression of CytR-regulated class III Escherichia coli promoters

cAMP receptor protein (CRP) and CytR mediate positive and negative control of nine genes in Escherichia coli, most of which are involved in nucleoside catabolism and recycling. Five promoters share a common architecture in which tandem CRP sites flank an intervening CytR operator (CytO). CytR and CRP bind cooperatively to these promoters to form a three-protein, DNA-bound complex that controls activation and repression, the levels of which vary markedly among the promoters. To understand the specific combinatorial control mechanisms that are responsible for this outcome, we have used quantitative DNase I footprinting to generate individual site isotherms for each site of protein-DNA interaction. The intrinsic affinities of each transcription factor for its respective site and the specific patterns of cooperativity and competition underlying the molecular interactions at each promoter were determined by a global analysis of these titration data. Here we present results obtained for nupGP and tsxP2, adding to results published previously for deoP2, udpP, and cddP. These data allowed us to correlate the reported levels of activation, repression, and induction with the ligation states of these five promoters under physiologically relevant conditions. A general pattern of transcriptional regulation emerges that allows for complex patterns of regulation in this seemingly simple system.

Structure-based mutational study of an archaeal DNA ligase towards improvement of ligation activity

DNA ligases catalyze the joining of strand breaks in duplex DNA. The DNA ligase of Pyrococcus furiosus (PfuLig), which architecturally resembles the human DNA ligase I (hLigI), comprises an N-terminal DNA-binding domain, a middle adenylylation domain, and a C-terminal oligonucleotide-binding (OB)-fold domain. Here we addressed the C-terminal helix in the OB-fold domain of PfuLig by mutational analysis. The crystal structure of PfuLig revealed that this helix stabilizes a closed conformation of the enzyme by forming several ionic interactions with the adenylylation domain. The C-terminal helix is oriented differently in hLigI when DNA is bound; this suggested that disruption of its interaction with the adenylylation domain might facilitate the binding of DNA substrates. We indeed identified one of its residues, Asp540, as being critical for ligation efficiency. The D540R mutation improved the overall ligation activity relative to the wild-type enzyme, and at lower temperatures; this is relevant to applications such as ligation amplification reactions. Physical and biochemical analyses indicated that the improved ligation activity of the D540R variant arises from effects on the ligase adenylylation step and on substrate DNA binding in particular.

Comparative genomics of CytR, an unusual member of the LacI family of transcription factors

CytR is a transcription regulator from the LacI family, present in some gamma-proteobacteria including Escherichia coli and known not only for its cellular role, control of transport and utilization of nucleosides, but for a number of unusual structural properties. The present study addressed three related problems: structure of CytR-binding sites and motifs, their evolutionary conservation, and identification of new members of the CytR regulon. While the majority of CytR-binding sites are imperfect inverted repeats situated between binding sites for another transcription factor, CRP, other architectures were observed, in particular, direct repeats. While the similarity between sites for different genes in one genome is rather low, and hence the consensus motif is weak, there is high conservation of orthologous sites in different genomes (mainly in the Enterobacteriales) arguing for the presence of specific CytR-DNA contacts. On larger evolutionary distances candidate CytR sites may migrate but the approximate distance between flanking CRP sites tends to be conserved, which demonstrates that the overall structure of the CRP-CytR-DNA complex is gene-specific. The analysis yielded candidate CytR-binding sites for orthologs of known regulon members in less studied genomes of the Enterobacteriales and Vibrionales and identified a new candidate member of the CytR regulon, encoding a transporter named NupT (YcdZ).

From steroid receptors to cytokines: the thermodynamics of self-associating systems

Since 1987, the Gibbs Conference on Biothermodynamics has maintained a focus on understanding the quantitative aspects of gene regulatory systems. These studies coupled rigorous techniques with exact theory to dissect the linked reactions associated with bacterial and lower eukaryotic gene regulation. However, only in the last ten years has it become possible to apply this approach to clinically relevant, human gene regulatory systems. Here we summarize our work on the thermodynamics of human steroid receptors and their interactions with multi-site promoter sequences, highlighting results not available from more traditional biochemical and structural approaches. Noting that the Gibbs Conference has also served as a vehicle to promote the broader use of thermodynamics in understanding biology, we then discuss collaborative work on the hydrodynamics of a cytokine implicated in tumor suppression, prostate derived factor (PDF).

Structural analysis of bacteriophage T4 DNA replication: a review in the Virology Journal series on bacteriophage T4 and its relatives

The bacteriophage T4 encodes 10 proteins, known collectively as the replisome, that are responsible for the replication of the phage genome. The replisomal proteins can be subdivided into three activities; the replicase, responsible for duplicating DNA, the primosomal proteins, responsible for unwinding and Okazaki fragment initiation, and the Okazaki repair proteins. The replicase includes the gp43 DNA polymerase, the gp45 processivity clamp, the gp44/62 clamp loader complex, and the gp32 single-stranded DNA binding protein. The primosomal proteins include the gp41 hexameric helicase, the gp61 primase, and the gp59 helicase loading protein. The RNaseH, a 5' to 3' exonuclease and T4 DNA ligase comprise the activities necessary for Okazaki repair. The T4 provides a model system for DNA replication. As a consequence, significant effort has been put forth to solve the crystallographic structures of these replisomal proteins. In this review, we discuss the structures that are available and provide comparison to related proteins when the T4 structures are unavailable. Three of the ten full-length T4 replisomal proteins have been determined; the gp59 helicase loading protein, the RNase H, and the gp45 processivity clamp. The core of T4 gp32 and two proteins from the T4 related phage RB69, the gp43 polymerase and the gp45 clamp are also solved. The T4 gp44/62 clamp loader has not been crystallized but a comparison to the E. coli gamma complex is provided. The structures of T4 gp41 helicase, gp61 primase, and T4 DNA ligase are unknown, structures from bacteriophage T7 proteins are discussed instead. To better understand the functionality of T4 DNA replication, in depth structural analysis will require complexes between proteins and DNA substrates. A DNA primer template bound by gp43 polymerase, a fork DNA substrate bound by RNase H, gp43 polymerase bound to gp32 protein, and RNase H bound to gp32 have been crystallographically determined. The preparation and crystallization of complexes is a significant challenge. We discuss alternate approaches, such as small angle X-ray and neutron scattering to generate molecular envelopes for modeling macromolecular assemblies.

An unusual pattern of CytR and CRP binding energetics at Escherichia coli cddP suggests a unique blend of class I and class II mediated activation

Two transcription factors, CRP and CytR, mediate positive and negative control of nine cistrons involved in nucleoside catabolism and recycling in Escherichia coli. The ability of multiple transcription factors to combine in different ways to confer differential gene regulation is of significant interest in both prokaryotic and eukaryotic gene regulation. Analysis of cooperative interactions between CytR and CRP at the deoP2 and udpP promoters has implicated the importance of promoter architecture in controlling repression and induction. These studies have also identified competition between CytR and CRP as an additional contributor to differential regulation. The pattern and energetics of CytR and CRP interactions at the cdd promoter, the most strongly activated of the CytR-regulated promoters, have been delineated using DNase I footprinting. Surprisingly, CRP has greater affinity for the promoter proximal site at cddP, CRP1, than for the distal site, CRP2, in contrast to promoters studied previously. This difference is a major contributor to unusually high CRP-mediated activation of cddP. Additionally, while cytidine binding to CytR nearly eliminates the pairwise interactions between CytR and CRP bound at CRP1, it has little effect on pairwise cooperativity between CytR and CRP bound at CRP2 or as a consequence on the overall cooperativity of the three-protein complex in which CRP is bound to both sites. The effect of cytidine binding on cooperativity differs between the three promoters studied thus far. We propose that the different patterns of interaction reflect the spacing between CytR half-sites and the location of the CytR operator in relation to the two CRP sites.

Linked equilibria in regulation of transcription initiation

Assembly of transcriptional regulatory complexes often involves multiple binding processes and these binding processes are frequently coupled to one another. Small molecule binding can promote or inhibit DNA-binding or protein-protein interactions. DNA binding may be coupled to protein association. Finally, proteins may bind cooperatively to multiple sites in a transcriptional regulatory region. The level of transcription initiation at a promoter reflects the assembly of regulatory complexes in a transcription control region. Quantitative mechanistic understanding of regulatory complex assembly requires dissection of the assembly process into its constituent interactions followed by measurements of linkage between the individual binding processes. Methods and approaches to achieving this quantitative understanding of transcription regulation are outlined in this chapter.

Flexibility and adaptability in binding of E. coli cytidine repressor to different operators suggests a role in differential gene regulation

Interactions between DNA-bound transcription factors CytR and CRP regulate the promoters of the Escherichia coli CytR regulon. A distinctive feature of the palindromic CytR operators is highly variable length central spacers (0-9 bp). Previously we demonstrated distinct modes of CytR binding to operators that differ in spacer length. These different modes are characterized by opposite enthalpic and entropic contributions at 25 degrees C. Of particular note were radically different negative DeltaCp values suggesting variable contribution from coupled protein folding and/or DNA structural transitions. We proposed that the CytR DNA binding-domain adopts either a more rigid or flexible DNA-bound conformation in response to the different spacer lengths. More recently, similar effects were shown to contribute to discrimination between operator and non-specific DNA binding by LacR, a CytR homolog. Here we have extended the thermodynamic analysis to the remaining natural CytR operators plus a set of synthetic operators designed to isolate spacing as the single variable. The thermodynamic results show a broad and monotonic range of effects that are primarily dependent on spacer length. The magnitude of effects suggests participation by more than the DNA-binding domain. 15N HSQC NMR and CD spectral analyses were employed to characterize the structural basis for these effects. The results indicate that while CytR forms a well-ordered structure in solution, it is highly dynamic. We propose a model in which a large ensemble of native state conformations narrows upon binding, to an extent governed by operator spacing. This in turn is expected to constrain intermolecular interactions in the CytR-CRP-DNA complex, thus generating operator-specific effects on repression and induction of transcription.

CpxR/OmpR interplay regulates curli gene expression in response to osmolarity in Escherichia coli

Curli fibers could be described as a virulence factor able to confer adherence properties to both abiotic and eukaryotic surfaces. The ability to adapt rapidly to changing environmental conditions through signal transduction pathways is crucial for the growth and pathogenicity of bacteria. OmpR was shown to activate csgD expression, resulting in curli production. The CpxR regulator was shown to negatively affect curli gene expression when binding to its recognition site that overlaps the csgD OmpR-binding site. This study was undertaken to clarify how the interplay between the two regulatory proteins, OmpR and CpxR, can affect the transcription of the curli gene in response to variation of the medium osmolarity. Band-shift assays with purified CpxR proteins indicate that CpxR binds to the csgD promoter region at multiple sites that are ideally positioned to explain the csg repression activity of CpxR. To understand the physiological meaning of this in vitro molecular phenomenon, we analyzed the effects of an osmolarity shift on the two-component pathway CpxA/CpxR. We establish here that the Cpx pathway is activated at both transcriptional and posttranscriptional levels in response to a high osmolarity medium and that CpxR represses csgD expression in high-salt-content medium, resulting in low curli production. However, csgD repression in response to high sucrose content is not mediated by CpxR but by the global regulatory protein H-NS. Therefore, multiple systems (EnvZ/OmpR, Cpx, Rcs, and H-NS) appear to be involved in sensing environmental osmolarity, leading to sophisticated regulation of the curli genes.

Identification of the CRP regulon using in vitro and in vivo transcriptional profiling

The Escherichia coli cyclic AMP receptor protein (CRP) is a global regulator that controls transcription initiation from more than 100 promoters by binding to a specific DNA sequence within cognate promoters. Many genes in the CRP regulon have been predicted simply based on the presence of DNA-binding sites within gene promoters. In this study, we have exploited a newly developed technique, run-off transcription/microarray analysis (ROMA) to define CRP-regulated promoters. Using ROMA, we identified 176 operons that were activated by CRP in vitro and 16 operons that were repressed. Using positive control mutants in different regions of CRP, we were able to classify the different promoters into class I or class II/III. A total of 104 operons were predicted to contain Class II CRP-binding sites. Sequence analysis of the operons that were repressed by CRP revealed different mechanisms for CRP inhibition. In contrast, the in vivo transcriptional profiles failed to identify most CRP-dependent regulation because of the complexity of the regulatory network. Analysis of these operons supports the hypothesis that CRP is not only a regulator of genes required for catabolism of sugars other than glucose, but also regulates the expression of a large number of other genes in E.coli. ROMA has revealed 152 hitherto unknown CRP regulons.

Unique ligation properties of eukaryotic NAD+-dependent DNA ligase from Melanoplus sanguinipes entomopoxvirus

The eukaryotic Melanoplus sanguinipes entomopoxvirus (MsEPV) genome reveals a homologous sequence to eubacterial nicotinamide adenine dinucleotide (NAD(+))-dependent DNA ligases [J. Virol. 73 (1999) 533]. This 522-amino acid open reading frame (ORF) contains all conserved nucleotidyl transferase motifs but lacks the zinc finger motif and BRCT domain found in conventional eubacterial NAD(+) ligases. Nevertheless, cloned MsEPV ligase seals DNA nicks in a NAD(+)-dependent fashion, while adenosine 5'-monophosphate (ATP) cannot serve as an adenylation cofactor. The ligation activity of MsEPV ligase requires Mg(2+) or Mn(2+). MsEPV ligase seals sticky ends efficiently, but has little activity on 1-nucleotide gap or blunt-ended DNA substrates even in the presence of polyethylene glycol. In comparison, bacterial NAD(+)-dependent ligases seal blunt-ended DNA substrates in the presence of polyethylene glycol. MsEPV DNA ligase readily joins DNA nicks with mismatches at either side of the nick junction, except for mismatches at the nick junction containing an A base in the template strand (A/A, G/A, and C/A). MsEPV NAD(+)-dependent DNA ligase can join DNA probes on RNA templates, a unique property that distinguishes this enzyme from other conventional bacterial NAD(+) DNA ligases. T4 ATP-dependent DNA ligase shows no detectable mismatch ligation at the 3' side of the nick but substantial 5' T/G mismatch ligation on an RNA template. In contrast, MsEPV ligase joins mismatches at the 3' side of the nick more frequently than at the 5' side of the nick on an RNA template. The complementary specificities of these two enzymes suggest alternative primer design for genomic profiling approaches that use allele-specific detection directly from RNA transcripts.

Role of protein-protein bridging interactions on cooperative assembly of DNA-bound CRP-CytR-CRP complex and regulation of the Escherichia coli CytR regulon

The unlinked operons that comprise the Escherichia coli CytR regulon are controlled coordinately through interactions between two gene regulatory proteins, the cAMP receptor protein (CRP) and the cytidine repressor (CytR). CytR controls the balance between CRP-mediated recruitment and activation of RNA polymerase and transcriptional repression. Cooperative interactions between CytR, when bound to an operator (CytO) located upstream of a CytR-regulated promoter, and CRP, when bound to flanking tandem promoters, are critical to the regulatory role of CytR. When CytR binds cytidine, cooperativity is reduced resulting in increased transcriptional activity. However, this cytidine-mediated effect varies among promoters, suggesting that coupling between cytidine binding to CytR and CytR-CRP association is sensitive to promoter structure. To investigate the chemical and structural basis for these effects, we investigated how cytidine binding affects association between CytR and CRP in solution and how it affects the binding of CytR deletion mutants lacking the DNA binding HTH domain, with tandem CRP dimers bound to either udpP or deoP2. Deletion mutants that, as we show here, retain the native functions of the allosteric, inducer-binding domain but do not bind DNA were expressed and purified. We refer to these as Core domain. Despite only weak association between CytR and CRP in solution, our results demonstrate the formation of a relatively stable complex in which the Core domain forms a protein bridge between tandem CRP dimers when bound to either udpP or deoP2. The DeltaG(o) for bridge complex formation is about -7.8 kcal/mol. This is well in excess of that required to account for cooperativity (-2.5 to -3 kcal/mol). The bridge complexes are significantly destabilized by cytidine binding, and to the same extent in both promoter complexes (DeltaDeltaG(o) approximately +2 kcal/mol). Even with this destabilization, DeltaG(o) for bridge complex formation by cytidine-liganded Core domain is still sufficient by itself to account for cooperativity. These findings demonstrate that direct coupling between cytidine binding to CytR and CytR-CRP association does not account for promoter-specific effects on cooperativity. Instead, cytidine binding must induce a CytR conformation that is more rigid or in some other way less tolerant of the variation in the geometric arrangement of operator sites between different promoters.

Thermodynamics of E. coli cytidine repressor interactions with DNA: distinct modes of binding to different operators suggests a role in differential gene regulation

Interactions between the Escherichia coli cytidine repressor protein (CytR) and its operator sites at the different promoters that comprise the CytR regulon, play an important role in the regulation of these promoters. The natural operators are palindromes separated by variable length central spacers (0-9 bp). We have suggested that this variability affects the flexibility of CytR-DNA contacts, thereby affecting the critical protein-protein interactions between CytR and the cAMP receptor protein (CRP) that underlie differential repression and activation of CytR-regulated genes. To assess this hypothesis, we investigated the thermodynamics of CytR binding to the natural operator sequences found in udpP and deoP2. To separate effects due to spacing from effects due to the differing sequences of the recognition half-sites of these two operators, we also investigated CytR binding to artificial hybrid operators, in which the half-site sequences of udpP and deoP2 were exchanged. Thermodynamic parameters, DeltaS(o), DeltaH(o) and DeltaC(o)(p), were determined by van't Hoff analysis of CytR binding, monitored by changes in the steady-state fluorescence anisotropy of dye-conjugated, operator-containing oligonucleotides. Large differences in thermodynamics were observed that depend primarily on the central spacer rather than the sequences of the recognition half-sites. Binding to operators with deoP2 spacing results in a very large, negative DeltaC(o)(p). Association is strongly favored enthalpically and strongly disfavored entropically at ambient temperature. By contrast, binding to operators with udpP spacing results in a small, negative DeltaC(o)(p). Association is weakly favored both enthalpically and entropically at ambient temperature. A difference of such magnitude in DeltaDeltaC(o)(p) has not been reported previously for specific binding of a transcription factor to different sites. The identical salt dependence of CytR binding to deoP2 and udpP operators indicates that ion-dependent processes do not contribute significantly to this difference. Thus, the different thermodynamic effects appear to reflect distinctly different modes of site-specific DNA binding. We discuss similarities to operator binding by CytR homologs among LacI family repressors, and we consider how different CytR binding modes might affect interactions with other components of the gene regulatory machinery that contribute to differential gene regulation.

Nucleotide sequence, heterologous expression and novel purification of DNA ligase from Bacillus stearothermophilus(1)

The gene for DNA ligase (EC 6.5.1.2) from thermophilic bacterium Bacillus stearothermophilus NCA1503 has been cloned and the complete nucleotide sequence determined. The ligase gene encodes a protein 670 amino acids in length. The gene was overexpressed in Escherichia coli and the enzyme has been purified to homogeneity. Preliminary characterisation confirms that it is a thermostable, NAD(+)-dependent DNA ligase.

Role of multiple CytR binding sites on cooperativity, competition, and induction at the Escherichia coli udp promoter

The CytR repressor fulfills dual roles as both a repressor of transcription from promoters of the Escherichia coli CytR regulon and a co-activator in some circumstances. Transcription is repressed by a three-protein complex (cAMP receptor protein (CRP)-CytR-CRP) that is stabilized by cooperative interactions between CRP and CytR. However, cooperativity also means that CytR can recruit CRP and, by doing so, can act as a co-activator. The central role of cooperativity in regulation is highlighted by the fact that binding of the inducer, cytidine, to CytR is coupled to CytR-CRP cooperativity; this underlies the mechanism for induction. Similar interactions at the different promoters of the CytR regulon coordinate expression of the transport proteins and enzymes required for nucleoside catabolism but also provide differential expression of these genes. A fundamental question in both prokaryotic and eukaryotic gene regulation is how combinatorial mechanisms of this sort regulate differential expression. Recently, we showed that CytR binds specifically to multiple sites in the E. coli deoP promoter, thereby providing competition for CRP binding to CRP operator site 1 (CRP1) and CRP2 as well as cooperativity. The effect of the competition at this promoter is to negate the role of CytR in recruiting CRP. Here, we have used quantitative footprint and mobility shift analysis to investigate CRP and CytR binding to the E. coli udp promoter. Here too, we find that CytR both cooperates and competes for CRP binding. However, consistent with both the distribution of CytR recognition motifs in the sequence of the promoter and the regulation of the promoter, the competition is limited to CRP2. When cytidine binds to CytR, the effect on cooperativity is very different at the udp promoter than at the deoP2 promoter. Cooperativity with CRP at CRP1 is nearly eliminated, but the effect on CytR-CRP2 cooperativity is negligible. These results are discussed in relation to the current structural model of CytR in which the core, inducer-binding domain is tethered to the helix-turn-helix, DNA-binding domain via flexible peptide linkers.

Catabolic repression of secB expression is positively controlled by cyclic AMP (cAMP) receptor protein-cAMP complexes at the transcriptional level

SecB, a protein export-specific chaperone, enhances the export of a subset of proteins across cytoplasmic membranes of Escherichia coli. Previous studies showed that the synthesis of SecB is repressed by the presence of glucose in the medium. The derepression of SecB requires the products of both the cya and crp genes, indicating that secB expression is under the control of catabolic repression. In this study, two secB-specific promoters were identified. In addition, 5' transcription initiation sites from these two promoters were determined by means of secB-lacZ fusions and primer extension. The distal P1 promoter appeared to be independent of carbon sources, whereas the proximal P2 promoter was shown to be subject to control by the cyclic AMP (cAMP) receptor protein (CRP)-cAMP complexes. Gel-mobility shift studies showed that this regulation results from direct interaction between the secB P2 promoter region and the CRP-cAMP complex. Moreover, the CRP binding site on the secB gene was determined by DNase I footprinting and further substantiated by mutational analysis. The identified secB CRP binding region is centered at the -61.5 region of the secB gene and differed from the putative binding sites predicted by computer analysis.

Cooperative non-specific DNA binding by octamerizing lambda cI repressors: a site-specific thermodynamic analysis

Relationships between dimerization and site-specific binding have been characterized previously for wild-type and mutant cI repressors at the right operator (OR) of bacteriophage lambda DNA. However, the roles of higher-order oligomers (tetramers and octamers) that are also formed from these cI molecules have remained elusive. In this study, a clear correlation has been established between repressor oligomerization and non-specific DNA-binding activity. A modification of the quantitative DNase I footprint titration technique has been used to evaluate the degree of saturation of non-specific, OR-flanking lambda DNA by cI repressor oligomers. With the exception of one mutant, only those repressors capable of octamerizing were found to exhibit non-specific DNA-binding activity. The non-specific interaction was accurately modeled using either a one-dimensional, univalent, site-specific Ising lattice approximation, or a more traditional, multivalent lattice approach. It was found that non-specific DNA-binding by repressor oligomers is highly cooperative and energetically independent from site-specific binding at OR. Furthermore, the coupling free energy resolved for non-specific binding was similar to that of site-specific binding for each repressor, suggesting that similar structural elements may mediate the cooperative component of both binding processes. It is proposed that the state of assembly of the repressor molecule modulates its relative affinity for specific and non-specific DNA sequences. These specificities are allosterically regulated by the transmission of assembly-state information from the C-terminal domain, which mediates self-association and cooperativity, to the N-terminal domain, which primarily mediates DNA-binding. While dimers have a high affinity for their cognate sites within OR, tetramers and octamers may preferentially recognize non-specific DNA sequences. The concepts and findings developed in this study may facilitate quantitative characterization of the relationships between specific, and non-specific binding in other systems that utilize multiple modes of DNA-binding cooperativity.

Some repressors of bacterial transcription

For a long time, repression of transcription in Escherichia coli was thought to be generally caused by one repressor binding to one operator. Recent work has indicated the frequent presence of auxiliary operators and helper proteins. The recent solution of the X-ray structures of Lac and Pur repressors were breakthroughs; yet, it has become painfully clear that important aspects of repression are still not understood.

Allosteric mechanism of induction of CytR-regulated gene expression. Cytr repressor-cytidine interaction

Transcription from cistrons of the Escherichia coli CytR regulon is activated by E. coli cAMP receptor protein (CRP) and repressed by a multiprotein complex composed of CRP and CytR. De-repression results when CytR binds cytidine. CytR is a homodimer and a LacI family member. A central question for all LacI family proteins concerns the allosteric mechanism that couples ligand binding to the protein-DNA and protein-protein interactions that regulate transcription. To explore this mechanism for CytR, we analyzed nucleoside binding in vitro and its coupling to cooperative CytR binding to operator DNA. Analysis of the thermodynamic linkage between sequential cytidine binding to dimeric CytR and cooperative binding of CytR to deoP2 indicates that de-repression results from just one of the two cytidine binding steps. To test this conclusion in vivo, CytR mutants that have wild-type repressor function but are cytidine induction-deficient (CID) were identified. Each has a substitution for Asp281 or neighboring residue. CID CytR281N was found to bind cytidine with three orders of magnitude lower affinity than wild-type CytR. Other CytR mutants that do not exhibit the CID phenotype were found to bind cytidine with affinity similar to wild-type CytR. The rate of transcription regulated by heterodimeric CytR composed of one CytR281N and one wild-type subunit was compared with that regulated by wild-type CytR under inducing conditions. The data support the conclusion that the first cytidine binding step alone is sufficient to induce.