Strengthened arm-dimerization domain interactions in AraC

Arabinose Alters Both Local and Distal H-D Exchange Rates in the Escherichia coli AraC Transcriptional Regulator

In the absence of arabinose, the dimeric Escherichia coli regulatory protein of the l-arabinose operon, AraC, represses expression by looping the DNA between distant half-sites. Binding of arabinose to the dimerization domains forces AraC to preferentially bind two adjacent DNA half-sites, which stimulates RNA polymerase transcription of the araBAD catabolism genes. Prior genetic and biochemical studies hypothesized that arabinose allosterically induces a helix-coil transition of a linker between the dimerization and DNA binding domains that switches the AraC conformation to an inducing state [Brown, M. J., and Schleif, R. F. (2019) Biochemistry, preceding paper in this issue (DOI: 10.1021/acs.biochem.9b00234)]. To test this hypothesis, hydrogen-deuterium exchange mass spectrometry was utilized to identify structural regions involved in the conformational activation of AraC by arabinose. Comparison of the hydrogen-deuterium exchange kinetics of individual dimeric dimerization domains and the full-length dimeric AraC protein in the presence and absence of arabinose reveals a prominent arabinose-induced destabilization of the amide hydrogen-bonded structure of linker residues (I₁₆₇ and N₁₆₈). This destabilization is demonstrated to result from an increased probability to form a helix capping motif at the C-terminal end of the dimerizing α-helix of the dimerization domain that preceeds the interdomain linker. These conformational changes could allow for quaternary repositioning of the DNA binding domains required for induction of the araBAD promoter through rotation of peptide backbone dihedral angles of just a couple of residues. Subtle changes in exchange rates are also visible around the arabinose binding pocket and in the DNA binding domain.

Analysis of amino acid substitutions in AraC variants that respond to triacetic acid lactone

The Escherichia coli regulatory protein AraC regulates expression of ara genes in response to l-arabinose. In efforts to develop genetically encoded molecular reporters, we previously engineered an AraC variant that responds to the compound triacetic acid lactone (TAL). This variant (named "AraC-TAL1") was isolated by screening a library of AraC variants, in which five amino acid positions in the ligand-binding pocket were simultaneously randomized. Screening was carried out through multiple rounds of alternating positive and negative fluorescence-activated cell sorting. Here we show that changing the screening protocol results in the identification of different TAL-responsive variants (nine new variants). Individual substituted residues within these variants were found to primarily act cooperatively toward the gene expression response. Finally, X-ray diffraction was used to solve the crystal structure of the apo AraC-TAL1 ligand-binding domain. The resolved crystal structure confirms that this variant takes on a structure nearly identical to the apo wild-type AraC ligand-binding domain (root-mean-square deviation 0.93 Å), suggesting that AraC-TAL1 behaves similar to wild-type with regard to ligand recognition and gene regulation. Our results provide amino acid sequence-function data sets for training and validating AraC modeling studies, and contribute to our understanding of how to design new biosensors based on AraC.

Mutational analysis of the N-terminal domain of UreR, the positive transcriptional regulator of urease gene expression

The Escherichia coli plasmid-encoded urease, a virulence factor in human and animal infections of the urinary and gastroduodenal tracts, is induced when the substrate urea is present in the growth medium. Urea-dependent urease expression is mediated at the transcriptional level by the AraC-like activator UreR. Previous work has shown that a peptide representing the N-terminal 194 amino-acid residues of UreR binds urea at a single site, full-length UreR forms an oligomer, and the oligomerization motif is thought to reside in the N-terminal portion of the molecule. The C-terminal domain of UreR contains two helix-turn-helix motifs presumed to be necessary for DNA binding. In this study, we exploited mutational analyses at the N-terminal domain of UreR to determine if this domain dimerizes similar to other AraC family members. UreR mutants were analyzed for the ability to activate transcription of lacZ from an ureDp-lacZ transcriptional fusion. A construct encoding the N-terminal 194 amino acids of UreR, eluted as an oligomer by gel filtration and had a dominant negative phenotype over the wild-type ureR allele. We hypothesize that this dominant negative phenotype results from the formation of inactive heterodimers between wild-type and truncated UreR. Dominant negative analysis and cross-linking assays demonstrated that E. coli UreR is active as a dimer and dimerization occurs within the first 180 residues.

Active role of the interdomain linker of AraC

Mutations in the interdomain linker of the gene for the AraC regulatory protein of Escherichia coli that severely interfere with the protein's ability either to repress or to activate transcription have been found. These mutations have relatively small effects on the dimerization domain's ability to bind arabinose or to dimerize the protein or on the DNA-binding domain's affinity for a single DNA half-site. The linker mutations, however, dramatically change the affinity of AraC for binding to two direct-repeat DNA half-sites. Less dramatically, the induction-deficient linker variants also display altered DNA sequence selectivity. These results show that changing the sequence of the interdomain linker can profoundly affect the dimerization domain-DNA-binding domain interactions in AraC. The smaller effects on the functions of the individual domains could be the direct result of the linker alterations but more likely are the indirect result of the altered dimerization domain-DNA-binding domain interactions. In summary, the linker does not simply function as a passive and flexible connector between the domains of AraC but, instead, is more directly involved in the protein's dimerization domain-DNA-binding domain interactions.

AraC protein, regulation of the l-arabinose operon in Escherichia coli, and the light switch mechanism of AraC action

This review covers the physiological aspects of regulation of the arabinose operon in Escherichia coli and the physical and regulatory properties of the operon's controlling gene, araC. It also describes the light switch mechanism as an explanation for many of the protein's properties. Although many thousands of homologs of AraC exist and regulate many diverse operons in response to many different inducers or physiological states, homologs that regulate arabinose-catabolizing genes in response to arabinose were identified. The sequence similarities among them are discussed in light of the known structure of the dimerization and DNA-binding domains of AraC.

Computational predictions of the mutant behavior of AraC

An algorithm implemented in Rosetta correctly predicts the folding capabilities of the 17-residue N-terminal arm of the AraC gene regulatory protein when arabinose is bound to the protein and the dramatically different structure of this arm when arabinose is absent. The transcriptional activity of 43 mutant AraC proteins with alterations in the arm sequences was measured in vivo and compared with their predicted folding properties. Seventeen of the mutants possessed regulatory properties that could be directly compared with folding predictions. Sixteen of the 17 mutants were correctly predicted. The algorithm predicts that the N-terminal arm sequences of AraC homologs fold to the Escherichia coli AraC arm structure. In contrast, it predicts that random sequences of the same length and many partially randomized E. coli arm sequences do not fold to the E. coli arm structure. The high level of success shows that relatively "simple" computational methods can in some cases predict the behavior of mutant proteins with good reliability.

Elucidating residue roles in engineered variants of AraC regulatory protein

The AraC regulatory protein was previously engineered to control gene expression specifically in response to D-arabinose and not the native effector L-arabinose (Tang et al., J Am Chem Soc 2008;130:5267-5271). Mutations were targeted in the ligand-binding pocket and on the AraC N-terminal arm, which plays an important role in maintaining repressing or activating conformations in the absence or presence of effector, respectively. In this study, we analyze the contributions of individual mutations toward the overall mutant functions in an attempt to streamline future AraC design efforts. For a variety of point mutants, we quantify the induced expression response to D-arabinose (level of leaky expression, induction fold, half-maximal dose response, and effector specificity) and the binding affinity of the purified ligand-binding domain for D-arabinose. We find that mutations introduced in the N-terminal arm (design Position 8) strengthen the induction response, most likely by weakening interactions with the DNA-binding domain, but are not involved in ligand binding. Meanwhile, binding pocket mutations occurring further away from the arm (Positions 80 and 82) primarily contribute to maintaining repression in the absence of effector and do not show response to D-arabinose without the accompanying mutations. Combinations of mutations cooperatively couple molecular recognition to transcriptional activation, demonstrating the complexity of the AraC regulatory switch and the power of combinatorial protein design to alter effector specificity while maintaining regulatory function.

A DNA-assisted binding assay for weak protein-protein interactions

We describe a new method used for quantitating weak interactions between proteins in which the weak interaction is "assisted" by a known DNA-DNA interaction. Oligonucleotides, which are conjugated to proteins of interest, contain short complementary DNA sequences that provide additional binding energy for protein-protein interactions. A stretch of unpaired bases links the protein to the hybridizing DNA sequence to allow formation of both protein-protein and DNA-DNA interactions with minimal structural interference. We validated the DNA-assisted binding method using heterodimerizing coiled-coil proteins. The method was then used to measure the predicted weak interaction between two domains of the Escherichia coli L-arabinose operon regulatory protein AraC. The interaction between domains has the expected magnitude (K(d)=0.37 mM) in the absence of arabinose. Upon addition of arabinose, we detected a weaker and unexpected interaction, which may necessitate modification of the proposed mechanism of AraC. The DNA-assisted binding method may also prove useful in the study of other weak protein-protein interactions.

Constitutive mutations in the Escherichia coli AraC protein

The Escherichia coli AraC protein represses and induces the araBAD operon in response to the absence or presence of l-arabinose. Constitutive mutations in the AraC gene no longer require the presence of l-arabinose to convert AraC from its repressing to its inducing state. Such mutations were isolated directly by virtue of their constitutivity or by their resistance to the nonmetabolizable arabinose analog, d-fucose. The majority of the constitutive mutations lie within the same residues of the N-terminal regulatory arm of AraC. Two, however, were found in the core of the dimerization domain. As predicted by the light switch mechanism of AraC, constitutive mutations increase the susceptibility of the N-terminal arms to digestion by trypsin or chymotrypsin, suggesting that these mutations weaken or disrupt the arm structure required for repression by AraC. Fluorescence, circular dichroism, and cysteine reactivity measurements show that the constitutive mutations in the core of the dimerization domain lead to a weakening of the support for the arms and reduce the stability of the minus-arabinose arm structure. These mutations also weaken the interaction between the two-helix bundle and the beta-barrel subdomains of the dimerization domain and reduce the structural stability of the beta-barrels.

Differences in the mechanism of the allosteric l-rhamnose responses of the AraC/XylS family transcription activators RhaS and RhaR

Proteins in the largest subset of AraC/XylS family transcription activators, including RhaS and RhaR, have C-terminal domains (CTDs) that mediate DNA-binding and transcription activation, and N-terminal domains (NTDs) that mediate dimerization and effector binding. The mechanism of the allosteric effector response in this family has been identified only for AraC. Here, we investigated the mechanism by which RhaS and RhaR respond to their effector, l-rhamnose. Unlike AraC, N-terminal truncations suggested that RhaS and RhaR do not use an N-terminal arm to inhibit activity in the absence of effector. We used random mutagenesis to isolate RhaS and RhaR variants with enhanced activation in the absence of l-rhamnose. NTD substitutions largely clustered around the predicted l-rhamnose-binding pockets, suggesting that they mimic the structural outcome of effector binding to the wild-type proteins. RhaS-CTD substitutions clustered in the first HTH motif, and suggested that l-rhamnose induces improved DNA binding. In contrast, RhaR-CTD substitutions clustered at a single residue in the second HTH motif, at a position consistent with improved RNAP contacts. We propose separate allosteric mechanisms for the two proteins: Without l-rhamnose, RhaS does not effectively bind DNA while RhaR does not effectively contact RNAP. Upon l-rhamnose binding, both proteins undergo structural changes that enable transcription activation.

DNA tape measurements of AraC

A new method for measuring distances between points in the AraC–DNA complex was developed and applied. It utilizes variable lengths of single-stranded DNA that connect double-stranded regions containing the two half-site binding sequences of AraC. These distances plus the protein interdomain linker distances are compatible with two classes of structure for the dimeric AraC gene regulatory protein. In one class, the N-terminal regulatory arm of one dimerization domain is capable of interacting with the DNA-binding domain on the same polypeptide chain for a cis interaction. In the other class, the possible arm-DNA-binding domain interaction is trans, where it adds to the dimerization interface.

The AraC/XylS regulator TxtR modulates thaxtomin biosynthesis and virulence in Streptomyces scabies

Streptomyces scabies is the best studied of those streptomycetes that cause an economically important disease known as potato scab. The phytotoxin thaxtomin is made exclusively by these pathogens and is required for virulence. Here we describe regulation of thaxtomin biosynthesis by TxtR, a member of the AraC/XylS family of transcriptional regulators. The txtR gene is imbedded in the thaxtomin biosynthetic pathway and is located on a conserved pathogenicity island in S. scabies, S. turgidiscabies and S. acidiscabies. Thaxtomin biosynthesis was abolished and virulence was almost eliminated in the txtR deletion mutant of S. scabies 87.22. Accumulation of thaxtomin biosynthetic gene (txtA, txtB, txtC, nos) transcripts was reduced compared with the wild-type S. scabies 87.22. NOS-dependent nitric oxide production by S. scabies was also reduced in the mutant. The TxtR protein bound cellobiose, an inducer of thaxtomin production, and transcription of txtR and thaxtomin biosynthetic genes was upregulated in response to cellobiose. TxtR is the first example of an AraC/XylS family protein regulated by cellobiose. Together, these data suggest that cellobiose, the smallest oligomer of cellulose, may signal the availability of expanding plant tissue, which is the site of action of thaxtomin.

Transcription activation by the DNA-binding domain of the AraC family protein RhaS in the absence of its effector-binding domain

The Escherichia coli L-rhamnose-responsive transcription activators RhaS and RhaR both consist of two domains, a C-terminal DNA-binding domain and an N-terminal dimerization domain. Both function as dimers and only activate transcription in the presence of L-rhamnose. Here, we examined the ability of the DNA-binding domains of RhaS (RhaS-CTD) and RhaR (RhaR-CTD) to bind to DNA and activate transcription. RhaS-CTD and RhaR-CTD were both shown by DNase I footprinting to be capable of binding specifically to the appropriate DNA sites. In vivo as well as in vitro transcription assays showed that RhaS-CTD could activate transcription to high levels, whereas RhaR-CTD was capable of only very low levels of transcription activation. As expected, RhaS-CTD did not require the presence of L-rhamnose to activate transcription. The upstream half-site at rhaBAD and the downstream half-site at rhaT were found to be the strongest of the known RhaS half-sites, and a new putative RhaS half-site with comparable strength to known sites was identified. Given that cyclic AMP receptor protein (CRP), the second activator required for full rhaBAD expression, cannot activate rhaBAD expression in a DeltarhaS strain, it was of interest to test whether CRP could activate transcription in combination with RhaS-CTD. We found that RhaS-CTD allowed significant activation by CRP, both in vivo and in vitro, although full-length RhaS allowed somewhat greater CRP activation. We conclude that RhaS-CTD contains all of the determinants necessary for transcription activation by RhaS.

Specific interactions by the N-terminal arm inhibit self-association of the AraC dimerization domain

Deletion of the regulatory N-terminal arms of the AraC protein from its dimerization domain fragments increases the susceptibility of the dimerization domain to form a series of higher order polymers by indefinite self-association. We investigated how the normal presence of the arm inhibits this self-association. One possibility is that arms can act as an entropic bristles to interfere with the approach of other macromolecules, thereby decreasing collision frequencies. We examined the repulsive effect of flexible arms by measuring the rate of trypsin cleavage of a specially constructed ubiquitin-arm protein. Adding an arm to ubiquitin or increasing its length produced only a modest repulsive effect. This suggests that arms such as the N-terminal arm of AraC do not reduce self-association by entropic exclusion. We consequently tested the hypothesis that the arm on AraC reduces self-association by binding to the core of the dimerization domain even in the absence of arabinose. The behaviors of dimerization domain mutants containing deletions or alterations in the N-terminal arms substantiate this hypothesis. Apparently, interactions between the N-terminal arm and the dimerization domain core position the arm to interfere with the protein-protein contacts necessary for self-association.

Structure and properties of a truely apo form of AraC dimerization domain

The arabinose-binding pockets of wild type AraC dimerization domains crystallized in the absence of arabinose are occupied with the side chains of Y31 from neighboring domains. This interaction leads to aggregation at high solution concentrations and prevents determination of the structure of truely apo AraC. In this work we found that the aggregation does not significantly occur at physiological concentrations of AraC. We also found that the Y31V mutation eliminates the self-association, but does not affect regulation properties of the protein. At the same time, the mutation allows crystallization of the dimerization domain of the protein with only solvent in the arabinose-binding pocket. Using a distance difference method suitable for detecting and displaying even minor structural variation among large groups of similar structures, we find that there is no significant structural change in the core of monomers of the AraC dimerization domain resulting from arabinose, fucose, or tyrosine occupancy of the ligand-binding pocket. A slight change is observed in the relative orientation of monomers in the dimeric form of the domain upon the binding of arabinose but its significance cannot yet be assessed.

Mutational analysis of the Escherichia coli melR gene suggests a two-state concerted model to explain transcriptional activation and repression in the melibiose operon

Transcription of the Escherichia coli melAB operon is regulated by the MelR protein, an AraC family member whose activity is modulated by the binding of melibiose. In the absence of melibiose, MelR is unable to activate the melAB promoter but autoregulates its own expression by repressing the melR promoter. Melibiose triggers MelR-dependent activation of the melAB promoter and relieves MelR-dependent repression of the melR promoter. Twenty-nine single amino acid substitutions in MelR that result in partial melibiose-independent activation of the melAB promoter have been identified. Combinations of different substitutions result in almost complete melibiose-independent activation of the melAB promoter. MelR carrying each of the single substitutions is less able to repress the melR promoter, while MelR carrying some combinations of substitutions is completely unable to repress the melR promoter. These results argue that different conformational states of MelR are responsible for activation of the melAB promoter and repression of the melR promoter. Supporting evidence for this is provided by the isolation of substitutions in MelR that block melibiose-dependent activation of the melAB promoter while not changing melibiose-independent repression of the melR promoter. Additional experiments with a bacterial two-hybrid system suggest that interactions between MelR subunits differ according to the two conformational states.

Leucines 193 and 194 at the N-terminal domain of the XylS protein, the positive transcriptional regulator of the TOL meta-cleavage pathway, are involved in dimerization

Members of the AraC/XylS family of transcriptional regulators are usually organized in two domains: a conserved domain made up of 100 amino acids and frequently located at the C-terminal end, involved in DNA binding; and an N-terminal nonconserved domain involved in signal recognition, as is the case for regulators involved in the control of carbon metabolism (R. Tobes and J. L. Ramos, Nucleic Acids Res. 30:318-321, 2002). The XylS protein, which is extremely insoluble, controls expression of the meta-cleavage pathway for alkylbenzoate metabolism. We fused the N-terminal end of XylS to the maltose-binding protein (MBP) in vitro and found in glutaraldehyde cross-linking assays that the protein dimerized. Experiments with a chimeric N-terminal XylS linked to a 'LexA protein showed that the dimer was stabilized in the presence of alkylbenzoates. Sequence alignments with AraC and UreR allowed us to identify three residues, Leu193, Leu194, and Ile205, as potentially being involved in dimerization. Site-directed mutagenesis of XylS in which each of the above residues was replaced with Ala revealed that Leu193 and Leu194 were critical for activity and that a chimera in which LexA was linked to the N terminus of XylSLeu193Ala or XylSLeu194Ala was not functional. Dimerization of the chimeras MBP-N-XylSLeu193Ala and MBP-N-XylSLeu194Ala was not observed in cross-linking assays with glutaraldehyde.

AraC protein: a love-hate relationship

In the bacterium Escherichia coli, the AraC protein positively and negatively regulates expression of the proteins required for the uptake and catabolism of the sugar L-arabinose. This essay describes how work from my laboratory on this system spanning more than thirty years has aided our understanding of positive regulation, revealed DNA looping (a mechanism that explains many action-at-a-distance phenomena) and, more recently, has uncovered the mechanism by which arabinose shifts AraC from a state where it prefers to bind to two well-separated DNA half-sites and form a DNA loop to a state where it binds to two adjacent half-sites and activates transcription. This work required learning how to assay, purify, and work with a protein possessing highly uncooperative biochemical properties. Present work is focussed on understanding arabinose-responsive mechanism in atomic detail and is also directed towards understanding protein structure and function well enough to be able to engineer the allosteric mechanism seen in AraC onto other proteins.

Biophysical evidence of arm-domain interactions in AraC

We report development of a method for the direct measurement of the interaction between the N-terminal arm and the remainder of the dimerization domain in the Escherichia coli AraC protein, the regulator of the l-arabinose operon. The interaction was measured using surface plasmon resonance to monitor the association between the immobilized peptide arm and the dimerization domain, truncated of its arm, in solution. As expected from genetic and physiological data, the interaction is strongly stimulated by l-arabinose and is insensitive to sugars like d-glucose or d-galactose. Alterations in the sequence of the arm which physiological experiments predict either to strengthen or weaken the arm produce the expected responses.