Insights into signal transduction revealed by the low resolution structure of the FixJ response regulator

Identification of wysPII as an Activator of Morphological Development in Streptomyces albulus CK-15

Wuyiencin is produced by Streptomyces albulus var. wuyiensis and used widely in agriculture to control a variety of fungal diseases, such as cucumber downy mildew, strawberry powdery mildew, and tomato gray mold. As an industrially-produced biopesticide, reducing production costs is very important for popularization of this approach. To obtain a rapidly growing strain that effectively shortens the fermentation time, we investigated the effects of knockout and overexpression of the wysPII gene, a member of the LuxR regulatory gene family, in S. albulus strain CK-15. The ΔwysPII mutant exhibited a reduced rate of growth and sporulation. The time taken to reach the greatest mycelial biomass was approximately 18 h shorter in the ooPII (wysPII overexpressing) strain compared with that of the wild-type (WT) strain. In addition, the time to reach the greatest wuyiencin production was 56 h in the ooPII strain compared with 62 h in the WT strain. Furthermore, wysPII was shown to act as an activator of morphological development without affecting wuyiencin production. Thus, the ooPII strain can be used to reduce costs and increase efficiency in industrial fermentation processes for wuyiencin production.

Architecture of the complete oxygen-sensing FixL-FixJ two-component signal transduction system

The symbiotic nitrogen-fixing bacterium Bradyrhizobium japonicum is critical to the agro-industrial production of soybean because it enables the production of high yields of soybeans with little use of nitrogenous fertilizers. The FixL and FixJ two-component system (TCS) of this bacterium ensures that nitrogen fixation is only stimulated under conditions of low oxygen. When it is not bound to oxygen, the histidine kinase FixL undergoes autophosphorylation and transfers phosphate from adenosine triphosphate (ATP) to the response regulator FixJ, which, in turn, stimulates the expression of genes required for nitrogen fixation. We purified full-length B. japonicum FixL and FixJ proteins and defined their structures individually and in complex using small-angle x-ray scattering, crystallographic, and in silico modeling techniques. Comparison of active and inactive forms of FixL suggests that intramolecular signal transduction is driven by local changes in the sensor domain and in the coiled-coil region connecting the sensor and histidine kinase domains. We also found that FixJ exhibits conformational plasticity not only in the monomeric state but also in tetrameric complexes with FixL during phosphotransfer. This structural characterization of a complete TCS contributes both a mechanistic and evolutionary understanding to TCS signal relay, specifically in the context of the control of nitrogen fixation in root nodules.

Crystal structure of the response regulator spr1814 from Streptococcus pneumoniae reveals unique interdomain contacts among NarL family proteins

Spr1814 belongs to the NarL/FixJ subfamily of signal transduction response regulators (RR), and has been predicted to regulate the neighboring ABC transporter, which translocates antibiotic molecules in Streptococcus pneumoniae. Here, we report the crystal structure of full-length unphosphorylated spr1814 at 1.7Å resolution. The asymmetric unit contains two spr1814 molecules, which display very different conformations. Through comparisons with other RRs structures, we concluded that one molecule adopts a general inactive conformation, whereas the other molecule adopts an intermediate conformation. The superposition of each molecule showed that rotational change of the effector domain occurred in intermediate conformational state, implying that domain rearrangement could occur upon phosphorylation.

Fur-dependent MrkHI regulation of type 3 fimbriae in Klebsiella pneumoniae CG43

Type 3 fimbriae play a crucial role in Klebsiella pneumoniae biofilm formation, but the mechanism of the regulation of the type 3 fimbrial operon is largely unknown. In K. pneumoniae CG43, three regulatory genes, mrkH, mrkI and mrkJ, are located downstream of the type 3 fimbrial genes mrkABCDF. The production of the major pilin MrkA is abolished by the deletion of mrkH or mrkI but slightly increased by the deletion of mrkJ. Additionally, quantitative RT-PCR and a promoter-reporter assay of mrkHI verified that the transcription of mrkHI was activated by MrkI, suggesting autoactivation of mrkHI transcription. In addition, sequence analysis of the mrkH promoter region revealed a putative ferric uptake regulator (Fur) box. Deletion of fur decreased the transcription of mrkH, mrkI and mrkA. The expression of type 3 fimbriae and bacterial biofilm formation were also reduced by the deletion of fur. Moreover, a recombinant Fur was found to be able to bind both promoters, with higher affinity for P(mrkH) than P(mrkA), implying that Fur controls type 3 fimbriae expression via MrkHI. We also proved that iron availability can influence type 3 fimbriae activity.

Quorum sensing in Acinetobacter: an emerging pathogen

Acinetobacter is emerging as one of the major nosocomial infectious pathogens, facilitated by tolerance to desiccation and multidrug resistance. Quorum sensing (autoinducer-receptor mechanism) plays role in biofilm formation in Acinetobacter, though its role in regulation of other virulence factors is yet to be established. Phylogenetic studies indicate that Acinetobacter baumannii is closely related to Burkholderia ambifaria but its quorum sensing genes (abaI and abaR) were acquired horizontally from Halothiobacillus neapolitanus. The prospects of quorum quenching to control the infections caused by Acinetobacter have also been discussed.

Brucella melitensis VjbR and C12-HSL regulons: contributions of the N-dodecanoyl homoserine lactone signaling molecule and LuxR homologue VjbR to gene expression

Background

Quorum sensing is a communication system that regulates gene expression in response to population density and often regulates virulence determinants. Deletion of the luxR homologue vjbR highly attenuates intracellular survival of Brucella melitensis and has been interpreted to be an indication of a role for QS in Brucella infection. Confirmation for such a role was suggested, but not confirmed, by the demonstrated in vitro synthesis of an auto-inducer (AI) by Brucella cultures. In an effort to further delineate the role of VjbR to virulence and survival, gene expression under the control of VjbR and AI was characterized using microarray analysis.

Results

Analyses of wildtype B. melitensis and isogenic ΔvjbR transciptomes, grown in the presence and absence of exogenous N-dodecanoyl homoserine lactone (C₁₂-HSL), revealed a temporal pattern of gene regulation with variances detected at exponential and stationary growth phases. Comparison of VjbR and C₁₂-HSL transcriptomes indicated the shared regulation of 127 genes with all but 3 genes inversely regulated, suggesting that C₁₂-HSL functions via VjbR in this case to reverse gene expression at these loci. Additional analysis using a ΔvjbR mutant revealed that AHL also altered gene expression in the absence of VjbR, up-regulating expression of 48 genes and a luxR homologue blxR 93-fold at stationary growth phase. Gene expression alterations include previously un-described adhesins, proteases, antibiotic and toxin resistance genes, stress survival aids, transporters, membrane biogenesis genes, amino acid metabolism and transport, transcriptional regulators, energy production genes, and the previously reported fliF and virB operons.

Conclusions

VjbR and C₁₂-HSL regulate expression of a large and diverse number of genes. Many genes identified as virulence factors in other bacterial pathogens were found to be differently expressed, suggesting an important contribution to intracellular survival of Brucella. From these data, we conclude that VjbR and C₁₂-HSL contribute to virulence and survival by regulating expression of virulence mechanisms and thus controlling the ability of the bacteria to survive within the host cell. A likely scenario occurs via QS, however, operation of such a mechanism remains to be demonstrated.

The Frankia alni symbiotic transcriptome

The actinobacteria Frankia spp. are able to induce the formation of nodules on the roots of a large spectrum of actinorhizal plants, where they convert dinitrogen to ammonia in exchange for plant photosynthates. In the present study, transcriptional analyses were performed on nitrogen-replete free-living Frankia alni cells and on Alnus glutinosa nodule bacteria, using whole-genome microarrays. Distribution of nodule-induced genes on the genome was found to be mostly over regions with high synteny between three Frankia spp. genomes, while nodule-repressed genes, which were mostly hypothetical and not conserved, were spread around the genome. Genes known to be related to nitrogen fixation were highly induced, nif (nitrogenase), hup2 (hydrogenase uptake), suf (sulfur-iron cluster), and shc (hopanoids synthesis). The expression of genes involved in ammonium assimilation and transport was strongly modified, suggesting that bacteria ammonium assimilation was limited. Genes involved in particular in transcriptional regulation, signaling processes, protein drug export, protein secretion, lipopolysaccharide, and peptidoglycan biosynthesis that may play a role in symbiosis were also identified. We also showed that this Frankia symbiotic transcriptome was highly similar among phylogenetically distant plant families Betulaceae and Myricaceae. Finally, comparison with rhizobia transcriptome suggested that F. alni is metabolically more active in symbiosis than rhizobia.

Diversity and functional analysis of LuxR-type transcriptional regulators of cyclic lipopeptide biosynthesis in Pseudomonas fluorescens

Cyclic lipopeptides (CLPs) are produced by many Pseudomonas species and have several biological functions, including a role in surface motility, biofilm formation, virulence, and antimicrobial activity. This study focused on the diversity and role of LuxR-type transcriptional regulators in CLP biosynthesis in Pseudomonas species and, specifically, viscosin production by Pseudomonas fluorescens strain SBW25. Phylogenetic analyses showed that CLP biosynthesis genes in Pseudomonas strains are flanked by LuxR-type regulators that contain a DNA-binding helix-turn-helix domain but lack N-acylhomoserine lactone-binding or response regulator domains. For SBW25, site-directed mutagenesis of the genes coding for either of the two identified LuxR-type regulators, designated ViscAR and ViscBCR, strongly reduced transcript levels of the viscABC biosynthesis genes and resulted in a loss of viscosin production. Expression analyses further showed that a mutation in either viscAR or viscBCR did not substantially (change of <2.5-fold) affect transcription of the other regulator. Transformation of the DeltaviscAR mutant of SBW25 with a LuxR-type regulatory gene from P. fluorescens strain SS101 that produces massetolide, a CLP structurally related to viscosin, restored transcription of the viscABC genes and viscosin production. The results further showed that a functional viscAR gene was required for heterologous expression of the massetolide biosynthesis genes of strain SS101 in strain SBW25, leading to the production of both viscosin and massetolide. Collectively, these results indicate that the regulators flanking the CLP biosynthesis genes in Pseudomonas species represent a unique LuxR subfamily of proteins and that viscosin biosynthesis in P. fluorescens SBW25 is controlled by two LuxR-type transcriptional regulators.

Phosphorylation of A-Type ARR to function as negative regulator of cytokinin signal transduction

The plant hormone cytokinins regulate diverse aspects of plant growth and development. In Arabidopsis, a multi-step TCS system similar to bacterial and yeast TCS is used for cytokinin signaling. In a TCS system, a His sensor kinase perceives the signal by auto-phosphorylating on a His residue in response to an output signal, and the phosphate group is transferred to a conserved Asp residue in the receiver domain of the response regulator. The response regulator then modulates downstream signaling. Cytokinin multi-step TCS system utilizes an additional component, histidine-containing phosphotransfer domain protein (HPT) to transfer the phosphate group from a sensor kinase to a response regulator in the nucleus. The typical response regulators are classified into either type A or B. The type-B ARRs are transcription activators that act as positive regulators of cytokinin signaling, whereas most of the type-A ARRs are negative regulators of cytokinin signaling. Histidyl-aspartidyl phosphorelays are presumed to be essential for this cytokinin signal transduction in plants. Our studies have shown that ARR7, an A-type response regulator, negatively regulates cytokinin signaling in various aspects by acting as a transcriptional repressor and that the phosphorylation of ARR7 is required for these ARR7-regulated cytokinin-responses. Here I propose potential mechanisms by which the phosphorylation of ARRs is involved in regulating cytokinin- mediated gene expression, mainly based on biochemical and structural studies of bacterial response regulators. Protein-protein interaction and DNA-binding studies using the phosphorylated and the un-phosphorylated forms of the ARR proteins with their structural determination will provide molecular understanding of cytokinin-responsive gene regulation by ARRs.

A close-up view of the VraSR two-component system. A mediator of Staphylococcus aureus response to cell wall damage

Staphylococcus aureus remains a clinical scourge. Recent studies have revealed that S. aureus is capable of mounting a response to antibiotics that target cell wall peptidoglycan biosynthesis, such as beta-lactams and vancomycin. A phosphotransfer-mediated signaling pathway composed of a histidine protein kinase, VraS, and a response regulator protein, VraR, has been linked to the coordination of this response. Herein, we report for the first time on the signal transduction mechanism of the VraSR system. We found that VraS is capable of undergoing autophosphorylation in vitro and its phosphoryl group is rapidly transferred to VraR. In addition, phosphorylated VraR undergoes rapid dephosphorylation by VraS. Evidence is presented that VraR has adopted a novel strategy in regulating the output response of the VraSR-mediated signaling pathway. The VraR effector domain inhibits formation of inactive VraR dimers and, in doing so, it holds the regulatory domain into an intermediate active state. We show that only phosphorylation induces formation of the biological active VraR-dimer species. Furthermore, we propose that damage inflicted to cell wall peptidoglycan could be the main source of the stimuli that VraR responds to due to the tight control that VraS has on the phosphorylation state of VraR. Our findings provide for the first time insights into the molecular basis for the proposed role of VraSR as a "sentinel" system capable of rapidly sensing cell wall peptidoglycan damage and coordinating a response that enhances the resistance phenotype in S. aureus.

Crystal structures of the response regulator DosR from Mycobacterium tuberculosis suggest a helix rearrangement mechanism for phosphorylation activation

The response regulator DosR is essential for promoting long-term survival of Mycobacterium tuberculosis under low oxygen conditions in a dormant state and may be responsible for latent tuberculosis in one-third of the world's population. Here, we report crystal structures of full-length unphosphorylated DosR at 2.2 A resolution and its C-terminal DNA-binding domain at 1.7 A resolution. The full-length DosR structure reveals several features never seen before in other response regulators. The N-terminal domain of the full-length DosR structure has an unexpected (beta alpha)(4) topology instead of the canonical (beta alpha)(5) fold observed in other response regulators. The linker region adopts a unique conformation that contains two helices forming a four-helix bundle with two helices from another subunit, resulting in dimer formation. The C-terminal domain in the full-length DosR structure displays a novel location of helix alpha 10, which allows Gln199 to interact with the catalytic Asp54 residue of the N-terminal domain. In contrast, the structure of the DosR C-terminal domain alone displays a remarkable unstructured conformation for helix alpha 10 residues, different from the well-defined helical conformations in all other known structures, indicating considerable flexibility within the C-terminal domain. Our structures suggest a mode of DosR activation by phosphorylation via a helix rearrangement mechanism.

Phosphorylation of Arabidopsis response regulator 7 (ARR7) at the putative phospho-accepting site is required for ARR7 to act as a negative regulator of cytokinin signaling

Cytokinins are plant hormones that regulate diverse aspects of plant growth and development. Arabidopsis cytokinin signal transduction utilizes a multi-step two-component signaling (TCS) system by histidyl-aspartidyl phosphorelays. We here show that phosphorylation of ARR7, an A-type response regulator that acts as a negative regulator of cytokinin signaling, is required for its function in plants. Phosphorylation of ARR7 is inhibited in vitro by mutation in a putative phospho-accepting Asp residue into an Asn residue (ARR7(D85N)). While ectopic expression of ARR7 decreases root-growth inhibition, callus formation, and cytokinin-inducible gene expression, overexpression of ARR7 ( D85N ) at the similar level does not generate these phenotypes. ARR7(D85N) is localized to the nucleus and the half-life of this mutant protein is similar to that of ARR7 in Arabidopsis mesophyll protoplasts. These results suggest that the phosphorylation of ARR7 is necessary for ARR7-mediated cytokinin response.

Two-component systems of Mycobacterium tuberculosis: structure-based approaches

Mycobacterium tuberculosis contains few two-component systems compared to many other bacteria, possibly because it has more serine/threonine signaling pathways. Even so, these two-component systems appear to play an important role in early intracellular survival of the pathogen as well as in aspects of virulence. In this chapter, we discuss what has been learned about the mycobacterial two-component systems, with particular emphasis on knowledge gained from structural genomics projects.

Oxygen blocks the reaction of the FixL-FixJ complex with ATP but does not influence binding of FixJ or ATP to FixL

The RmFixL-RmFixJ oxygen signal transduction system ensures that a cascade of the Sinorhizobium meliloti nitrogen fixation genes is induced as the concentration of O2 drops below 50 microM in symbiotic nodules. Deoxy-RmFixL is a histidine protein kinase that catalyzes a phosphoryl transfer from ATP to the aspartate 54 residue of RmFixJ; RmFixJ is a response regulator that becomes activated as a transcription factor by phosphorylation. Association of O2 with a heme-binding domain in RmFixL triggers a conformational change that inhibits its kinase activity. Here we consider whether this inhibition is achieved by disrupting binding of either of the substrates, i.e., RmFixJ or ATP, to the RmFixL kinase. The ATP affinities of the oxy and deoxy states were compared via competition of ATP against TNP-nucleotide fluorophores. The influence of O2 on formation of the RmFixL-RmFixJ complex was investigated by fluorescence polarization. Oxygen dramatically inhibited the reaction of the RmFixL-RmFixJ complex with ATP but affected neither ATP binding (Kd approximately 100 microM) nor RmFixL-RmFixJ complex formation (Kd approximately 4 microM), indicating that inhibition of the kinase by the oxy-heme in RmFixL is achieved by inactivating the catalytic site, rather than by blocking the association of this enzyme with either of its substrates. An 8-fold enhancement of the rate of reaction of RmFixL with ATP in a deoxy-RmFixL-D54N RmFixJ complex, compared to that in isolated deoxy-RmFixL, exposes the strength of the allosteric effect of RmFixJ on the reaction. These results clarify the mechanistic roles of the signal and regulatory partner in this signal transduction system.

Solution structure of the C-terminal transcriptional activator domain of FixJ from Sinorhizobium meliloti and its recognition of the fixK promoter

FixJ is a response regulator of the two-component signal transduction pathway involved in the transcriptional activation of nitrogen fixation genes of Sinorhizobium meliloti. Upon phosphorylation, FixJ transcriptionally activates the fixK and nifA promoters. We identified a FixJ recognition sequence of 16 bp in the high affinity binding site of the fixK promoter by means of a gel shift assay. In addition, the solution structure of the truncated C-terminal DNA binding domain of FixJ (FixJC) was solved by NMR spectroscopy. FixJC contains five alpha-helices that encode a typical helix-turn-helix motif as a potential DNA binding core with the highest structural similarity toward the C-terminal DNA binding domain of NarL. The addition of the DNA fragment containing the recognition sequence of the high affinity FixJ binding site resulted in intermediate to slow exchange interactions on the NMR time scale in the spectrum of FixJC, while the exchange was rapid in the case of control DNA. These spectral data suggest that more than one molecule of FixJC binds to the recognition sequence, although FixJC alone is present in monomeric form in solution. This result is consistent with a scenario in which a transcriptionally active species of FixJ is a homodimer of the phosphorylated form.

An Active-like Structure in the Unphosphorylated StyR Response Regulator Suggests a Phosphorylation- Dependent Allosteric Activation Mechanism

StyR belongs to the FixJ subfamily of signal transduction response regulators; it controls transcription of the styABCD operon coding for styrene catabolism in Pseudomonas fluorescens ST. The crystal structure of unphosphorylated StyR is reported at 2.2 A resolution. StyR is composed of an N-terminal regulatory domain (StyR-N) and a C-terminal DNA binding domain (StyR-C). The two domains are separated by an elongated linker alpha helix (34 residues), a new feature in known response regulator structures. StyR-C is structured similarly to the DNA binding domain of the response regulator NarL. StyR-N shows structural reorganization of the phosphate receiving region involved in activation/homodimerization: specific residues adopt an "active-like" conformation, and the alpha4 helix, involved in dimerization of the homologous FixJ response regulator, is trimmed to just one helical turn. Overall, structural considerations suggest that phosphorylation may act as an allosteric switch, shifting a preexisting StyR equilibrium toward the active, dimeric, DNA binding form.

Analysis of the N-terminal DNA binding domain of the IS30 transposase

IS30 is the founding member of a large family of widely spread bacterial insertion sequences with closely related transposases. The N-terminal end of the IS30 transposase had been shown to retain sequence-specific DNA binding activity and to protect the IS30 terminal inverted repeats. Structural predictions revealed the presence of a helix-helix-turn-helix motif (H-HTH2) which, in the case of IS30, is preceded by an additional helix-turn-helix motif (HTH1). Analysis of deletion and point mutants in this region revealed that both motifs are important for IS30 transposition. IS30 exhibits two types of insertion specificity preferring either a 24 bp palindromic hot-spot (GOHS) or sequences resembling its ends [left and right terminal inverted repeat (IRL and IRR)]. Results are presented suggesting that the HTH1 region is required for GOHS targeting and interferes with the inverted repeat (IR) targeting. On the other hand, H-HTH2 appears to be required for both. The binding activities of the mutant proteins to the terminal IS30 IRs as measured by gel retardation correlated well with these results. Furthermore, close inspection of the H-HTH2 region revealed significant amino acid identity with a similar predicted secondary structure carried by the transcriptional regulator FixJ of Sinorhizobium meliloti and involved in FixJ binding to its target sequence. This suggests that FixJ and IS30 transposase share similar sequence-specific DNA binding mechanisms.

Data-driven docking for the study of biomolecular complexes

With the amount of genetic information available, a lot of attention has focused on systems biology, in particular biomolecular interactions. Considering the huge number of such interactions, and their often weak and transient nature, conventional experimental methods such as X-ray crystallography and NMR spectroscopy are not sufficient to gain structural insight into these. A wealth of biochemical and/or biophysical data can, however, readily be obtained for biomolecular complexes. Combining these data with docking (the process of modeling the 3D structure of a complex from its known constituents) should provide valuable structural information and complement the classical structural methods. In this review we discuss and illustrate the various sources of data that can be used to map interactions and their combination with docking methods to generate structural models of the complexes. Finally a perspective on the future of this kind of approach is given.

The uncoupling of oxygen sensing, phosphorylation signalling and transcriptional activation in oxygen sensor FixL and FixJ mutants

The rhizobial FixL/FixJ system, a member of the superfamily of bacterial two-component signal transducing systems, regulates the expression of nitrogen fixation-related genes by sensing environmental oxygen tension. Oxygen-free (deoxy) FixL is autophosphorylated at an invariant histidine residue with ATP, and the phosphoryl group is transferred to FixJ, leading to an enhancement in transcriptional activity at low oxygen tensions, but the histidine kinase activity of the oxygen-bound (oxy) form is inhibited. To investigate the mechanism of oxygen sensing, we established a FixL/FixJ-mediated PfixK-lacZ reporter system in Escherichia coli, and isolated FixL and FixJ mutations conferring an upregulation of lacZ gene expression on the reporter cells even under aerobic conditions. FixL mutant proteins, which contain single amino acid changes near the autophosphorylation site, showed elevated levels of autophosphorylation and a concomitant phosphoryl transfer to FixJ in the presence of oxygen, although their oxygen-binding affinities were unimpaired. These mutational analyses suggest that the autophosphorylation domain plays a crucial role in regulatory coupling between oxygen binding and kinase activity. FixJ mutants in helix alpha1 and strand beta5 of the N-terminal half exhibited the formation of a stable acyl phosphate bond. In contrast, those in helices alpha4 and alpha5 constitutively bound to the fixK promoter in a monomeric form, suggesting that the alpha4 and alpha5 helices may be involved in the post-phosphorylation/dimerization signal transfer to liberate the DNA-binding activity of the C-terminal domain, not only serving as a dimerization interface.