Structural basis of transcriptional regulation by UrtR in response to uric acid

Uric acid (UA)-responsive transcriptional regulators (UrtRs), which belong to the multiple antibiotic resistance regulator (MarR) superfamily, transcriptionally coordinate virulence and metabolism in bacteria by modulating interactions with operator DNA in response to UA. To elucidate the transcriptional regulatory mechanism of UrtR, we structurally analyzed UrtR proteins, including PecS, MftR, and HucR, alone and in complex with UA or DNA. UrtR contains a dimerization domain (DD) and a winged helix-turn-helix domain (wHTHD) and forms a homodimer primarily via the DD, as observed for other MarR superfamily proteins. However, UrtRs are characterized by a unique N-terminal α-helix, which contributes to dimerization and UA recognition. In the absence of UA, the UrtR dimer symmetrically binds to the operator double-stranded DNA (dsDNA) by inserting its α4 recognition helix and β-stranded wing within the wHTHD into the major and minor grooves of dsDNA, respectively. Upon exposure to UA, UrtR accommodates UA in the intersubunit pocket between the DD and wHTHD. UA binding induces a conformational change in the major groove-binding core element of the UrtR wHTHD, generating a DNA binding-incompatible structure. This local allosteric mechanism of UrtR completely differs from that generally observed in other MarR superfamily members, in which the entire wHTHD undergoes effector-responsive global shifts.

A dynamics association study of gut barrier and microbiota in hyperuricemia

Introduction

The intricate interplay between gut microbiota and hyperuricemia remains a subject of growing interest. However, existing studies only provided snapshots of the gut microbiome at single time points, the temporal dynamics of gut microbiota alterations during hyperuricemia progression and the intricate interplay between the gut barrier and microbiota remain underexplored. Our investigation revealed compelling insights into the dynamic changes in both gut microbiota and intestinal barrier function throughout the course of hyperuricemia.

Methods

The hyperuricemia mice (HY) were given intragastric administration of adenine and potassium oxalate. Gut microbiota was analyzed by 16S rRNA sequencing at 3, 7, 14, and 21 days after the start of the modeling process. Intestinal permeability as well as LPS, TNF-α, and IL-1β levels were measured at 3, 7, 14, and 21 days.

Results

We discovered that shifts in microbial community composition occur prior to the onset of hyperuricemia, key bacterial Bacteroidaceae, Bacteroides, and Blautia exhibited reduced levels, potentially fueling microbial dysbiosis as the disease progresses. During the course of hyperuricemia, the dynamic fluctuations in both uric acid levels and intestinal barrier function was accompanied with the depletion of key beneficial bacteria, including Prevotellaceae, Muribaculum, Parabacteroides, Akkermansia, and Bacteroides, and coincided with an increase in pathogenic bacteria such as Oscillibacter and Ruminiclostridium. This microbial community shift likely contributed to elevated lipopolysaccharide (LPS) and pro-inflammatory cytokine levels, ultimately promoting metabolic inflammation. The decline of Burkholderiaceae and Parasutterella was inversely related to uric acid levels, Conversely, key families Ruminococcaceae, Family_XIII, genera Anaeroplasma exhibited positive correlations with uric acid levels. Akkermansiaceae and Bacteroidaceae demonstrating negative correlations, while LPS-containing microbiota such as Desulfovibrio and Enterorhabdus exhibited positive correlations with intestinal permeability.

Conclusion

In summary, this study offers a dynamic perspective on the complex interplay between gut microbiota, uric acid levels, and intestinal barrier function during hyperuricemia progression. Our study suggested that Ruminiclostridium, Bacteroides, Akkermansiaceae, Bilophila, Burkholderiaceae and Parasutterella were the key bacteria that play vital rols in the progress of hyperuricemia and compromised intestinal barrier, which provide a potential avenue for therapeutic interventions in hyperuricemia.

The Global Regulator MftR Controls Virulence and Siderophore Production in Burkholderia thailandensis

Bacterial pathogens face iron limitation in a host environment. To overcome this challenge, they produce siderophores, small iron-chelating molecules.

Biosynthesis of the redox cofactor mycofactocin is controlled by the transcriptional regulator MftR and induced by long-chain acyl-CoA species

Mycofactocin (MFT) is a ribosomally synthesized and post-translationally-modified redox cofactor found in pathogenic mycobacteria. While MFT biosynthetic proteins have been extensively characterized, the physiological conditions under which MFT biosynthesis is required are not well understood. To gain insights into the mechanisms of regulation of MFT expression in Mycobacterium smegmatis mc²155, we investigated the DNA-binding and ligand-binding activities of the putative TetR-like transcription regulator, MftR. In this study, we demonstrated that MftR binds to the mft promoter region. We used DNase I footprinting to identify the 27 bp palindromic operator located 5′ to mftA and found it to be highly conserved in Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium ulcerans, and Mycobacterium marinum. To determine under which conditions the mft biosynthetic gene cluster (BGC) is induced, we screened for effectors of MftR. As a result, we found that MftR binds to long-chain acyl-CoAs with low micromolar affinities. To demonstrate that oleoyl-CoA induces the mft BGC in vivo, we re-engineered a fluorescent protein reporter system to express an MftA–mCherry fusion protein. Using this mCherry fluorescent readout, we show that the mft BGC is upregulated in M. smegmatis mc²155 when oleic acid is supplemented to the media. These results suggest that MftR controls expression of the mft BGC and that MFT production is induced by long-chain acyl-CoAs. Since MFT-dependent dehydrogenases are known to colocalize with acyl carrier protein/CoA-modifying enzymes, these results suggest that MFT might be critical for fatty acid metabolism or cell wall reorganization.

Impaired purine homeostasis plays a primary role in trimethoprim-mediated induction of virulence genes in Burkholderia thailandensis

One of the most commonly prescribed antibiotics against Burkholderia infections is co-trimoxazole, a cocktail of trimethoprim and sulfamethoxazole. Trimethoprim elicits an upregulation of the mal gene cluster, which encodes proteins involved in synthesis of the cytotoxic polyketide malleilactone; trimethoprim does so by increasing expression of the malR gene, which encodes the activator MalR. We report that B. thailandensis grown on trimethoprim exhibited increased virulence against Caenorhabditis elegans. This enhanced virulence correlated with an increase in expression of the mal gene cluster. Notably, inhibition of xanthine dehydrogenase by addition of allopurinol led to similar upregulation of malA and malR, with addition of trimethoprim or allopurinol also resulting in an equivalent intracellular accumulation of xanthine. Xanthine is a ligand for the transcription factor MftR that leads to attenuated DNA binding, and we show using chromatin immunoprecipitation that MftR binds directly to malR. Our gene expression data suggest that malR expression is repressed by both MftR and by a separate transcription factor, which also responds to a metabolite that accumulates on exposure to trimethoprim. Since allopurinol elicits a similar increase in malR/malA expression as trimethoprim, we suggest that impaired purine homeostasis plays a primary role in trimethoprim-mediated induction of malR and in turn malA.

Secondary metabolites from the Burkholderia pseudomallei complex: structure, ecology, and evolution

Bacterial secondary metabolites play important roles in promoting survival, though few have been carefully studied in their natural context. Numerous gene clusters code for secondary metabolites in the genomes of members of the Bptm group, made up of three closely related species with distinctly different lifestyles: the opportunistic pathogen Burkholderia pseudomallei, the non-pathogenic saprophyte Burkholderia thailandensis, and the host-adapted pathogen Burkholderia mallei. Several biosynthetic gene clusters are conserved across two or all three species, and this provides an opportunity to understand how the corresponding secondary metabolites contribute to survival in different contexts in nature. In this review, we discuss three secondary metabolites from the Bptm group: bactobolin, malleilactone (and malleicyprol), and the 4-hydroxy-3-methyl-2-alkylquinolines, providing an overview of each of their biosynthetic pathways and insight into their potential ecological roles. Results of studies on these secondary metabolites provide a window into how secondary metabolites contribute to bacterial survival in different environments, from host infections to polymicrobial soil communities.

Do Global Regulators Hold the Key to Production of Bacterial Secondary Metabolites?

The emergence of multiple antibiotic resistant bacteria has pushed the available pool of antibiotics to the brink. Bacterial secondary metabolites have long been a valuable resource in the development of antibiotics, and the genus Burkholderia has recently emerged as a source of novel compounds with antibacterial, antifungal, and anti-cancer activities. Genome mining has contributed to the identification of biosynthetic gene clusters, which encode enzymes that are responsible for synthesis of such secondary metabolites. Unfortunately, these large gene clusters generally remain silent or cryptic under normal laboratory settings, which creates a hurdle in identification and isolation of these compounds. Various strategies, such as changes in growth conditions and antibiotic stress, have been applied to elicit the expression of these cryptic gene clusters. Although a number of compounds have been isolated from different Burkholderia species, the mechanisms by which the corresponding gene clusters are regulated remain poorly understood. This review summarizes the activity of well characterized secondary metabolites from Burkholderia species and the role of local regulators in their synthesis, and it highlights recent evidence for the role of global regulators in controlling production of secondary metabolites. We suggest that targeting global regulators holds great promise for the awakening of cryptic gene clusters and for developing better strategies for discovery of novel antibiotics.

Biology of MarR family transcription factors and implications for targets of antibiotics against tuberculosis

The emergence of multidrug resistant (MDR) Mycobacterium tuberculosis strains and increased incidence of HIV coinfection fueled the difficulty in controlling tuberculosis (TB). MarR (multiple antibiotic resistance regulator) family transcription factors can regulate marRAB operon and are involved in resistance to multiple environmental stresses. We have summarized the structure, function, distribution, and regulation of the MarR family proteins, as well as their implications for novel targets for antibiotics, especially for tuberculosis.

MarR Family Transcription Factors from Burkholderia Species: Hidden Clues to Control of Virulence-Associated Genes

Species within the genus Burkholderia exhibit remarkable phenotypic diversity. Genomic plasticity, including genome reduction and horizontal gene transfer, has been correlated with virulence traits in several species. However, the conservation of virulence genes in species otherwise considered to have limited potential for infection suggests that phenotypic diversity may not be explained solely on the basis of genetic diversity. Instead, differential organization and control of gene regulatory networks may underlie many phenotypic differences. In this review, we evaluate how regulation of gene expression by members of the multiple antibiotic resistance regulator (MarR) family of transcription factors may contribute to shaping the physiological diversity of Burkholderia species, with a focus on the clinically relevant human pathogens. All Burkholderia species encode a relatively large number of MarR proteins, a feature common to bacteria that must respond to environmental changes such as those associated with host invasion. However, evolution of gene regulatory networks has likely resulted in orthologous transcription factors controlling disparate sets of genes. Adaptation to, and survival in, diverse habitats, including a human or plant host, is key to the success of Burkholderia species as (opportunistic) pathogens, and recent reports suggest that control of virulence-associated genes by MarR proteins features prominently among the survival strategies employed by these species. We suggest that identification of MarR regulons will contribute significantly to clarification of virulence determinants and phenotypic diversity.

A MarR family transcriptional regulator and subinhibitory antibiotics regulate type VI secretion gene clusters in Burkholderia pseudomallei

Burkholderia pseudomallei, the aetiological agent of melioidosis, is an inhabitant of soil and water in many tropical and subtropical regions worldwide. It possesses six distinct type VI secretion systems (T6SS-1 to T6SS-6), but little is known about most of them, as they are poorly expressed in laboratory culture media. A genetic screen was devised to locate a putative repressor of the T6SS-2 gene cluster and a MarR family transcriptional regulator, termed TctR, was identified. The inactivation of tctR resulted in a 50-fold increase in the expression of an hcp2-lacZ transcriptional fusion, indicating that TctR is a negative regulator of the T6SS-2 gene cluster. Surprisingly, the tctR mutation resulted in a significant decrease in the expression of an hcp6-lacZ transcriptional fusion. B. pseudomallei K96243 and a tctR mutant were grown to logarithmic phase in rich culture medium and RNA was isolated and sequenced in order to identify other genes regulated by TctR. The results identified seven gene clusters that were repressed by TctR, including T6SS-2, and three gene clusters that were significantly activated. A small molecule library consisting of 1120 structurally defined compounds was screened to identify a putative ligand (or ligands) that might bind TctR and derepress transcription of the T6SS-2 gene cluster. Seven compounds, six fluoroquinolones and one quinolone, activated the expression of hcp2-lacZ. Subinhibitory ciprofloxacin also increased the expression of the T6SS-3, T6SS-4 and T6SS-6 gene clusters. This study highlights the complex layers of regulatory control that B. pseudomallei utilizes to ensure that T6SS expression only occurs under very defined environmental conditions.

Global Awakening of Cryptic Biosynthetic Gene Clusters in Burkholderia thailandensis

Many bacteria encode biosynthetic proteins that produce a vast array of natural products. These compounds are often synthesized during host invasion as they function as virulence factors. In addition, such secondary metabolites have yielded numerous molecular scaffolds with pharmaceutical and clinical importance. The gene clusters that encode proteins responsible for synthesis of these compounds are typically silenced or "cryptic" under laboratory growth conditions, hampering discovery of novel lead compounds. We report here that MftR is a global repressor of secondary metabolite synthesis in Burkholderia thailandensis and that urate functions as a physiologically relevant inducer of gene expression. Biosynthetic gene clusters under MftR control include those associated with production of the antimicrobial bactobolins, the iron siderophore malleobactin, and the virulence factor malleilactone. MftR also controls additional genes associated with survival in a host environment, such as genes encoding components of the type III secretion system (T3SS) and proteins linked to anaerobic respiration. This observation not only has implications for understanding activation of gene regulatory networks during host invasion, but it also paves the way for isolation of novel therapeutic leads.

MarR family transcription factors: dynamic variations on a common scaffold

Members of the multiple antibiotic resistance regulator (MarR) family of transcription factors are critical for bacterial cells to respond to chemical signals and to convert such signals into changes in gene activity. Obligate dimers belonging to the winged helix-turn-helix protein family, they are critical for regulation of a variety of functions, including degradation of organic compounds and control of virulence gene expression. The conventional regulatory paradigm is based on a genomic locus in which the gene encoding the MarR protein is divergently oriented from a gene under its control; MarR binding to the intergenic region controls expression of both genes by changing the interaction of RNA polymerase with gene promoters. MarR protein oxidation or binding of a small molecule ligand adversely affects DNA binding, resulting in altered expression of the divergent genes. The generality of this simple paradigm, including the regulation of Escherichia coli MarR by direct binding of antibiotics, has been challenged by reports published in recent years. In addition, structural and biochemical analyses of ligand binding to numerous MarR homologs are converging to identify a shared ligand-binding "hot-spot". This review highlights recent research advances that point to shared features, yet at the same time highlights the remarkable flexibility with which members of this protein family implement responses to inducing signals. A more comprehensive understanding of protein function will pave the way towards the development of both antibacterial agents and biosensors that are based on MarR family proteins.

Efflux pump-mediated drug resistance in Burkholderia

Several members of the genus Burkholderia are prominent pathogens. Infections caused by these bacteria are difficult to treat because of significant antibiotic resistance. Virtually all Burkholderia species are also resistant to polymyxin, prohibiting use of drugs like colistin that are available for treatment of infections caused by most other drug resistant Gram-negative bacteria. Despite clinical significance and antibiotic resistance of Burkholderia species, characterization of efflux pumps lags behind other non-enteric Gram-negative pathogens such as Acinetobacter baumannii and Pseudomonas aeruginosa. Although efflux pumps have been described in several Burkholderia species, they have been best studied in Burkholderia cenocepacia and B. pseudomallei. As in other non-enteric Gram-negatives, efflux pumps of the resistance nodulation cell division (RND) family are the clinically most significant efflux systems in these two species. Several efflux pumps were described in B. cenocepacia, which when expressed confer resistance to clinically significant antibiotics, including aminoglycosides, chloramphenicol, fluoroquinolones, and tetracyclines. Three RND pumps have been characterized in B. pseudomallei, two of which confer either intrinsic or acquired resistance to aminoglycosides, macrolides, chloramphenicol, fluoroquinolones, tetracyclines, trimethoprim, and in some instances trimethoprim+sulfamethoxazole. Several strains of the host-adapted B. mallei, a clone of B. pseudomallei, lack AmrAB-OprA, and are therefore aminoglycoside and macrolide susceptible. B. thailandensis is closely related to B. pseudomallei, but non-pathogenic to humans. Its pump repertoire and ensuing drug resistance profile parallels that of B. pseudomallei. An efflux pump in B. vietnamiensis plays a significant role in acquired aminoglycoside resistance. Summarily, efflux pumps are significant players in Burkholderia drug resistance.

Ligand-binding pocket bridges DNA-binding and dimerization domains of the urate-responsive MarR homologue MftR from Burkholderia thailandensis

Members of the multiple antibiotic resistance regulator (MarR) family often regulate gene activity by responding to a specific ligand. In the absence of ligand, most MarR proteins function as repressors, while ligand binding causes attenuated DNA binding and therefore increased gene expression. Previously, we have shown that urate is a ligand for MftR (major facilitator transport regulator), which is encoded by the soil bacterium Burkholderia thailandensis. We show here that both mftR and the divergently oriented gene mftP encoding a major facilitator transport protein are upregulated in the presence of urate. MftR binds two cognate sites in the mftR-mftP intergenic region with equivalent affinity and sensitivity to urate. Mutagenesis of four conserved residues previously reported to be involved in urate binding to Deinococcus radiodurans HucR and Rhizobium radiobacter PecS significantly reduced protein stability and DNA binding affinity but not ligand binding. These data suggest that residues equivalent to those implicated in ligand binding to HucR and PecS serve structural roles and that MftR relies on distinct residues for ligand binding. MftR exhibits a two-step melting transition suggesting independent unfolding of the dimerization and DNA-binding regions; urate binding or mutations in the predicted ligand-binding sites result in one-step unfolding transitions. We suggest that MftR binds the ligand in a cleft between the DNA-binding lobes and the dimer interface but that the mechanism of ligand-mediated attenuation of DNA binding differs from that proposed for other urate-responsive MarR homologues. Since DNA binding by MftR is attenuated at 37 °C, our data also suggest that MftR responds to both ligand and a thermal upshift by attenuated DNA binding and upregulation of the genes under its control.

Streptomyces coelicolor encodes a urate-responsive transcriptional regulator with homology to PecS from plant pathogens

Many transcriptional regulators control gene activity by responding to specific ligands. Members of the multiple-antibiotic resistance regulator (MarR) family of transcriptional regulators feature prominently in this regard, and they frequently function as repressors in the absence of their cognate ligands. Plant pathogens such as Dickeya dadantii encode a MarR homolog named PecS that controls expression of a gene encoding the efflux pump PecM in addition to other virulence genes. We report here that the soil bacterium Streptomyces coelicolor also encodes a PecS homolog (SCO2647) that regulates a pecM gene (SCO2646). S. coelicolor PecS, which exists as a homodimer, binds the intergenic region between pecS and pecM genes with high affinity. Several potential PecS binding sites were found in this intergenic region. The binding of PecS to its target DNA can be efficiently attenuated by the ligand urate, which also quenches the intrinsic fluorescence of PecS, indicating a direct interaction between urate and PecS. In vivo measurement of gene expression showed that activity of pecS and pecM genes is significantly elevated after exposure of S. coelicolor cultures to urate. These results indicate that S. coelicolor PecS responds to the ligand urate by attenuated DNA binding in vitro and upregulation of gene activity in vivo. Since production of urate is associated with generation of reactive oxygen species by xanthine dehydrogenase, we propose that PecS functions under conditions of oxidative stress.

The transcriptional regulator TamR from Streptomyces coelicolor controls a key step in central metabolism during oxidative stress

Multiple antibiotic resistance regulator (MarR) family transcriptional regulators usually regulate gene activity by responding to specific ligands. Here we show that TamR (trans-aconitate methyltransferase regulator), a MarR homologue from Streptomyces coelicolor, functions in oxidative stress responses to regulate a key step in central metabolism. The gene encoding TamR is oriented divergently from the tam gene, which encodes trans-aconitate methyltransferase. Trans-aconitate methyltransferase methylates trans-aconitate, which is formed when cis-aconitate is released during aconitase-mediated isomerization of citrate to isocitrate; trans-aconitate, but not its methyl ester, is a potent inhibitor of aconitase. We show that TamR binds with high affinity to the intergenic region between the tamR and tam genes. Notably, trans-aconitate attenuates DNA-binding by TamR, as do citrate, cis-aconitate and isocitrate, which are the substrate, intermediate and product of aconitase respectively. In vivo, hydrogen peroxide and citrate induce significant upregulation of the tam (SCO3132), tamR (SCO3133) and aconitase (SCO5999) genes. Since oxidative stress leads to disassembly of the [4Fe-4S] cluster that is essential for aconitase activity, resulting in accumulation of citrate and release of cis-aconitate and its subsequent conversion to trans-aconitate, we propose that TamR mediates a novel regulatory function in which the inhibitory effects of trans-aconitate and accumulated citrate are alleviated.

MarR homologs with urate-binding signature

Bacteria that associate with living hosts require intricate mechanisms to detect and respond to host defenses. Part of the early host defense against invading bacteria is the production of reactive oxygen species, and xanthine oxidase is one of the main producers of such agents. The end-product of the enzymatic activity of xanthine oxidase, urate, was previously shown to be the natural ligand for Deinococcus radiodurans-encoded HucR and it was shown to attenuate DNA binding by Agrobacterium tumefaciens-encoded PecS and Burkholderia thailandensis-encoded MftR, all members of the multiple antibiotic resistance regulator (MarR) family. We here show that residues involved in binding urate and eliciting the DNA binding antagonism are conserved in a specific subset of MarR homologs. Although HucR controls endogenous urate levels by regulating a uricase gene, almost all other homologs are predicted to respond to exogenous urate levels and to regulate a transmembrane transport protein belonging to either the drug metabolite transporter (DMT) or the major facilitator superfamily (MFS), as further evidenced by the presence of conserved binding sites for the cognate transcription factor within the respective promoter regions. These data suggest the use of orthologous genes for different regulatory purposes. We propose the designation UrtR (urate responsive transcriptional regulator) for this distinct subfamily of MarR homologs based on their common mechanism of urate-mediated attenuation of DNA binding.