Leucine-regulated self-association of leucine-responsive regulatory protein (Lrp) from Escherichia coli

Shigella Senses the Environmental Cue Leucine to Promote its Virulence Gene Expression in the Colon

Shigella is a foodborne enteropathogenic bacteria that causes severe bacillary dysentery in humans. Shigella primarily colonizes the human colon and causes disease via invasion of colon epithelial cells. However, the signal regulatory mechanisms associated with its colonization and pathogenesis in the colon remain poorly defined. Here, we report a leucine-mediated regulatory mechanism that promotes Shigella virulence gene expression and invasion of colon epithelial cells. Shigella in response to leucine, which is highly abundant in the colon, via the leucine-responsive regulator Lrp and the binding of Lrp with leucine induces the expression of a newly identified small RNA SsrV. SsrV then activates the expression of virF and downstream invasion-related virulence genes by increasing the protein level of the LysR-type transcription regulator LrhA, therefore enabling Shigella invasion of colon epithelial cells. Shigella lacking ssrV displays impaired invasion ability. Collectively, these findings suggest that Shigella employs a leucine-responsive environmental activation mechanism to establish colonization and pathogenicity.

The Xenorhabdus nematophila LrhA transcriptional regulator modulates production of γ-keto-N-acyl amides with inhibitory activity against mutualistic host nematode egg hatching

Xenorhabdus nematophila is a symbiotic Gammaproteobacterium that produces diverse natural products that facilitate mutualistic and pathogenic interactions in their nematode and insect hosts, respectively. The interplay between X. nematophila secondary metabolism and symbiosis stage is tuned by various global regulators. An example of such a regulator is the LysR-type protein transcription factor LrhA, which regulates amino acid metabolism and is necessary for virulence in insects and normal nematode progeny production. Here, we utilized comparative metabolomics and molecular networking to identify small molecule factors regulated by LrhA and characterized a rare γ-ketoacid (GKA) and two new N-acyl amides, GKA-Arg (1) and GKA-Pro (2) which harbor a γ-keto acyl appendage. A lrhA null mutant produced elevated levels of compound 1 and reduced levels of compound 2 relative to wild type. N-acyl amides 1 and 2 were shown to be selective agonists for the human G-protein-coupled receptors (GPCRs) C3AR1 and CHRM2, respectively. The CHRM2 agonist 2 deleteriously affected the hatch rate and length of Steinernema nematodes. This work further highlights the utility of exploiting regulators of host-bacteria interactions for the identification of the bioactive small molecule signals that they control.

IMPORTANCE

Xenorhabdus bacteria are of interest due to their symbiotic relationship with Steinernema nematodes and their ability to produce a variety of natural bioactive compounds. Despite their importance, the regulatory hierarchy connecting specific natural products and their regulators is poorly understood. In this study, comparative metabolomic profiling was utilized to identify the secondary metabolites modulated by the X. nematophila global regulator LrhA. This analysis led to the discovery of three metabolites, including an N-acyl amide that inhibited the egg hatching rate and length of Steinernema carpocapsae nematodes. These findings support the notion that X. nematophila LrhA influences the symbiosis between X. nematophila and S. carpocapsae through N-acyl amide signaling. A deeper understanding of the regulatory hierarchy of these natural products could contribute to a better comprehension of the symbiotic relationship between X. nematophila and S. carpocapsae.

Exploring the molecular mechanisms of Lrp/AsnC-type transcription regulator DecR, an L-cysteine-responsive feast/famine regulatory protein

The Lrp/AsnC family of transcriptional regulators is commonly found in prokaryotes and is associated with the regulation of amino acid metabolism. However, it remains unclear how the L-cysteine-responsive Lrp/AsnC family regulator perceives and responds to L-cysteine. Here, we try to elucidate the molecular mechanism of the L-cysteine-responsive transcriptional regulator. Through 5'RACE and EMSA, we discovered a 15 bp incompletely complementary pair palindromic sequence essential for DecR binding, which differed slightly from the binding sequence of other Lrp/AsnC transcription regulators. Using alanine scanning, we identified the L-cysteine binding site on DecR and found that different Lrp/AsnC regulators adjust their binding pocket's side-chain residues to accommodate their specific effector. MD simulations were then conducted to explore how ligand binding influences the allosteric behavior of the protein. PCA and in silico docking revealed that ligand binding induced perturbations in the linker region, triggering conformational alterations and leading to the relocalization of the DNA-binding domains, enabling the embedding of the DNA-binding region of DecR into the DNA molecule, thereby enhancing DNA-binding affinity. Our findings can broaden the understanding of the recognition and regulatory mechanisms of the Lrp/AsnC-type transcription factors, providing a theoretical basis for further investigating the molecular mechanisms of other transcription factors.

Mycobacterium tuberculosis Rv2324 is a multifunctional feast/famine regulatory protein involved in growth, DNA replication and damage control

Feast/famine regulatory proteins (FFRPs) are multifunctional regulators. We show that Mtb Rv2324 is important for growth, survival, and countering DNA damage in Mycobacterium tuberculosis (Mtb). DNA-relaxation activity against linear and supercoiled substrates suggest its involvement in transcription activation, while its high affinity for recombination, replication and repair substrates suggest a role there too. Small-Angle-X-ray scattering supports the adoption of an 'open' quaternary association in response to amino-acid binding. Size-exclusion-chromatography and glutaraldehyde cross-linking identify the adoption of diverse oligomers modulated by amino-acid binding, and DNA interactions. We tested G52A, G101T and D104A mutants which correspond to highly conserved residues, distal to the DNA-binding site, and are important for amino acids binding. G101T exhibits increased DNA affinity, while G52A and D104A exhibit weak DNA-binding thereby suggesting that they mediate effector-binding, and DNA binding activities. Gain and loss-of-function studies show that Rv2324 overexpression promotes growth-rate, while its knock-down leads to retarded growth. Rv2324 down-regulation lowers Mtb survival inside resting and IFN-ϒ-activated macrophages. Rv2324 protects the pathogen from DNA damage, as evidenced by the reduction in the knockdown strain's survival following treatment with H2O2 and UV light. Overall, we show that Rv2324 plays a crucial role in regulating survival and growth of Mtb.

Escherichia coli Leucine-Responsive Regulatory Protein Bridges DNA In Vivo and Tunably Dissociates in the Presence of Exogenous Leucine

Feast-famine response proteins are a widely conserved class of global regulators in prokaryotes, the most highly studied of which is the Escherichia coli leucine-responsive regulatory protein (Lrp). Lrp senses the environmental nutrition status and subsequently regulates up to one-third of the genes in E. coli, either directly or indirectly. Lrp exists predominantly as octamers and hexadecamers (16mers), where leucine is believed to shift the equilibrium toward the octameric state. In this study, we analyzed the effects of three oligomerization state mutants of Lrp in terms of their ability to bind to DNA and regulate gene expression in response to exogenous leucine. We find that oligomerization beyond dimers is required for Lrp's regulatory activity and that, contrary to previous speculation, exogenous leucine modulates Lrp activity at its target promoters exclusively by inhibiting Lrp binding to DNA. We also show evidence that Lrp binding bridges DNA over length scales of multiple kilobases, revealing a new range of mechanisms for Lrp-mediated transcriptional regulation. IMPORTANCE Leucine-responsive regulatory protein (Lrp) is one of the most impactful regulators in E. coli and other bacteria. Lrp senses nutrient conditions and responds by controlling strategies for virulence, cellular motility, and nutrient acquisition. Despite its importance and being evolutionarily highly conserved across bacteria and archaea, several mysteries remain regarding Lrp, including how it actually responds to leucine to change its regulation of targets. Previous studies have led to the hypothesis that Lrp switches between two states, an octamer (8 Lrp molecules together) and a hexadecamer (16 Lrp molecules together), upon exposure to leucine; these are referred to as different oligomerization states. Here, we show that contrary to previous expectations, it is Lrp's propensity to bind DNA, rather than its oligomerization state, that is directly affected by leucine in the cell's environment. Our new understanding of Lrp activity will aid in identifying and disrupting pathways used by bacteria to cause disease.

Leucine-Responsive Regulatory Protein in Acetic Acid Bacteria Is Stable and Functions at a Wide Range of Intracellular pH Levels

Acetic acid bacteria grow while producing acetic acid, resulting in acidification of the culture. Limited reports elucidate the effect of changes in intracellular pH on transcriptional factors. In the present study, the intracellular pH of Komagataeibacter europaeus was monitored with a pH-sensitive green fluorescent protein, showing that the intracellular pH decreased from 6.3 to 4.7 accompanied by acetic acid production during cell growth. The leucine-responsive regulatory protein of K. europaeus (KeLrp) was used as a model to examine pH-dependent effects, and its properties were compared with those of the Escherichia coli ortholog (EcLrp) at different pH levels. The DNA-binding activities of EcLrp and KeLrp with the target DNA (Ec-ilvI and Ke-ilvI) were examined by gel mobility shift assays under various pH conditions. EcLrp showed the highest affinity with the target at pH 8.0 (K_d [dissociation constant], 0.7 μM), decreasing to a minimum of 3.4 μM at pH 4.0. Conversely, KeLrp did not show significant differences in binding affinity between pH 4 and 7 (K_d, 1.0 to 1.5 μM), and the highest affinity was at pH 5.0 (K_d, 1.0 μM). Circular dichroism spectroscopy revealed that the α-helical content of KeLrp was the highest at pH 5.0 (49%) and was almost unchanged while being maintained at >45% over a range of pH levels examined, while that of EcLrp decreased from its maximum (49% at pH 7.0) to its minimum (36% at pH 4.0). These data indicate that KeLrp is stable and functions over a wide range of intracellular pH levels. IMPORTANCE Lrp is a highly conserved transcriptional regulator found in bacteria and archaea and regulates transcriptions of various genes. The intracellular pH of acetic acid bacteria (AAB) changes accompanied by acetic acid production during cell growth. The Lrp of AAB K. europaeus (KeLrp) was structurally stable over a wide range of pH and maintained DNA-binding activity even at low pH compared with Lrp from E. coli living in a neutral environment. An in vitro experiment showed DNA-binding activity of KeLrp to the target varied with changes in pH. In AAB, change of the intracellular pH during a cell growth would be an important trigger in controlling the activity of Lrp in vivo.

The leucine-responsive regulatory proteins/feast-famine regulatory proteins: an ancient and complex class of transcriptional regulators in bacteria and archaea

Since the discovery of the Escherichia coli leucine-responsive regulatory protein (Lrp) almost 50 years ago, hundreds of Lrp homologs have been discovered, occurring in 45% of sequenced bacteria and almost all sequenced archaea. Lrp-like proteins are often referred to as the feast/famine regulatory proteins (FFRPs), reflecting their common regulatory roles. Acting as either global or local transcriptional regulators, FFRPs detect the environmental nutritional status by sensing small effector molecules (usually amino acids) and regulate the expression of genes involved in metabolism, virulence, motility, nutrient transport, stress tolerance, and antibiotic resistance to implement appropriate behaviors for the specific ecological niche of each organism. Despite FFRPs' complexity, a significant role in gene regulation, and prevalence throughout prokaryotes, the last comprehensive review on this family of proteins was published about a decade ago. In this review, we integrate recent notable findings regarding E. coli Lrp and other FFRPs across bacteria and archaea with previous observations to synthesize a more complete view on the mechanistic details and biological roles of this ancient class of transcription factors.

Functional characterization of asnC family transcriptional regulator in Pseudomonas aeruginosa PGPR2 during root colonization

Transcriptional regulators in bacteria are the crucial players in mediating communication between environmental cues and DNA transcription through a complex network process. Pseudomonas aeruginosa PGPR2 is an efficient root colonizer and a biocontrol strain. Previously, we identified that the transcriptional regulator, asnC, negatively regulates the corn root colonization of P. aeruginosa PGPR2. In a transposon insertion sequencing (INSeq) screen, the asnC insertion mutant was positively selected during root colonization, meaning the disruption of asnC improves the fitness of the P. aeruginosa PGPR2 strain for the root colonization. In this study, we constructed isogenic mutant of asnC family transcriptional regulator encoded by PGPR2_17510 by allele exchange mutagenesis. The ΔasnC mutant was able to efficiently colonize corn roots with a twofold increase in population when compared to the wild-type strain. Similarly, the mutant strain outcompeted the wild-type strain in a competition assay, where the mutant strain represented 90% of the total population recovered from the root. We compared the whole transcriptome of the wild-type and the ΔasnC mutant of P. aeruginosa PGPR2 when exposed to the corn root exudates. The RNA-Seq revealed that a total of 360 genes were differentially expressed in the ΔasnC strain of P. aeruginosa PGPR2. Inactivation of asnC transcriptional regulator resulted in the up-regulation of several genetic factors implicated in metabolism, uptake of nutrients, motility, stress response, and signal transduction, which could play crucial roles in root colonization. This notion was further validated by phenotypic characterization and quantification of transcription pattern of selected genes associated with metabolism, motility, and carbon catabolite repression between wild type and mutant strain, which was in agreement with transcriptome data. Similarly, ΔasnC strain formed increased biofilm on abiotic surface validating our RNA-seq analysis, where transcript levels of several genes associated with biofilm formation were up-regulated in the mutant strain. We report that the inactivation of an asnC family transcriptional regulator encoded by PGPR2_17510 enhances the root colonization and biofilm-forming ability of P. aeruginosa PGPR2. Together, our results provide evidence for the molecular adaptations that enable ΔasnC mutant strain to colonize on the corn roots and to form a biofilm.

A Thermosensitive, Phase-Variable Epigenetic Switch: pap Revisited

It has been more than a decade since the last comprehensive review of the phase-variable uropathogen-associated pyelonephritis-associated pilus (pap) genetic switch. Since then, important data have come to light, including additional factors that regulate pap expression, better characterization of H-NS regulation, the structure of the Lrp octamer in complex with pap regulatory DNA, the temperature-insensitive phenotype of a mutant lacking the acetyltransferase RimJ, evidence that key components of the regulatory machinery are acetylated, and new insights into the role of DNA binding by key regulators in shaping both the physical structure and regulatory state of the papI and papBA promoters. This review revisits pap, integrating these newer observations with older ones to produce a new model for the concerted behavior of this virulence-regulatory region.

Diversity, versatility and complexity of bacterial gene regulation mechanisms: opportunities and drawbacks for applications in synthetic biology

Gene expression occurs in two essential steps: transcription and translation. In bacteria, the two processes are tightly coupled in time and space, and highly regulated. Tight regulation of gene expression is crucial. It limits wasteful consumption of resources and energy, prevents accumulation of potentially growth inhibiting reaction intermediates, and sustains the fitness and potential virulence of the organism in a fluctuating, competitive and frequently stressful environment. Since the onset of studies on regulation of enzyme synthesis, numerous distinct regulatory mechanisms modulating transcription and/or translation have been discovered. Mostly, various regulatory mechanisms operating at different levels in the flow of genetic information are used in combination to control and modulate the expression of a single gene or operon. Here, we provide an extensive overview of the very diverse and versatile bacterial gene regulatory mechanisms with major emphasis on their combined occurrence, intricate intertwinement and versatility. Furthermore, we discuss the potential of well-characterized basal expression and regulatory elements in synthetic biology applications, where they may ensure orthogonal, predictable and tunable expression of (heterologous) target genes and pathways, aiming at a minimal burden for the host.

Competitive Repression of the artPIQM Operon for Arginine and Ornithine Transport by Arginine Repressor and Leucine-Responsive Regulatory Protein in Escherichia coli

Two out of the three major uptake systems for arginine in Escherichia coli are encoded by the artJ-artPIQM gene cluster. ArtJ is the high-affinity periplasmic arginine-specific binding protein (ArgBP-I), whereas artI encodes the arginine and ornithine periplasmic binding protein (AO). Both ArtJ and ArtI are supposed to combine with the inner membrane-associated ArtQMP₂ transport complex of the ATP-binding cassette-type (ABC). Transcription of artJ is repressed by arginine repressor (ArgR) and the artPIQM operon is regulated by the transcriptional regulators ArgR and Leucine-responsive regulatory protein (Lrp). Whereas repression by ArgR requires arginine as corepressor, repression of P_artP by Lrp is partially counteracted by leucine, its major effector molecule. We demonstrate that binding of dimeric Lrp to the artP control region generates four complexes with a distinct migration velocity, and that leucine has an effect on both global binding affinity and cooperativity in the binding. We identify the binding sites for Lrp in the artP control region, reveal interferences in the binding of ArgR and Lrp in vitro and demonstrate that the two transcription factors act as competitive repressors in vivo, each one being a more potent regulator in the absence of the other. This competitive behavior may be explained by the partial steric overlap of their respective binding sites. Furthermore, we demonstrate ArgR binding to an unusual position in the control region of the lrp gene, downstream of the transcription initiation site. From this unusual position for an ArgR-specific operator, ArgR has little direct effect on lrp expression, but interferes with the negative leucine-sensitive autoregulation exerted by Lrp. Direct arginine and ArgR-dependent repression of lrp could be observed with a 25-bp deletion mutant, in which the ArgR binding site was artificially moved to a position immediately downstream of the lrp transcription initiation site. This finding is reminiscent of a previous observation made for the carAB operon encoding carbamoylphosphate synthase, where ArgR bound in overlap with the downstream promoter P2 does not block transcription initiated 67 bp upstream at the P1 promoter, and further supports the hypothesis that ArgR does not act as an efficient roadblock.

Engineering of leucine-responsive regulatory protein improves spiramycin and bitespiramycin biosynthesis

Background

Bitespiramycin (BT) is produced by recombinant spiramycin (SP) producing strain Streptomyces spiramyceticus harboring a heterologous 4″-O-isovaleryltransferase gene (ist). Exogenous l-Leucine (l-Leu) could improve the production of BT. The orf2 gene found from the genomic sequence of S. spiramyceticus encodes a leucine-responsive regulatory protein (Lrp) family regulator named as SSP_Lrp. The functions of SSP_Lrp and l-Leu involved in the biosynthesis of spiramycin (SP) and BT were investigated in S. spiramyceticus.

Results

SSP_Lrp was a global regulator directly affecting the expression of three positive regulatory genes, bsm23, bsm42 and acyB2, in SP or BT biosynthesis. Inactivation of SSP_Lrp gene in S. spiramyceticus 1941 caused minor increase of SP production. However, SP production of the ΔSSP_Lrp-SP strain containing an SSP_Lrp deficient of putative l-Leu binding domain was higher than that of S. spiramyceticus 1941 (476.2 ± 3.1 μg/L versus 313.3 ± 25.2 μg/L, respectively), especially SP III increased remarkably. The yield of BT in ΔSSP_Lrp-BT strain was more than twice than that in 1941-BT. The fact that intracellular concentrations of branched-chain amino acids (BCAAs) decreased markedly in the ΔSSP_Lrp-SP demonstrated increasing catabolism of BCAAs provided more precursors for SP biosynthesis. Comparative analysis of transcriptome profiles of the ΔSSP_Lrp-SP and S. spiramyceticus 1941 found 12 genes with obvious differences in expression, including 6 up-regulated genes and 6 down-regulated genes. The up-regulated genes are related to PKS gene for SP biosynthesis, isoprenoid biosynthesis, a Sigma24 family factor, the metabolism of aspartic acid, pyruvate and acyl-CoA; and the down-regulated genes are associated with ribosomal proteins, an AcrR family regulator, and biosynthesis of terpenoid, glutamate and glutamine.

Conclusion

SSP_Lrp in S. spiramyceticus was a negative regulator involved in the SP and BT biosynthesis. The deletion of SSP_Lrp putative l-Leu binding domain was advantageous for production of BT and SP, especially their III components.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1086-0) contains supplementary material, which is available to authorized users.

Escherichia coli Lrp Regulates One-Third of the Genome via Direct, Cooperative, and Indirect Routes

The global regulator Lrp plays a crucial role in regulating metabolism, virulence, and motility in response to environmental conditions. Lrp has previously been shown to activate or repress approximately 10% of the genes in Escherichia coli However, the full spectrum of targets, and how Lrp acts to regulate them, have stymied earlier study. We have combined matched chromatin-immunoprecipitation sequencing (ChIP-seq) and RNA sequencing (RNA-seq) under nine physiological conditions to comprehensively map the binding and regulatory activity of Lrp as it directs responses to nutrient abundance. In addition to identifying hundreds of novel Lrp targets, we observe two new global trends, as follows: first, that Lrp will often bind to promoters in a poised position under conditions when it has no regulatory activity to enable combinatorial interactions with other regulators, and second, that nutrient levels induce a global shift in the equilibrium between less-sequence-specific and more-sequence-specific DNA binding. The overall regulatory behavior of Lrp, which as we now show extends to 38% of E. coli genes directly or indirectly under at least one condition, thus arises from the interaction between changes in Lrp binding specificity and cooperative action with other regulators.IMPORTANCE To survive, bacteria such as E. coli must rapidly respond to changing environmental conditions, including nutrient levels. A decrease in nutrient availability causes bacteria to stop rapid replication and enter stationary phase, where they perform limited to no cell division. The E. coli global regulatory protein Lrp has been previously implicated in modulating the expression of genes particularly important at this transition from rapid to slowed growth. Here, we monitor Lrp's DNA binding locations and effect on gene expression under three different nutrient conditions across three growth stages. We find that Lrp's role is even broader than previously suspected and that it appears to interact with many other bacterial regulators to perform its function in a condition-specific manner.

Alanine dehydrogenases in mycobacteria

Since NAD(H)-dependent L-alanine dehydrogenase (EC 1.1.4.1; Ald) was identified as one of the major antigens present in culture filtrates of Mycobacterium tuberculosis, many studies on the enzyme have been conducted. Ald catalyzes the reversible conversion of pyruvate to alanine with concomitant oxidation of NADH to NAD⁺ and has a homohexameric quaternary structure. Expression of the ald genes was observed to be strongly upregulated in M. tuberculosis and Mycobacterium smegmatis grown in the presence of alanine. Furthermore, expression of the ald genes in some mycobacteria was observed to increase under respiration-inhibitory conditions such as oxygen-limiting and nutrient-starvation conditions. Upregulation of ald expression by alanine or under respiration-inhibitory conditions is mediated by AldR, a member of the Lrp/AsnC family of transcriptional regulators. Mycobacterial Alds were demonstrated to be the enzymes required for utilization of alanine as a nitrogen source and to help mycobacteria survive under respiration-inhibitory conditions by maintaining cellular NADH/NAD⁺ homeostasis. Several inhibitors of Ald have been developed, and their application in combination with respiration-inhibitory antitubercular drugs such as Q203 and bedaquiline was recently suggested.

Evolution of a global regulator: Lrp in four orders of γ-Proteobacteria

Background

Bacterial global regulators each regulate the expression of several hundred genes. In Escherichia coli, the top seven global regulators together control over half of all genes. Leucine-responsive regulatory protein (Lrp) is one of these top seven global regulators. Lrp orthologs are very widely distributed, among both Bacteria and Archaea. Surprisingly, even within the phylum γ-Proteobacteria (which includes E. coli), Lrp is a global regulator in some orders and a local regulator in others. This raises questions about the evolution of Lrp and, more broadly, of global regulators.

Results

We examined Lrp sequences from four bacterial orders of the γ-Proteobacteria using phylogenetic and Logo analyses. The orders studied were Enterobacteriales and Vibrionales, in which Lrp plays a global role in tested species; Pasteurellales, in which Lrp is a local regulator in the tested species; and Alteromonadales, an order closely related to the other three but in which Lrp has not yet been studied. For comparison, we analyzed the Lrp paralog AsnC, which in all tested cases is a local regulator. The Lrp and AsnC phylogenetic clusters each divided, as expected, into subclusters representing the Enterobacteriales, Vibrionales, and Pasteuralles. However the Alteromonadales did not yield coherent clusters for either Lrp or AsnC. Logo analysis revealed signatures associated with globally- vs. locally- acting Lrp orthologs, providing testable hypotheses for which portions of Lrp are responsible for a global vs. local role. These candidate regions include both ends of the Lrp polypeptide but not, interestingly, the highly-conserved helix-turn-helix motif responsible for DNA sequence specificity.

Conclusions

Lrp and AsnC have conserved sequence signatures that allow their unambiguous annotation, at least in γ-Proteobacteria. Among Lrp orthologs, specific residues correlated with global vs. local regulatory roles, and can now be tested to determine which are functionally relevant and which simply reflect divergence. In the Alteromonadales, it appears that there are different subgroups of Lrp orthologs, one of which may act globally while the other may act locally. These results suggest experiments to improve our understanding of the evolution of bacterial global regulators.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0685-1) contains supplementary material, which is available to authorized users.

The Global Transcription Factor Lrp Controls Virulence Modulation in Xenorhabdus nematophila

The bacterium Xenorhabdus nematophila engages in phenotypic variation with respect to pathogenicity against insect larvae, yielding both virulent and attenuated subpopulations of cells from an isogenic culture. The global regulatory protein Lrp is necessary for X. nematophila virulence and immunosuppression in insects, as well as colonization of the mutualistic host nematode Steinernema carpocapsae, and mediates expression of numerous genes implicated in each of these phenotypes. Given the central role of Lrp in X. nematophila host associations, as well as its involvement in regulating phenotypic variation pathways in other bacteria, we assessed its function in virulence modulation. We discovered that expression of lrp varies within an isogenic population, in a manner that correlates with modulation of virulence. Unexpectedly, although Lrp is necessary for optimal virulence and immunosuppression, cells expressing high levels of lrp were attenuated in these processes relative to those with low to intermediate lrp expression. Furthermore, fixed expression of lrp at high and low levels resulted in attenuated and normal virulence and immunosuppression, respectively, and eliminated population variability of these phenotypes. These data suggest that fluctuating lrp expression levels are sufficient to drive phenotypic variation in X. nematophila.

IMPORTANCE

Many bacteria use cell-to-cell phenotypic variation, characterized by distinct phenotypic subpopulations within an isogenic population, to cope with environmental change. Pathogenic bacteria utilize this strategy to vary antigen or virulence factor expression. Our work establishes that the global transcription factor Lrp regulates phenotypic variation in the insect pathogen Xenorhabdus nematophila, leading to attenuation of virulence and immunosuppression in insect hosts. Unexpectedly, we found an inverse correlation between Lrp expression levels and virulence: high levels of expression of Lrp-dependent putative virulence genes are detrimental for virulence but may have an adaptive advantage in other aspects of the life cycle. Investigation of X. nematophila phenotypic variation facilitates dissection of this phenomenon in the context of a naturally occurring symbiosis.

Regulation Mechanism of the ald Gene Encoding Alanine Dehydrogenase in Mycobacterium smegmatis and Mycobacterium tuberculosis by the Lrp/AsnC Family Regulator AldR

In the presence of alanine, AldR, which belongs to the Lrp/AsnC family of transcriptional regulators and regulates ald encoding alanine dehydrogenase in Mycobacterium smegmatis, changes its quaternary structure from a homodimer to an octamer with an open-ring conformation. Four AldR-binding sites (O2, O1, O4, and O3) with a consensus sequence of GA/T-N2-NWW/WWN-N2-A/TC were identified upstream of the M. smegmatis ald gene by means of DNase I footprinting analysis. O2, O1, and O4 are required for the induction of ald expression by alanine, while O3 is directly involved in the repression of ald expression. In addition to O3, both O1 and O4 are also necessary for full repression of ald expression in the absence of alanine, due to cooperative binding of AldR dimers to O1, O4, and O3. Binding of a molecule of the AldR octamer to the ald control region was demonstrated to require two AldR-binding sites separated by three helical turns between their centers and one additional binding site that is in phase with the two AldR-binding sites. The cooperative binding of AldR dimers to DNA requires three AldR-binding sites that are aligned with a periodicity of three helical turns. The aldR gene is negatively autoregulated independently of alanine. Comparative analysis of ald expression of M. smegmatis and Mycobacterium tuberculosis in conjunction with sequence analysis of both ald control regions led us to suggest that the expression of the ald genes in both mycobacterial species is regulated by the same mechanism.

IMPORTANCE

In mycobacteria, alanine dehydrogenase (Ald) is the enzyme required both to utilize alanine as a nitrogen source and to grow under hypoxic conditions by maintaining the redox state of the NADH/NAD(+) pool. Expression of the ald gene was reported to be regulated by the AldR regulator that belongs to the Lrp/AsnC (feast/famine) family, but the underlying mechanism was unknown. This study revealed the regulation mechanism of ald in Mycobacterium smegmatis and Mycobacterium tuberculosis. Furthermore, a generalized arrangement pattern of cis-acting regulatory sites for Lrp/AsnC (feast/famine) family regulators is suggested in this study.

Expanded roles of leucine-responsive regulatory protein in transcription regulation of the Escherichia coli genome: Genomic SELEX screening of the regulation targets

Leucine-responsive regulatory protein (Lrp) is a transcriptional regulator for the genes involved in transport, biosynthesis and catabolism of amino acids in Escherichia coli. In order to identify the whole set of genes under the direct control of Lrp, we performed Genomic SELEX screening and identified a total of 314 Lrp-binding sites on the E. coli genome. As a result, the regulation target of Lrp was predicted to expand from the hitherto identified genes for amino acid metabolism to a set of novel target genes for utilization of amino acids for protein synthesis, including tRNAs, aminoacyl-tRNA synthases and rRNAs. Northern blot analysis indicated alteration of mRNA levels for at least some novel targets, including the aminoacyl-tRNA synthetase genes. Phenotype MicroArray of the lrp mutant indicated significant alteration in utilization of amino acids and peptides, whilst metabolome analysis showed variations in the concentration of amino acids in the lrp mutant. From these two datasets we realized a reverse correlation between amino acid levels and cell growth rate: fast-growing cells contain low-level amino acids, whilst a high level of amino acids exists in slow-growing cells. Taken together, we propose that Lrp is a global regulator of transcription of a large number of the genes involved in not only amino acid transport and metabolism, but also amino acid utilization.

The structure, function and properties of sirohaem decarboxylase--an enzyme with structural homology to a transcription factor family that is part of the alternative haem biosynthesis pathway

Some bacteria and archaea synthesize haem by an alternative pathway, which involves the sequestration of sirohaem as a metabolic intermediate rather than as a prosthetic group. Along this pathway the two acetic acid side-chains attached to C12 and C18 are decarboxylated by sirohaem decarboxylase, a heterodimeric enzyme composed of AhbA and AhbB, to give didecarboxysirohaem. Further modifications catalysed by two related radical SAM enzymes, AhbC and AhbD, transform didecarboxysirohaem into Fe-coproporphyrin III and haem respectively. The characterization of sirohaem decarboxylase is reported in molecular detail. Recombinant versions of Desulfovibrio desulfuricans, Desulfovibrio vulgaris and Methanosarcina barkeri AhbA/B have been produced and their physical properties compared. The D. vulgaris and M. barkeri enzyme complexes both copurify with haem, whose redox state influences the activity of the latter. The kinetic parameters of the D. desulfuricans enzyme have been determined, the enzyme crystallized and its structure has been elucidated. The topology of the enzyme reveals that it shares a structural similarity to the AsnC/Lrp family of transcription factors. The active site is formed in the cavity between the two subunits and a AhbA/B-product complex with didecarboxysirohaem has been obtained. A mechanism for the decarboxylation of the kinetically stable carboxyl groups is proposed.

Environmental adaptability and stress tolerance of Laribacter hongkongensis: a genome-wide analysis

Background

Laribacter hongkongensis is associated with community-acquired gastroenteritis and traveler's diarrhea and it can reside in human, fish, frogs and water. In this study, we performed an in-depth annotation of the genes in its genome related to adaptation to the various environmental niches.

Results

L. hongkongensis possessed genes for DNA repair and recombination, basal transcription, alternative σ-factors and 109 putative transcription factors, allowing DNA repair and global changes in gene expression in response to different environmental stresses. For acid stress, it possessed a urease gene cassette and two arc gene clusters. For alkaline stress, it possessed six CDSs for transporters of the monovalent cation/proton antiporter-2 and NhaC Na⁺:H⁺ antiporter families. For heavy metals acquisition and tolerance, it possessed CDSs for iron and nickel transport and efflux pumps for other metals. For temperature stress, it possessed genes related to chaperones and chaperonins, heat shock proteins and cold shock proteins. For osmotic stress, 25 CDSs were observed, mostly related to regulators for potassium ion, proline and glutamate transport. For oxidative and UV light stress, genes for oxidant-resistant dehydratase, superoxide scavenging, hydrogen peroxide scavenging, exclusion and export of redox-cycling antibiotics, redox balancing, DNA repair, reduction of disulfide bonds, limitation of iron availability and reduction of iron-sulfur clusters are present. For starvation, it possessed phosphorus and, despite being asaccharolytic, carbon starvation-related CDSs.

Conclusions

The L. hongkongensis genome possessed a high variety of genes for adaptation to acid, alkaline, temperature, osmotic, oxidative, UV light and starvation stresses and acquisition of and tolerance to heavy metals.

Recognition of DNA by the helix-turn-helix global regulatory protein Lrp is modulated by the amino terminus

The AsnC/Lrp family of regulatory proteins links bacterial and archaeal transcription patterns to metabolism. In Escherichia coli, Lrp regulates approximately 400 genes, over 200 of them directly. In earlier studies, lrp genes from Vibrio cholerae, Proteus mirabilis, and E. coli were introduced into the same E. coli background and yielded overlapping but significantly different regulons. These differences were seen despite amino acid sequence identities of 92% (Vibrio) and 98% (Proteus) to E. coli Lrp, including complete conservation of the helix-turn-helix motifs. The N-terminal region contains many of the sequence differences among these Lrp orthologs, which led us to investigate its role in Lrp function. Through the generation of hybrid proteins, we found that the N-terminal diversity is responsible for some of the differences between orthologs in terms of DNA binding (as revealed by mobility shift assays) and multimerization (as revealed by gel filtration, dynamic light scattering, and analytical ultracentrifugation). These observations indicate that the N-terminal tail plays a significant role in modulating Lrp function, similar to what is seen for a number of other regulatory proteins.

An overview of prokaryotic transcription factors : a summary of function and occurrence in bacterial genomes

Transcriptional initiation is arguably the most important control point for gene expression. It is regulated by a combination of factors, including DNA sequence and its three-dimensional topology, proteins and small molecules. In this chapter, we focus on the trans-acting factors of bacterial regulation. Initiation begins with the recruitment of the RNA polymerase holoenzyme to a specific locus upstream of the gene known as its promoter. The sigma factor, which is a component of the holoenzyme, provides the most fundamental mechanisms for orchestrating broad changes in gene expression state. It is responsible for promoter recognition as well as recruiting the holoenzyme to the promoter. Distinct sigma factors compete with for binding to a common pool of RNA polymerases, thus achieving condition-dependent differential expression. Another important class of bacterial regulators is transcription factors, which activate or repress transcription of target genes typically in response to an environmental or cellular trigger. These factors may be global or local depending on the number of genes and range of cellular functions that they target. The activities of both global and local transcription factors may be regulated either at a post-transcriptional level via signal-sensing protein domains or at the level of their own expression. In addition to modulating polymerase recruitment to promoters, several global factors are considered as "nucleoid-associated proteins" that impose structural constraints on the chromosome by altering the conformation of the bound DNA, thus influencing other processes involving DNA such as replication and recombination. This chapter concludes with a discussion of how regulatory interactions between transcription factors and their target genes can be represented as a network.

Unexpected coregulator range for the global regulator Lrp of Escherichia coli and Proteus mirabilis

The Lrp/AsnC family of transcription factors links gene regulation to metabolism in bacteria and archaea. Members of this family, collectively, respond to a wide range of amino acids as coregulators. In Escherichia coli, Lrp regulates over 200 genes directly and is well known to respond to leucine and, to a somewhat lesser extent, alanine. We focused on Lrp from Proteus mirabilis and E. coli, orthologs with 98% identity overall and identical helix-turn-helix motifs, for which a previous study nevertheless found functional differences. Sequence differences between these orthologs, within and adjacent to the amino acid-responsive RAM domain, led us to test for differential sensitivity to coregulatory amino acids. In the course of this investigation, we found, via in vivo reporter fusion assays and in vitro electrophoretic mobility shift experiments, that E. coli Lrp itself responded to a broader range of amino acids than was previously appreciated. In particular, for both the E. coli and P. mirabilis orthologs, Lrp responsiveness to methionine was similar in magnitude to that to leucine. Both Lrp orthologs are also fairly sensitive to Ile, His, and Thr. These observations suggest that Lrp ties gene expression in the Enterobacteriaceae rather extensively to physiological status, as reflected in amino acid pools. These findings also have substantial implications for attempts to model regulatory architecture from transcriptome measurements or to infer such architecture from genome sequences, and they suggest that even well-studied regulators deserve ongoing exploration.

Activation of motility by sensing short-chain fatty acids via two steps in a flagellar gene regulatory cascade in enterohemorrhagic Escherichia coli

The regulated expression of virulence genes is critical for successful infection by an intestinal pathogen. Bacteria rely on sensing environmental signals to find preferable niches and reach the infectious state. Orally ingested enterohemorrhagic Escherichia coli (EHEC) travels through the gastrointestinal tract and encounters a variety of environmental factors, some of which act as triggering signals for the induction of virulence genes. Butyrate, one of the main short-chain fatty acids (SCFAs), is such a signal, enhancing the expression of genes for intimate attachment and type III secretion. We further explored the role of SCFAs and found a positive effect of SCFAs on flagellar expression. Although EHEC did not produce flagella when grown in Dulbecco's modified Eagle's medium (DMEM), a tissue culture medium that enhances virulence gene expression, the addition of SCFAs to the medium induced the production of flagella, and the EHEC bacteria became motile. Among SCFAs, butyrate simultaneously activates both virulence and flagellar genes. Flagella did not affect initial adherence, and they were not expressed in adherent bacteria during microcolony formation. SCFAs activated flagellar genes via two regulatory steps. Butyrate activated the flhDC regulatory genes through leucine-responsive regulatory protein (Lrp), which is also a regulator of virulence genes. However, butyrate, acetate, and propionate also activated downstream genes independently of flhDC activation. Consequently, when encountering increased concentrations of SCFAs, which are abundant in acetate, in the intestine, EHEC first activates flagellar production and motility, followed by genes involved in adherence and type III secretion, which leads to efficient adherence in a preferable niche.

Transcriptional control by two leucine-responsive regulatory proteins in Halobacterium salinarum R1

Background

Archaea combine bacterial-as well as eukaryotic-like features to regulate cellular processes. Halobacterium salinarum R1 encodes eight leucine-responsive regulatory protein (Lrp)-homologues. The function of two of them, Irp (OE3923F) and lrpA1 (OE2621R), were analyzed by gene deletion and overexpression, including genome scale impacts using microarrays.

Results

It was shown that Lrp affects the transcription of multiple target genes, including those encoding enzymes involved in amino acid synthesis, central metabolism, transport processes and other regulators of transcription. In contrast, LrpA1 regulates transcription in a more specific manner. The aspB3 gene, coding for an aspartate transaminase, was repressed by LrpA1 in the presence of L-aspartate. Analytical DNA-affinity chromatography was adapted to high salt, and demonstrated binding of LrpA1 to its own promoter, as well as L-aspartate dependent binding to the aspB3 promoter.

Conclusion

The gene expression profiles of two archaeal Lrp-homologues report in detail their role in H. salinarum R1. LrpA1 and Lrp show similar functions to those already described in bacteria, but in addition they play a key role in regulatory networks, such as controlling the transcription of other regulators. In a more detailed analysis ligand dependent binding of LrpA1 was demonstrated to its target gene aspB3.

The leucine-responsive regulatory protein, Lrp, modulates microcin J25 intrinsic resistance in Escherichia coli by regulating expression of the YojI microcin exporter

Many Escherichia coli K-12 strains display an intrinsic resistance to the peptide antibiotic microcin J25. In this study, we present results showing that the leucine-responsive regulatory protein, Lrp, is involved in this phenotype by acting as a positive regulator of YojI, a chromosomally encoded efflux pump which expels microcin out of cells. Exogenous leucine antagonizes the effect of Lrp, leading to a diminished expression of the pump and an increased susceptibility to microcin J25.

Autoregulated expression of the gene coding for the leucine-responsive protein, Lrp, a global regulator in Salmonella enterica serovar Typhimurium

In this study, the lrp gene encoding the leucine-responsive regulatory protein (Lrp) in Salmonella enterica serovar Typhimurium was found to be negatively autoregulated. Its transcription start site was determined by primer extension analysis, showing that the lrp promoter is located at a different site to that inferred previously from the S. Typhimurium genome sequence. Chromosomal DNA fragments that include the promoter region were bound by purified Lrp protein in vitro, producing up to four distinct protein-DNA complexes. DNase I footprinting identified regions that were protected by the protein in vitro as well as bases that became hypersensitive to DNase I treatment following Lrp binding. A clear pattern of periodic hypersensitivity was detected between positions -130 and +15 that was consistent with wrapping of the DNA around Lrp in a nucleoprotein complex that includes the putative promoter region. Lrp-DNA interaction in this region was fully consistent with the observed repression of lrp transcription by this protein. Leucine was found to modulate Lrp-mediated autorepression by remodelling the Lrp-DNA nucleoprotein complex.

Competitive Lrp and Dam assembly at the pap regulatory region: implications for mechanisms of epigenetic regulation

Escherichia coli DNA adenine methyltransferase (Dam) and Leucine-responsive regulatory protein (Lrp) are key regulators of the pap operon, which codes for the pilus proteins necessary for uropathogenic E. coli cellular adhesion. The pap operon is regulated by a phase variation mechanism in which the methylation states of two GATC sites in the pap regulatory region and the binding position of Lrp determine whether the pilus genes are expressed. The post-replicative reassembly of Dam, Lrp, and the local regulator PapI onto a hemimethylated pap intermediate is a critical step of the phase variation switching mechanism and is not well understood. We show that Lrp, in the presence and in the absence of PapI and nonspecific DNA, specifically protects pap regulatory GATC sites from Dam methylation when allowed to compete with Dam for assembly on unmethylated and hemimethylated pap DNA. The methylation protection is dependent upon the concentration of Lrp and does not occur with non-regulatory GATC sites. Our data suggest that only at low Lrp concentrations will Dam compete effectively for binding and methylation of the proximal GATC site, leading to a phase switch resulting in the expression of pili.

Crystal structure of glutamine receptor protein from Sulfolobus tokodaii strain 7 in complex with its effector L-glutamine: implications of effector binding in molecular association and DNA binding

Genome analyses have revealed that members of the Lrp/AsnC family of transcriptional regulators are widely distributed among prokaryotes, including both bacteria and archaea. These regulatory proteins are involved in cellular metabolism in both global and specific manners, depending on the availability of the exogenous amino acid effectors. Here we report the first crystal structure of glutamine receptor protein (Grp) from Sulfolobus tokodaii strain 7, in the ligand-free and glutamine-bound (Grp-Gln) forms. Although the overall structures of both molecules are similar, a significant conformational change was observed at the ligand [L-glutamine (Gln)] binding site in the effector domain, which may be essential for further stabilization of the octameric structure, and in turn for facilitating DNA binding. In addition, we predicted promoter for the grp gene, and these analyses suggested the importance of cooperative binding to the protein. To gain insights into the ligand-induced conformational changes, we mutated all of the ligand-binding residues in Grp, and revealed the importance of Gln binding by biochemical and structural analyses. Further structural analyses showed that Y77 is crucial for ligand binding, and that the residues T132 and T134, which are highly conserved among the Lrp family of proteins, fluctuates between the active and inactive conformations, thus affecting protein oligomerization for DNA binding.

The leucine-responsive regulatory protein, Lrp, activates transcription of the fim operon in Salmonella enterica serovar typhimurium via the fimZ regulatory gene

The fim operon of Salmonella enterica serovar Typhimurium encodes type 1 fimbriae. The expression of fim is controlled in response to environmental signals through a complex regulatory cascade involving the proteins FimW, FimY, and FimZ and a genetic locus, fimU, that encodes a rare arginine tRNA. We discovered that a knockout mutation in lrp, the gene that codes for the leucine-responsive regulatory protein (Lrp), inhibited fim transcription. The loss of fim gene expression was accompanied by a corresponding loss of the mannose-sensitive hemagglutination that is a characteristic of type 1 fimbriae. Normal type 1 fimbrial expression was restored following the introduction into the knockout mutant of a plasmid carrying a functional copy of the lrp gene. Electrophoretic mobility shift analysis revealed no interactions between purified Lrp protein and the regulatory region of the fimA, fimU, or fimW gene. Instead, Lrp produced protein-DNA complexes with the regulatory region of the fimZ gene, and the nature of these complexes was leucine sensitive. DNase I footprinting showed that Lrp binds within a region between -65 and -170 with respect to the fimZ transcription start site, consistent with the binding and wrapping of the DNA in this upstream region. Ectopic expression of the fimZ gene from an inducible promoter caused Lrp-independent type 1 fimbriation in serovar Typhimurium. These data show that Lrp makes a positive contribution to fim gene expression through direct interaction with the fimZ promoter region, possibly by antagonizing the binding of the H-NS global repressor protein.

Mechanistic insights from the crystal structures of a feast/famine regulatory protein from Mycobacterium tuberculosis H37Rv

Rv3291c gene from Mycobacterium tuberculosis codes for a transcriptional regulator belonging to the (leucine responsive regulatory protein/regulator of asparigine synthase C gene product) Lrp/AsnC-family. We have identified a novel effector-binding site from crystal structures of the apo protein, complexes with a variety of amino acid effectors, X-ray based ligand screening and qualitative fluorescence spectroscopy experiments. The new effector site is in addition to the structural characterization of another distinct site in the protein conserved in the related AsnC-family of regulators. The structures reveal that the ligand-binding loops of two crystallographically independent subunits adopt different conformations to generate two distinct effector-binding sites. A change in the conformation of the binding site loop 100–106 in the B subunit is apparently necessary for octameric association and also allows the loop to interact with a bound ligand in the newly identified effector-binding site. There are four sites of each kind in the octamer and the protein preferentially binds to aromatic amino acids. While amino acids like Phe, Tyr and Trp exhibit binding to only one site, His exhibits binding to both sites. Binding of Phe is accompanied by a conformational change of 3.7 Å in the 75–83 loop, which is advantageously positioned to control formation of higher oligomers. Taken together, the present studies suggest an elegant control mechanism for global transcription regulation involving binding of ligands to the two sites, individually or collectively.

The Structure and Transcriptional Analysis of a Global Regulator from Neisseria meningitidis

Neisseria meningitidis, a causative agent of bacterial meningitis, has a relatively small repertoire of transcription factors, including NMB0573 (annotated AsnC), a member of the Lrp-AsnC family of regulators that are widely expressed in both Bacteria and Archaea. In the present study we show that NMB0573 binds to l-leucine and l-methionine and have solved the structure of the protein with and without bound amino acids. This has shown, for the first time that amino acid binding does not induce significant conformational changes in the structure of an AsnC/Lrp regulator although it does appear to stabilize the octameric assembly of the protein. Transcriptional profiling of wild-type and NMB0573 knock-out strains of N. meningitidis has shown that NMB0573 is associated with an adaptive response to nutrient poor conditions reflected in a reduction in major surface protein expression. On the basis of its structure and the transcriptional response, we propose that NMB0573 is a global regulator in Neisseria controlling responses to nutrient availability through indicators of general amino acid abundance: leucine and methionine.

The role of high affinity non-specific DNA binding by Lrp in transcriptional regulation and DNA organization

Transcriptional regulatory proteins typically bind specific DNA sequences with approximately 10(3)-10(7)-fold higher affinity than non-specific DNA and this discrimination is essential for their in vivo function. Here we show that the bacterial leucine-responsive regulatory protein (Lrp) does not follow this trend and has a approximately 20-400-fold binding discrimination between specific and non-specific DNA sequences. We suggest that the dual function of Lrp to regulate genes and to organize DNA utilizes this unique property. A approximately 20-fold decrease in binding affinity from specific DNA is dependent upon cryptic binding sites, including the sequence GN(2-3)TTT and A-tracts. Removal of these sites still results in high binding affinity, only approximately 70-fold weaker than that of specific sites. Similar to Lrp's binding of specific sites in the pap and ilvIH promoters, Lrp binds cooperatively to non-specific DNA; thus, protein/protein interactions are important for both specific and non-specific DNA binding. When considering this cooperativity of Lrp binding, the binding selectivity to specific sites may increase to a maximum of approximately 400-fold. Neither leucine nor the pap-specific local regulator PapI alter Lrp's non-specific binding affinity or cooperative binding of non-specific DNA. We hypothesize that Lrp combines low sequence discrimination and relatively high intracellular protein concentrations to ensure its ability to regulate the transcription of specific genes while also functioning as a nucleoid-associated protein. Modeling of Lrp binding data and comparison to other proteins with regulatory and nucleoid-associated properties suggests similar mechanisms.

Structure of the Escherichia coli leucine-responsive regulatory protein Lrp reveals a novel octameric assembly

The structure of Escherichia coli leucine-responsive regulatory protein (Lrp) co-crystallized with a short duplex oligodeoxynucleotide reveals a novel quaternary assembly in which the protein octamer forms an open, linear array of four dimers. In contrast, structures of the Lrp homologs LrpA, LrpC and AsnC crystallized in the absence of DNA show that these proteins instead form highly symmetrical octamers in which the four dimers form a closed ring. Although the DNA is disordered within the Lrp crystal, comparative analyses suggest that the observed differences in quaternary state may arise from DNA interactions during crystallization. Interconversion of these conformations, possibly in response to DNA or leucine binding, provides an underlying mechanism to alter the relative spatial orientation of the DNA-binding domains. Breaking of the closed octamer symmetry may be a common essential step in the formation of active DNA complexes by all members of the Lrp/AsnC family of transcriptional regulatory proteins.

Epigenetic gene regulation in the bacterial world

Like many eukaryotes, bacteria make widespread use of postreplicative DNA methylation for the epigenetic control of DNA-protein interactions. Unlike eukaryotes, however, bacteria use DNA adenine methylation (rather than DNA cytosine methylation) as an epigenetic signal. DNA adenine methylation plays roles in the virulence of diverse pathogens of humans and livestock animals, including pathogenic Escherichia coli, Salmonella, Vibrio, Yersinia, Haemophilus, and Brucella. In Alphaproteobacteria, methylation of adenine at GANTC sites by the CcrM methylase regulates the cell cycle and couples gene transcription to DNA replication. In Gammaproteobacteria, adenine methylation at GATC sites by the Dam methylase provides signals for DNA replication, chromosome segregation, mismatch repair, packaging of bacteriophage genomes, transposase activity, and regulation of gene expression. Transcriptional repression by Dam methylation appears to be more common than transcriptional activation. Certain promoters are active only during the hemimethylation interval that follows DNA replication; repression is restored when the newly synthesized DNA strand is methylated. In the E. coli genome, however, methylation of specific GATC sites can be blocked by cognate DNA binding proteins. Blockage of GATC methylation beyond cell division permits transmission of DNA methylation patterns to daughter cells and can give rise to distinct epigenetic states, each propagated by a positive feedback loop. Switching between alternative DNA methylation patterns can split clonal bacterial populations into epigenetic lineages in a manner reminiscent of eukaryotic cell differentiation. Inheritance of self-propagating DNA methylation patterns governs phase variation in the E. coli pap operon, the agn43 gene, and other loci encoding virulence-related cell surface functions.

Feast/famine regulatory proteins (FFRPs): Escherichia coli Lrp, AsnC and related archaeal transcription factors

Feast/famine regulatory proteins comprise a diverse family of transcription factors, which have been referred to in various individual identifications, including Escherichia coli leucine-responsive regulatory protein and asparagine synthase C gene product. A full length feast/famine regulatory protein consists of the N-terminal DNA-binding domain and the C-domain, which is involved in dimerization and further assembly, thereby producing, for example, a disc or a chromatin-like cylinder. Various ligands of the size of amino acids bind at the interface between feast/famine regulatory protein dimers, thereby altering their assembly forms. Also, the combination of feast/famine regulatory protein subunits forming the same assembly is altered. In this way, a small number of feast/famine regulatory proteins are able to regulate a large number of genes in response to various environmental changes. Because feast/famine regulatory proteins are shared by archaea and eubacteria, the genome-wide regulation by feast/famine regulatory proteins is traceable back to their common ancestor, being the prototype of highly differentiated transcription regulatory mechanisms found in organisms nowadays.

Structural insight into gene transcriptional regulation and effector binding by the Lrp/AsnC family

The Lrp/AsnC family of transcriptional regulatory proteins is found in both archaea and bacteria. Members of the family influence cellular metabolism in both a global (Lrp) and specific (AsnC) manner, often in response to exogenous amino acid effectors. In the present study we have determined both the first bacterial and the highest resolution structures for members of the family. Escherichia coli AsnC is a specific gene regulator whose activity is triggered by asparagine binding. Bacillus subtilis LrpC is a global regulator involved in chromosome condensation. Our AsnC-asparagine structure is the first for a regulator-effector complex and is revealed as an octameric disc. Key ligand recognition residues are identified together with a route for ligand access. The LrpC structure reveals a stable octamer supportive of a topological role in dynamic DNA packaging. The structures yield significant clues to the functionality of Lrp/AsnC-type regulators with respect to ligand binding and oligomerization states as well as to their role in specific and global DNA regulation.

Role of LrpC from Bacillus subtilis in DNA transactions during DNA repair and recombination

Bacillus subtilis LrpC is a sequence-independent DNA-binding and DNA-bending protein, which binds both single-stranded (ss) and double-stranded (ds) DNA and facilitates the formation of higher order protein–DNA complexes in vitro. LrpC binds at different sites within the same DNA molecule promoting intramolecular ligation. When bound to separate molecules, it promotes intermolecular ligation, and joint molecule formation between a circular ssDNA and a homologous ssDNA-tailed linear dsDNA. LrpC binding showed a higher affinity for 4-way (Holliday) junctions in their open conformation, when compared with curved dsDNA. Consistent with these biochemical activities, an lrpC null mutant strain rendered cells sensitive to DNA damaging agents such as methyl methanesulfonate and 4-nitroquinoline-1-oxide, and showed a segregation defect. These findings collectively suggest that LrpC may be involved in DNA transactions during DNA repair and recombination.

Modulation of the sensitivity of FimB recombination to branched-chain amino acids and alanine in Escherichia coli K-12

Phase variation of type 1 fimbriae of Escherichia coli requires the site-specific recombination of a short invertible element. Inversion is catalyzed by FimB (switching in either direction) or FimE (inversion mainly from on to off) and is influenced by auxiliary factors integration host factor (IHF) and leucine-responsive regulatory protein (Lrp). These proteins bind to sites (IHF site II and Lrp sites 1 and 2) within the invertible element to stimulate recombination, presumably by bending the DNA to enhance synapses. Interaction of Lrp with a third site (site 3) cooperatively with sites 1 and 2 (termed complex 1) impedes recombination. Inversion is stimulated by the branched-chain amino acids (particularly leucine) and alanine, and according to a current model, the amino acids promote the selective loss of Lrp from site 3 (complex 2). Here we show that the central portion of the fim invertible element, situated between Lrp site 3 and IHF site II, is dispensable for FimB recombination but that this region is also required for full amino acid stimulation of inversion. Further work reveals that the region is likely to contain multiple regulatory elements. Lrp site 3 is shown to bind the regulatory protein with low affinity, and a mutation that enhances binding to this element is found both to diminish the stimulatory effects of IVLA on FimB recombination and to inhibit recombination in the absence of the amino acids. The results obtained emphasize the importance of Lrp site 3 as a control element but also highlight the complexity of the regulatory system that affects this site.

Overexpression, purification, crystallization and preliminary X-ray diffraction analysis of the C-terminal domain of Ss-LrpB, a transcription regulator from Sulfolobus solfataricus

Ss-LrpB from Sulfolobus solfataricus P2 belongs to the bacterial/archaeal superfamily of Lrp-like (leucine-responsive regulatory protein-like) transcription regulators. The N-terminal DNA-binding domain contains a HTH motif and the C-terminal domain contains an alphabeta-sandwich (betaalphabetabetaalphabeta fold). The C-terminal domain was overexpressed in Escherichia coli, purified and crystallized using the hanging-drop vapour-diffusion method. The crystals belong to space group P2(1)2(1)2, with unit-cell parameters a = 59.35, b = 74.86, c = 38.08 A and a data set was collected to 2.0 A resolution. Structure determination using a selenomethionine derivative is under way.

Stochastic modeling of the phase-variable pap operon regulation in uropathogenic Escherichia coli

Regulation of the pap operon in uropathogenic Escherichia coli is phase variable. This phase variation arises from competition between regulatory proteins at two sites within the regulatory region, GATC(dist) and GATC(prox). We have used the available literature data to design a stochastic model of the molecular interactions of pap regulation and expression during growth in a non-glucose environment at 37 degrees C. The resulting wild-type model is consistent with reported data. The wild-type model served as a basis for two "in silico" mutant models for investigating the role of key regulatory components, the GATC(dist) binding site and the PapI interaction with Lrp at the GATC(prox) site. Our results show that competition at GATC(dist) is required for phase variation, as previously reported. However, our results suggest that removal of competition at GATC(dist) does not affect initial state dependence. Additionally, the PapI involvement in Lrp translocation from GATC(prox) to GATC(dist) is required for the initial state dependence but not for phase variation. Our results also predict that pap expression is maximized at low growth rates and minimized at high growth rates. These predictions provide a basis for further experimental investigation.

Cooperative binding of the leucine-responsive regulatory protein (Lrp) to DNA

The leucine-responsive regulatory protein (Lrp) of Escherichia coli activates expression of a number of operons and represses expression of others. For some members of the Lrp regulon, exogenous leucine mitigates the effect of Lrp, for some it potentiates the effect of Lrp, and for others it has no effect on Lrp action. For the ilvIH operon that we study, Lrp activates expression in vivo and mediates the repression of the operon by exogenous leucine. We studied Lrp-1, a leucine-insensitive variant, to investigate mechanisms by which leucine alters Lrp action as an activator of ilvIH expression. The Asp114Glu change did not have much effect on the amount of total Lrp-1 in cells but decreased the amount of free Lrp-1 two- to threefold. Lrp monomers associate to form octamers and hexadecamers (hexadecamer form predominates at micromolar concentrations; Kd=5.27x10(-8) M), and leucine promotes the dissociation of Lrp hexadecamer to a leucine-bound octamer. By contrast, Lrp-1 exists primarily as an octamer in solution (equilibrium dissociation constant 6.5x10(-5) M) and leucine had little effect on the equilibrium. Thus, the hexadecameric form that Lrp assumes in the absence of DNA is not required for activation of the ilvIH operon. Both leucine and the lrp-1 mutation reduced the apparent affinity of Lrp binding to ilvIH DNA (contains two groups of binding sites separated by 136 bp) but they have different effects on intrinsic binding affinity and binding cooperativity. Whereas leucine reduced intrinsic binding affinities and interactions of Lrps bound at upstream and downstream regions of ilvIH DNA, it increased cooperative dimer-dimer interactions of Lrps bound to two adjacent sites. By contrast, the lrp-1 mutation did not have much effect on intrinsic binding affinities but it decreased cooperative adjacent dimer-dimer interactions and enhanced interactions of Lrps bound at upstream and downstream regions of ilvIH DNA. Our analysis is consistent with the idea that leucine enhances dimer-dimer interactions that contribute to octamer formation, concomitantly reducing dimer-dimer interactions that contribute to the longer range interactions of Lrps that are required for activation of the ilvIH promoter.

Influence of environmental cues on transcriptional regulation of foo and clp coding for F165(1) and CS31A adhesins in Escherichia coli

F165(1) (foo) and CS31A (clp) are bacterial adhesins synthesized by Escherichia coli strains associated with diarrhea and septicemia in piglets and calves. They belong to the P-regulatory family and as such are subject to a phase variation control mediated by Lrp (leucine responsive regulatory protein) and regulators homologous to PapI. Analysis of expression of transcriptional fusions between the fooB or fooI promoters and lacZ showed that Lrp is an activator of foo and fooI transcription, whereas it represses clp transcription. Furthermore, foo phase variation leads to a large majority of phase-ON cells, whereas clp phase variation leads to a majority of phase-OFF cells. We compared the influence of several environmental cues on foo and clp expression, with special attention to the effects of leucine and alanine known to be mediated by Lrp. Inhibition or significant repression of foo and clp transcription was observed at low temperature, in LB medium, and in the presence of glucose, alanine, or leucine. Glucose repression of foo but not of clp was totally relieved by addition of cAMP. Osmolarity and pH had little effect. Alanine but not leucine, and LB medium inhibited foo and clp phase variation, locking cells in the OFF phase. Low temperature inhibited clp phase variation and altered the switch frequency of foo phase variation, leading to more phase-OFF cells. Glucose altered the phase variation of both operons, increasing the number of phase-OFF cells in the population. The regulation pattern of foo and clp is consistent with F165(1) and CS31A production in low nutrient environments, even at moderately acidic pH or high osmolarity.

Escherichia coli evolution during stationary phase

The process of evolution by natural selection has been known for a century and a half, yet the mechanics of selection are still poorly understood. In most cases where natural selection has been studied, the genetic and physiological bases of fitness variation that result in population changes were not identified, leaving only a partial understanding of selection. Starved cultures of the bacterium Escherichia coli present a model system with which to address the genetic and physiological bases of natural selection. This is a model system that also reflects the prevalent state of bacteria in the natural world; due to intense competition for nutrients, microorganisms spend the majority of their lives under starvation conditions. Genetic analyses of a single survivor of starvation identified four adaptive mutations(1). Investigation of these mutations has revealed insights into the molecular and physiological bases of evolution during prolonged starvation stress.

The archaeal feast/famine regulatory protein: potential roles of its assembly forms for regulating transcription

The classification feast/famine regulatory proteins (FFRPs) encompasses archaeal DNA-binding proteins with Escherichia coli transcription factors, the leucine-responsive regulatory protein and the asparagine synthase C gene product. In this paper, we describe two forms of the archaeal FFRP FL11 (pot0434017), both assembled from dimers. When crystallized, a helical cylinder is formed with six dimers per turn. In contrast, in solution, disks are formed, most likely consisting of four dimers each; an observation by cryoelectron microscopy. Whereas each dimer binds a 13-bp sequence, different forms will discriminate between promoters, based on the numbers of repeating 13-bp sequences, and types of linkers inserted between them, which are either of 7-8 or approximately 18 bp. The amino acid sequences of these FFRPs are designed to form the same type of 3D structures, and the transition between their assembly forms is regulated by interaction with small molecules. These considerations lead us to propose a possible mechanism for regulating a number of genes by varying assembly forms and by combining different FFRPs into these assemblies, responding to environmental changes.