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.

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 (Kd [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 (Kd, 1.0 to 1.5 µM), and the highest affinity was at pH 5.0 (Kd, 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 Transcriptional Regulator Lrp Contributes to Toxin Expression, Sporulation, and Swimming Motility in Clostridium difficile.

Clostridium difficile is a Gram-positive, spore-forming bacterium, and major cause of nosocomial diarrhea. Related studies have identified numerous factors that influence virulence traits such as the production of the two primary toxins, toxin A (TcdA) and toxin B (TcdB), as well as sporulation, motility, and biofilm formation. However, multiple putative transcriptional regulators are reportedly encoded in the genome, and additional factors are likely involved in virulence regulation. Although the leucine-responsive regulatory protein (Lrp) has been studied extensively in Gram-negative bacteria, little is known about its function in Gram-positive bacteria, although homologs have been identified in the genome. This study revealed that disruption of the lone lrp homolog in C. difficile decelerated growth under nutrient-limiting conditions, increased TcdA and TcdB production. Lrp was also found to negatively regulate sporulation while positively regulate swimming motility in strain R20291, but not in strain 630. The C. difficile Lrp appeared to function through transcriptional repression or activation. In addition, the lrp mutant was relatively virulent in a mouse model of infection. The results of this study collectively demonstrated that Lrp has broad regulatory function in C. difficile toxin expression, sporulation, motility, and pathogenesis.

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.

New Targets and Cofactors for the Transcription Factor LrpA from Mycobacterium tuberculosis.

Rv3291c (MtbLrpA), a transcriptional regulator, belongs to the leucine-responsive regulatory protein (Lrp) family and is thought to play an important role in Mycobacterium tuberculosis persistence. In this study, we verified 17 novel potential binding sites for MtbLrpA by in vitro binding assays on the basis of previous predictions from an in silico analysis and bacterial one-hybrid (BIH) reporter system. Amino acids, such as tyrosine, phenylalanine, tryptophan, and histidine, strongly affect the binding affinity of MtbLrpA, and vitamins, including B1, B3, B6, VC, B7, B9, B12, VA, and VK3, also decrease MtbLrpA binding affinity. This is the first report regarding that an Lrp-like protein can sense vitamins as an environmental signal. Vitamin supplementation to the environment can change the expression level of the target genes, which provides a potential mechanism for tuberculosis supplementary treatment with vitamins.

DNA adenine methyltransferase facilitated diffusion is enhanced by protein-DNA "roadblock" complexes that induce DNA looping.

The genomes of all cells are intimately associated with proteins, which are important for compaction, scaffolding, and gene regulation. Here we show that pre-existing protein-DNA complexes (roadblocks) diminish and-interestingly-enhance the ability of particular sequence-specific proteins to move along DNA to locate their binding sites. We challenge the bacterial DNA adenine methyltransferase (Dam, recognizes 5'-GATC-3') with tightly bound EcoRV ENase-DNA complexes, which bend DNA. A single EcoRV roadblock does not alter processive (multiple modifications) methylation by Dam. This result disfavors a reliance on heavily touted mechanisms involving sliding or short hops for Dam. Specific conformations of two EcoRV roadblocks cause an increase in processivity. The histone-like leucine-responsive regulatory protein (Lrp) binds DNA nonspecifically as an octamer, and also increases Dam's processivity. These results can be explained by our prior demonstration that Dam moves over large regions (>300 bp) within a single DNA molecule using an "intersegmental hopping" mechanism. This mechanism involves the protein hopping between looped DNA segments. Both roadblock systems can cause the DNA to loop and therefore facilitate intersegmental hopping. For Lrp, this only occurs when the Dam sites are separated (by >134bp) such that they can be looped around the protein. Intersegmental hopping may well be a general mechanism for proteins that navigate long distances along compacted DNA. Unlike Dam, EcoRI ENase (recognizes 5'-GAATTC-3') relies extensively on a sliding mechanism, and as expected, Lrp decreases its processivity. Our systematic use of protein roadblocks provides a powerful strategy to differentiate between site location mechanisms.

How to build a pathogen detector: structural basis of NB-LRR function.

Many plant disease resistance (R) proteins belong to the family of nucleotide-binding-leucine rich repeat (NB-LRR) proteins. NB-LRRs mediate recognition of pathogen-derived effector molecules and subsequently activate host defence. Their multi-domain structure allows these pathogen detectors to simultaneously act as sensor, switch and response factor. Structure-function analyses and the recent elucidation of the 3D structures of subdomains have provided new insight in how these different functions are combined and what the contribution is of the individual subdomains. Besides interdomain contacts, interactions with chaperones, the proteasome and effector baits are required to keep NB-LRRs in a signalling-competent, yet auto-inhibited state. In this review we explore operational models of NB-LRR functioning based on recent advances in understanding their structure.

The development and application of a single-cell biosensor for the detection of l-methionine and branched-chain amino acids.

The detection and quantification of specific metabolites in single bacterial cells is a major goal for industrial biotechnology. We have developed a biosensor based on the transcriptional regulator Lrp that detects intracellular l-methionine and branched-chain amino acids in Corynebacterium glutamicum. In assays, fluorescence output showed a linear relationship with cytoplasmic concentrations of the effector amino acids. In increasing order, the affinity of Lrp for the amino acids is l-valine, l-isoleucine, l-leucine and l-methionine. The sensor was applied for online monitoring and analysis of cell-to-cell variability of l-valine production by the pyruvate dehydrogenase-deficient C. glutamicum strain ΔaceE. Finally, the sensor system was successfully used in a high-throughput (HT) FACS screen for the isolation of amino acid-producing mutants after random mutagenesis of a non-producing wild type strain. These applications illustrate how one of nature's sensor devices - transcriptional regulators - can be used for the analysis, directed evolution and HT screening for microbial strain development.

Molecular and spatial constraints on NB-LRR receptor signaling.

In plants, a large polymorphic family of intracellular NB-LRR receptors lies at the heart of robust resistance to diverse pathogens and mechanisms by which these versatile molecular switches operate in effector-triggered immunity are beginning to emerge. We outline recent advances in our understanding of NB-LRR receptor signaling leading to disease resistance. Themes covered are (i) NB-LRR molecular constraining forces and their intimate relationship with receptor activation in different parts of the cell, (ii) cooperativity between NB-LRR proteins and the formation of higher order NB-LRR signaling complexes, and (iii) the spatial separation of different resistance branches within cells. Finally, we examine evidence for dynamic signaling across cell compartments in coordinating diverse immune outputs.

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.

Regulatory and pathogenesis roles of Mycobacterium Lrp/AsnC family transcriptional factors.

Lrp/AsnC (leucine-responsive regulatory protein/asparagine synthase C products) family transcriptional regulators, widespread among bacteria and archaea, is also known as feast/famine regulatory protein (FFRPs). They regulate multiple cellular metabolisms globally (Lrp) or specifically (AsnC), such as amino acid metabolism, pili synthesis, DNA transactions during DNA repair and recombination, and also might be implicated in persistence. To better understanding of the pathogenesis of M. tuberculosis, based on our lab's work on this transcriptional factor family, these progresses are summarized, with special focus on that of Mycobacterium via comparative genomics.

The Lrp family of transcription regulators in archaea.

Archaea possess a eukaryotic-type basal transcription apparatus that is regulated by bacteria-like transcription regulators. A universal and abundant family of transcription regulators are the bacterial/archaeal Lrp-like regulators. The Lrp family is one of the best studied regulator families in archaea, illustrated by investigations of proteins from the archaeal model organisms: Sulfolobus, Pyrococcus, Methanocaldococcus, and Halobacterium. These regulators are extremely versatile in their DNA-binding properties, response to effector molecules, and molecular regulatory mechanisms. Besides being involved in the regulation of the amino acid metabolism, they also regulate central metabolic processes. It appears that these regulatory proteins are also involved in large regulatory networks, because of hierarchical regulations and the possible combinatorial use of different Lrp-like proteins. Here, we discuss the recent developments in our understanding of this important class of regulators.

Hybrid Ptr2-like activators of archaeal transcription.

Methanocaldococcus jannaschii Ptr2, a member of the Lrp/AsnC family of bacterial DNA-binding proteins, is an activator of its eukaryal-type core transcription apparatus. In Lrp-family proteins, an N-terminal helix-turn-helix DNA-binding and dimerizing domain is joined to a C-terminal effector and multimerizing domain. A cysteine-scanning surface mutagenesis shows that the C-terminal domain of Ptr2 is responsible for transcriptional activation; two types of DNA binding-positive but activation-defective mutants are found: those unable to recruit the TBP and TFB initiation factors to the promoter, and those failing at a post-recruitment step. Transcriptional activation through the C-terminal Ptr2 effector domain is exploited in a screen of other Lrp effector domains for activation capability by constructing hybrid proteins with the N-terminal DNA-binding domain of Ptr2. Two hybrid proteins are effective activators: Ptr-H10, fusing the effector domain of Pyrococcus furiosus LrpA, and Ptr-H16, fusing the P. furiosus ORF1231 effector domain. Both new activators exhibit distinguishing characteristics: unlike octameric Ptr2, Ptr-H10 is a dimer; unlike Ptr2, the octameric Ptr-H16 poorly recruits TBP to the promoter, but more effectively co-recruits TFB with TBP. In contrast, the effector domain of Ptr1, the M. jannaschii Ptr2 paralogue, yields only very weak activation.

Limited functional conservation of a global regulator among related bacterial genera: Lrp in Escherichia, Proteus and Vibrio.

BACKGROUND: Bacterial genome sequences are being determined rapidly, but few species are physiologically well characterized. Predicting regulation from genome sequences usually involves extrapolation from better-studied bacteria, using the hypothesis that a conserved regulator, conserved target gene, and predicted regulator-binding site in the target promoter imply conserved regulation between the two species. However many compared organisms are ecologically and physiologically diverse, and the limits of extrapolation have not been well tested. In E. coli K-12 the leucine-responsive regulatory protein (Lrp) affects expression of approximately 400 genes. Proteus mirabilis and Vibrio cholerae have highly-conserved lrp orthologs (98% and 92% identity to E. coli lrp). The functional equivalence of Lrp from these related species was assessed. RESULTS: Heterologous Lrp regulated gltB, livK and lrp transcriptional fusions in an E. coli background in the same general way as the native Lrp, though with significant differences in extent. Microarray analysis of these strains revealed that the heterologous Lrp proteins significantly influence only about half of the genes affected by native Lrp. In P. mirabilis, heterologous Lrp restored swarming, though with some pattern differences. P. mirabilis produced substantially more Lrp than E. coli or V. cholerae under some conditions. Lrp regulation of target gene orthologs differed among the three native hosts. Strikingly, while Lrp negatively regulates its own gene in E. coli, and was shown to do so even more strongly in P. mirabilis, Lrp appears to activate its own gene in V. cholerae. CONCLUSION: The overall similarity of regulatory effects of the Lrp orthologs supports the use of extrapolation between related strains for general purposes. However this study also revealed intrinsic differences even between orthologous regulators sharing >90% overall identity, and 100% identity for the DNA-binding helix-turn-helix motif, as well as differences in the amounts of those regulators. These results suggest that predicting regulation of specific target genes based on genome sequence comparisons alone should be done on a conservative basis.

Physical organization and phylogenetic analysis of acdR as leucine-responsive regulator of the 1-aminocyclopropane-1-carboxylate deaminase gene acdS in phytobeneficial Azospirillum lipoferum 4B and other Proteobacteria.

The phytostimulatory alphaproteobacterium Azospirillum lipoferum 4B exhibits the plant-beneficial gene acdS, which enables deamination of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Here, we show that acdS is in the vicinity of acdR, a homolog to leucine-responsive regulator lrp, in A. lipoferum 4B and most other acdS+ Proteobacteria. Unlike in Beta- and Gammaproteobacteria, acdS (and acdR) is preferentially located on symbiotic islands and plasmids in Alphaproteobacteria. In A. lipoferum 4B, acdS was mapped on a 750-kb plasmid that is lost during phenotypic variation, whereas other phytobeneficial genes such as nifH (associative nitrogen fixation) are maintained. In Proteobacteria, the phylogenies of acdR and acdS were largely but not totally congruent, despite physical proximity of the genes, regardless of whether DNA or deduced protein sequences were used. Potential Lrp, cAMP receptor protein (CRP) and fumarate-nitrate reduction regulator (FNR) binding sites were evidenced in the acdS promoter regions of strain 4B and most of 46 other acdS+ Proteobacteria. Indeed, transcriptional and enzymatic analyses done in vitro pointed to the involvement of Lrp- and FNR-like transcriptional up-regulation of ACC deaminase activity in A. lipoferum 4B. This is the first synteny, phylogenetic, and functional analysis of factors modulating acdS expression in Azospirillum plant growth-promoting rhizobacterium.

Crystal structure of Mycobacterium tuberculosis LrpA, a leucine-responsive global regulator associated with starvation response.

The bacterial leucine-responsive regulatory protein (Lrp) is a global transcriptional regulator that controls the expression of many genes during starvation and the transition to stationary phase. The Mycobacterium tuberculosis gene Rv3291c encodes a 150-amino acid protein (designated here as Mtb LrpA) with homology with Escherichia coli Lrp. The crystal structure of the native form of Mtb LrpA was solved at 2.1 A. The Mtb LrpA structure shows an N-terminal DNA-binding domain with a helix-turn-helix (HTH) motif, and a C-terminal regulatory domain. In comparison to the complex of E. coli AsnC with asparagine, the effector-binding pocket (including loop 100-106) in LrpA appears to be largely preserved, with hydrophobic substitutions consistent with its specificity for leucine. The effector-binding pocket is formed at the interface between adjacent dimers, with an opening to the core of the octamer as in AsnC, and an additional substrate-access channel opening to the outer surface of the octamer. Using electrophoretic mobility shift assays, purified Mtb LrpA protein was shown to form a protein-DNA complex with the lat promoter, demonstrating that the lat operon is a direct target of LrpA. Using computational analysis, a putative motif is identified in this region that is also present upstream of other operons differentially regulated under starvation. This study provides insights into the potential role of LrpA as a global regulator in the transition of M. tuberculosis to a persistent state.

Purification, crystallization and preliminary X-ray crystallographic analysis of ST1022, a putative member of the Lrp/AsnC family of transcriptional regulators isolated from Sulfolobus tokodaii strain 7.

The Lrp/AsnC family of transcriptional regulators, also known as feast/famine transcriptional regulators, are widely distributed among bacteria and archaea. This family of proteins are likely to be involved in cellular metabolism, with exogenous amino acids functioning as effectors. Here, the crystallization and preliminary X-ray diffraction analysis of ST1022, a member of the Lrp/AsnC family of proteins, is reported with and without exogenous glutamine as the effector molecule. The crystals of native ST1022 and of the putative complex belong to the tetragonal space group I422, with unit-cell parameters a = b = 103.771, c = 73.297 A and a = b = 103.846, c = 73.992 A, respectively. Preliminary X-ray diffraction data analysis and molecular-replacement solution revealed the presence of one monomer per asymmetric unit.

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.

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.

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.