Cmr is a redox-responsive regulator of DosR that contributes to M. tuberculosis virulence.

Mycobacterium tuberculosis (MTb) is the causative agent of pulmonary tuberculosis (TB). MTb colonizes the human lung, often entering a non-replicating state before progressing to life-threatening active infections. Transcriptional reprogramming is essential for TB pathogenesis. In vitro, Cmr (a member of the CRP/FNR super-family of transcription regulators) bound at a single DNA site to act as a dual regulator of cmr transcription and an activator of the divergent rv1676 gene. Transcriptional profiling and DNA-binding assays suggested that Cmr directly represses dosR expression. The DosR regulon is thought to be involved in establishing latent tuberculosis infections in response to hypoxia and nitric oxide. Accordingly, DNA-binding by Cmr was severely impaired by nitrosation. A cmr mutant was better able to survive a nitrosative stress challenge but was attenuated in a mouse aerosol infection model. The complemented mutant exhibited a ∼2-fold increase in cmr expression, which led to increased sensitivity to nitrosative stress. This, and the inability to restore wild-type behaviour in the infection model, suggests that precise regulation of the cmr locus, which is associated with Region of Difference 150 in hypervirulent Beijing strains of Mtb, is important for TB pathogenesis.

Genomic mapping of cAMP receptor protein (CRP Mt) in Mycobacterium tuberculosis: relation to transcriptional start sites and the role of CRPMt as a transcription factor.

Chromatin immunoprecipitation identified 191 binding sites of Mycobacterium tuberculosis cAMP receptor protein (CRP(Mt)) at endogenous expression levels using a specific α-CRP(Mt) antibody. Under these native conditions an equal distribution between intragenic and intergenic locations was observed. CRP(Mt) binding overlapped a palindromic consensus sequence. Analysis by RNA sequencing revealed widespread changes in transcriptional profile in a mutant strain lacking CRP(Mt) during exponential growth, and in response to nutrient starvation. Differential expression of genes with a CRP(Mt)-binding site represented only a minor portion of this transcriptional reprogramming with ∼ 19% of those representing transcriptional regulators potentially controlled by CRP(Mt). The subset of genes that are differentially expressed in the deletion mutant under both culture conditions conformed to a pattern resembling canonical CRP regulation in Escherichia coli, with binding close to the transcriptional start site associated with repression and upstream binding with activation. CRP(Mt) can function as a classical transcription factor in M. tuberculosis, though this occurs at only a subset of CRP(Mt)-binding sites.

Cyclic-AMP and bacterial cyclic-AMP receptor proteins revisited: adaptation for different ecological niches.

Escherichia coli cyclic-AMP receptor protein (CRP) represents one of the paradigms of bacterial gene regulation. Yet despite decades of intensive study, new information continues to emerge that prompts reassessment of this classic regulatory system. Moreover, in recent years CRPs from several other bacterial species have been characterized, allowing the general applicability of the CRP paradigm to be tested. Here the properties of the E. coli, Mycobacterium tuberculosis and Pseudomonas putida CRPs are considered in the context of the ecological niches occupied by these bacteria. It appears that the cyclic-AMP-CRP regulatory system has been adapted to respond to distinct external and internal inputs across a broad sensitivity range that is, at least in part, determined by bacterial lifestyles.

Genomics of DNA cytosine methylation in Escherichia coli reveals its role in stationary phase transcription.

DNA cytosine methylation regulates gene expression in mammals. In bacteria, its role in gene expression and genome architecture is less understood. Here we perform high-throughput sequencing of bisulfite-treated genomic DNA from Escherichia coli K12 to describe, for the first time, the extent of cytosine methylation of bacterial DNA at single-base resolution. Whereas most target sites (C(m)CWGG) are fully methylated in stationary phase cells, many sites with an extended CC(m)CWGG motif are only partially methylated in exponentially growing cells. We speculate that these partially methylated sites may be selected, as these are slightly correlated with the risk of spontaneous, non-synonymous conversion of methylated cytosines to thymines. Microarray analysis in a cytosine methylation-deficient mutant of E. coli shows increased expression of the stress response sigma factor RpoS and many of its targets in stationary phase. Thus, DNA cytosine methylation is a regulator of stationary phase gene expression in E. coli.

Global analysis of the regulon of the transcriptional repressor LexA, a key component of SOS response in Mycobacterium tuberculosis.

The DNA damage response is crucial for bacterial survival. The transcriptional repressor LexA is a key component of the SOS response, the main mechanism for the regulation of DNA repair genes in many bacteria. In contrast, in mycobacteria gene induction by DNA damage is carried out by two mechanisms; a relatively small number of genes are thought to be regulated by LexA, and a larger number by an alternate, independent mechanism. In this study we have used ChIP-seq analysis to identify 25 in vivo LexA-binding sites, including nine regulating genes not previously known to be part of this regulon. Some of these binding sites were found to be internal to the predicted open reading frame of the gene they are thought to regulate; experimental analysis has confirmed that these LexA-binding sites regulate the expression of the expected genes, and transcriptional start site analysis has found that their apparent relative location is due to misannotation of these genes. We have also identified novel binding sites for LexA in the promoters of genes that show no apparent DNA damage induction, show positive regulation by LexA, and those encoding small RNAs.

Genomic analysis of DNA binding and gene regulation by homologous nucleoid-associated proteins IHF and HU in Escherichia coli K12.

IHF and HU are two heterodimeric nucleoid-associated proteins (NAP) that belong to the same protein family but interact differently with the DNA. IHF is a sequence-specific DNA-binding protein that bends the DNA by over 160°. HU is the most conserved NAP, which binds non-specifically to duplex DNA with a particular preference for targeting nicked and bent DNA. Despite their importance, the in vivo interactions of the two proteins to the DNA remain to be described at a high resolution and on a genome-wide scale. Further, the effects of these proteins on gene expression on a global scale remain contentious. Finally, the contrast between the functions of the homo- and heterodimeric forms of proteins deserves the attention of further study. Here we present a genome-scale study of HU- and IHF binding to the Escherichia coli K12 chromosome using ChIP-seq. We also perform microarray analysis of gene expression in single- and double-deletion mutants of each protein to identify their regulons. The sequence-specific binding profile of IHF encompasses ∼30% of all operons, though the expression of <10% of these is affected by its deletion suggesting combinatorial control or a molecular backup. The binding profile for HU is reflective of relatively non-specific binding to the chromosome, however, with a preference for A/T-rich DNA. The HU regulon comprises highly conserved genes including those that are essential and possibly supercoiling sensitive. Finally, by performing ChIP-seq experiments, where possible, of each subunit of IHF and HU in the absence of the other subunit, we define genome-wide maps of DNA binding of the proteins in their hetero- and homodimeric forms.

Direct and indirect effects of H-NS and Fis on global gene expression control in Escherichia coli.

Nucleoid-associated proteins (NAPs) are global regulators of gene expression in Escherichia coli, which affect DNA conformation by bending, wrapping and bridging the DNA. Two of these--H-NS and Fis--bind to specific DNA sequences and structures. Because of their importance to global gene expression, the binding of these NAPs to the DNA was previously investigated on a genome-wide scale using ChIP-chip. However, variation in their binding profiles across the growth phase and the genome-scale nature of their impact on gene expression remain poorly understood. Here, we present a genome-scale investigation of H-NS and Fis binding to the E. coli chromosome using chromatin immunoprecipitation combined with high-throughput sequencing (ChIP-seq). By performing our experiments under multiple time-points during growth in rich media, we show that the binding regions of the two proteins are mutually exclusive under our experimental conditions. H-NS binds to significantly longer tracts of DNA than Fis, consistent with the linear spread of H-NS binding from high- to surrounding lower-affinity sites; the length of binding regions is associated with the degree of transcriptional repression imposed by H-NS. For Fis, a majority of binding events do not lead to differential expression of the proximal gene; however, it has a significant indirect effect on gene expression partly through its effects on the expression of other transcription factors. We propose that direct transcriptional regulation by Fis is associated with the interaction of tandem arrays of Fis molecules to the DNA and possible DNA bending, particularly at operon-upstream regions. Our study serves as a proof-of-principle for the use of ChIP-seq for global DNA-binding proteins in bacteria, which should become significantly more economical and feasible with the development of multiplexing techniques.

Recruitment of the earliest component of the bacterial flagellum to the old cell division pole by a membrane-associated signal recognition particle family GTP-binding protein.

The specialised signal recognition particle family guanosine 5c-triphosphate (GTP)-binding protein FlhF is required for the correct localisation of flagella in several bacterial species. Here, we characterise the regions of Vibrio cholerae FlhF that are required for its function and targeting to the old cell pole, and we present evidence for a mechanism by which FlhF establishes flagellum polar localisation. Substitution of residues in FlhF nucleotide-binding motifs reduced GTP binding and the efficiency of flagellum biogenesis, and caused flagellum mislocalisation. However, replacement of conserved putative catalytic residues (D(321), R(324), and Q(330)) had no effect, suggesting that while GTP binding influences FlhF function, GTPase activity might not be essential. FlhF associated with the inner membrane in the absence of other flagellar proteins, and a functional FlhF-green fluorescent protein fusion was targeted to the old cell pole where the flagellum is localised. FlhF targeting to the pole was intrinsic, as no other flagellar proteins were needed. Neither the FlhF C-terminal GTP-binding region nor the N-terminal 166-residue B-region was required for polar localisation, though they were essential for FlhF function. Deletion of the central 108-residue N-region of FlhF, comprising alpha-helices N1-N4, did however severely reduce the efficiency of FlhF polar targeting, as well as FlhF function. The intrinsic localisation of FlhF to the old cell pole membrane suggested that FlhF might function at an early stage of flagellum assembly; to test this, we assessed the effect of FlhF on the localisation of the earliest flagellar structural component, the membrane-supramembrane ring protein FliF. Recruitment of FliF to the pole required only FlhF and no other flagellar proteins. FliF polar targeting was abolished in the absence of FlhF and by deletion of the FlhF B-domain or GTP-binding region. Our data indicate that FlhF establishes the site of flagellum assembly at the old cell pole membrane by recruiting the earliest flagellar structural component FliF.

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

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