PhoP-PhoQ-regulated loci are required for enhanced bile resistance in Salmonella spp. (1/1176)

As enteric pathogens, Salmonella spp. are resistant to the actions of bile. Salmonella typhimurium and Salmonella typhi strains were examined to better define the bile resistance phenotype. The MICs of bile for wild-type S. typhimurium and S. typhi were 18 and 12%, respectively, and pretreatment of log-phase S. typhimurium with 15% bile dramatically increased bile resistance. Mutant strains of S. typhimurium and S. typhi lacking the virulence regulator PhoP-PhoQ were killed at significantly lower bile concentrations than wild-type strains, while strains with constitutively active PhoP were able to survive prolonged incubation with bile at concentrations of >60%. PhoP-PhoQ was shown to mediate resistance specifically to the bile components deoxycholate and conjugated forms of chenodeoxycholate, and the protective effect was not generalized to other membrane-active agents. Growth of both S. typhimurium and S. typhi in bile and in deoxycholate resulted in the induction or repression of a number of proteins, many of which appeared identical to PhoP-PhoQ-activated or -repressed products. The PhoP-PhoQ regulon was not induced by bile, nor did any of the 21 PhoP-activated or -repressed genes tested play a role in bile resistance. However, of the PhoP-activated or -repressed genes tested, two (prgC and prgH) were transcriptionally repressed by bile in the medium independent of PhoP-PhoQ. These data suggest that salmonellae can sense and respond to bile to increase resistance and that this response likely includes proteins that are members of the PhoP regulon. These bile- and PhoP-PhoQ-regulated products may play an important role in the survival of Salmonella spp. in the intestine or gallbladder.  (+info)

Induction of the soxRS regulon of Escherichia coli by superoxide. (2/1176)

The soxRS regulon orchestrates a multifaceted defense against oxidative stress, by inducing the transcription of approximately 15 genes. The induction of this regulon by redox agents, known to mediate O-2 production, led to the view that O-2 is one signal to which it responds. However, redox cycling agents deplete cellular reductants while producing O-2, and one may question whether the regulon responds to the depletion of some cytoplasmic reductant or to O-2, or both. We demonstrate that raising [O-2] by mutational deletion of superoxide dismutases and/or by addition of paraquat, both under aerobic conditions, causes induction of a member of the soxRS regulon and that a mutational defect in soxRS eliminates that induction. This establishes that O-2, directly or indirectly, can cause induction of this defensive regulon.  (+info)

Mutational analysis of the phoD promoter in Bacillus subtilis: implications for PhoP binding and promoter activation of Pho regulon promoters. (3/1176)

The PhoP-PhoR two-component regulatory system controls the phosphate deficiency response in B. subtilis. A number of Pho regulon genes which require PhoP approximately P for activation or repression have been identified. The studies reported here were initiated to understand the PhoP-DNA interaction necessary for Pho promoter regulation. The regulatory region of phoD was characterized in detail using oligo-directed mutagenesis, DNase I footprinting, and in vivo transcription assays. These data reveal basic principles of PhoP binding relevant to PhoP's interaction with other Pho regulon promoters. Our results show that: (i) a dimer of PhoP approximately P is able to bind two consensus repeats in a stable fashion; (ii) PhoP binding is highly cooperative within the core promoter region, which is located from -66 to -17 on the coding strand and contains four TT(A/T/C)ACA-like repeats; (iii) specific bases comprising the TT(A/T/C)ACA consensus are essential for transcriptional activation, but the specific base pairs of the intervening sequences separating the consensus repeats are not important for either PhoP binding or promoter activation; (iv) the spacing between two consensus repeats within a putative dimer binding site in the core region is important for both PhoP binding and promoter activation; (v) the exact spacing between two dimer binding sites within the core region is important for promoter activation but less so for PhoP binding affinity, as long as the repeats are on the same face of the helix; and (vi) the 5' secondary binding region is important for coordinated PhoP binding to the core binding region, making it nearly essential for promoter activation.  (+info)

Nitroreductase A is regulated as a member of the soxRS regulon of Escherichia coli. (4/1176)

Nitroreductase A catalyzes the divalent reduction of nitro compounds, quinones, and dyes by NADPH. In this paper, nitroreductase A is induced in Escherichia coli by exposure to paraquat in a manner that depends on the expression of soxR. Nitroreductase activity was only slightly induced by paraquat in a strain bearing a mutational defect in the gene encoding nitroreductase A, but it was approximately 3-fold induced in the parental strain. Nitroreductase A thus appears to be a member of the soxRS regulon and probably contributes to the defenses against oxidative stress by minimizing the redox cycling attendant upon the univalent reduction of nitro compounds, quinones, and dyes.  (+info)

Acarbose, a pseudooligosaccharide, is transported but not metabolized by the maltose-maltodextrin system of Escherichia coli. (5/1176)

The pseudooligosaccharide acarbose is a potent inhibitor of amylases, glucosidases, and cyclodextrin glycosyltransferase and is clinically used for the treatment of so-called type II or insulin-independent diabetes. The compound consists of an unsaturated aminocyclitol, a deoxyhexose, and a maltose. The unsaturated aminocyclitol moiety (also called valienamine) is primarily responsible for the inhibition of glucosidases. Due to its structural similarity to maltotetraose, we have investigated whether acarbose is recognized as a substrate by the maltose/maltodextrin system of Escherichia coli. Acarbose at millimolar concentrations specifically affected the growth of E. coli K-12 on maltose as the sole source of carbon and energy. Uptake of radiolabeled maltose was competitively inhibited by acarbose, with a Ki of 1.1 microM. Maltose-grown cells transported radiolabeled acarbose, indicating that the compound is recognized as a substrate. Studying the interaction of acarbose with purified maltoporin in black lipid membranes revealed that the kinetics of acarbose binding to LamB is asymmetric. The on-rate of acarbose is approximately 30 times lower when the molecule enters the pore from the extracellular side than when it enters from the periplasmic side. Acarbose could not be utilized as a carbon source since the compound alone was not a substrate of amylomaltase (MalQ) and was only poorly attacked by maltodextrin glucosidase (MalZ).  (+info)

In vivo transcription of the Escherichia coli oxyR regulon as a function of growth phase and in response to oxidative stress. (6/1176)

Simultaneous expression of seven genes in Escherichia coli was measured by a reverse transcription-multiplex PCR fluorescence procedure. Genes studied were (i) oxyR (transcriptional regulator); (ii) katG, dps, gorA, and ahpCF (controlled by OxyR); (iii) sodA (controlled by SoxRS); and (iv) trxA (not related to OxyR or SoxRS). Except for trxA, transcription of all genes was activated during the course of growth of wild-type bacteria, though notable variations were observed with respect to both the time and extent of activation. Whereas oxyR, katG, dps, and gorA were activated during exponential growth, ahpCF and sodA were stimulated in stationary phase. Maximal induction ranged from 4.6- to 86.5-fold, for gorA and dps, respectively. Treatment with H2O2 stimulated expression of the genes (katG, dps, ahpCF, and gorA) previously identified as members of the OxyR regulon, except for oxyR itself. Induction by H2O2 was a remarkably rapid and reversible process that took place in an OxyR-dependent and sigmaS-independent manner. NaCl induced expression of the genes controlled by OxyR, including the oxyR locus. This transcriptional up-regulation was preserved in a strain with the DeltaoxyR::kan mutation, but it was abolished (ahpCF) or significantly reduced (oxyR and dps) in a strain with the rpoS::Tn10 mutation, potentially reflecting positive transcriptional regulation of the oxyR regulon by sigmaS. Expression of trxA was not increased either by H2O2 stress or by a shift to high-osmolarity conditions.  (+info)

Identification and transcriptional analysis of new members of the sigmaB regulon in Bacillus subtilis. (7/1176)

Bacillus subtilis responds to various stimuli (heat, ethanol and salt stress, energy starvation) with the induction of general stress proteins (GSPs). Most of them belong to the stress and stationary-phase regulon controlled by the alternative sigma factor sigmaB. The majority of sigmaB-dependent proteins are thought to provide a precautionary general stress resistance in stressed or starved cells. In this report, the identification and transcriptional analysis of nine new members of the sigmaB regulon are described. The biochemical function was not determined for any of the proteins encoded by the nine new sigmaB-dependent stress genes, however, similarities to proteins in the databases allowed a distinction between proteins with putative (i-iv) and unknown (v) function. The putative functions of BmrU, YcdF, YdaD, YdaP, YhdN and YocK underline the suggested protective role of sigmaB-dependent GSPs and also elucidate new areas where sigmaB might play an important role. (i) The finding that the bmrUR operon is under sigmaB control indicates that the elimination of multidrug compounds might be a new function in multiple stress resistance. (ii) YcdF and YdaD resemble NAD(P)-dependent dehydrogenases. Both proteins could be involved in the generation of NAD(P)H and therefore in the maintenance of the intracellular redox balance under stress. (iii) The ydaP gene might belong to the increasing number of sigmaB-dependent genes whose orthologues are under the control of sigmas in Escherichia coli, indicating that both regulons may fulfil similar functions. (iv) YhdN shows weak similarities to potassium ion channel proteins and YocK shows resemblance to the DnaK suppressor protein DksA. (v) Three new sigmaB-dependent genes (ydaE, ydaG and yfkM) encoding proteins with still unknown functions were also described. Further analyses of corresponding mutants might allow a first prediction of their function within the framework of the general stress regulon.  (+info)

A HilA-independent pathway to Salmonella typhimurium invasion gene transcription. (8/1176)

Salmonella typhimurium invasion of nonphagocytic cells requires the expression of a type III secretion system (TTSS) encoded within Salmonella pathogenicity island 1 (SPI1). TTSS gene transcription is activated in response to environmental signals and requires transcriptional regulators encoded within (HilA) and outside (SirA) SPI1. Two unique loci, sirB and sirC, which contribute to SPI1 gene transcription were defined. sirC is an SPI1-encoded transcription factor of the AraC family that contributes to the invasive phenotype. sirB is required for maximal expression of sirC and consists of two open reading frames located near kdsA, a gene involved in lipopolysaccharide biosynthesis. sirC expression, unlike expression of other SPI1 genes, does not require HilA. Overexpression of sirC or sirA restores expression of a subset of SPI1 genes, including invF and sspC, in the absence of HilA. These data define roles for SirC and SirA as part of a HilA-independent pathway to SPI1 gene expression. We postulate that HilA-independent activation of inv expression is important for efficient assembly and function of the SPI1 TTSS.  (+info)

• 1
• 2
• 3
• 4
• 5
• 6
• 7
• 8
• 9
• 10