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Chromobacterium violaceum presents a distinctive phenotypic characteristic, the production of a deep violet pigment named violacein. Although the physiological function of this pigment is not well understood, the sequencing of the genome of this bacterium has given some insight into the mechanisms and control of violacein production. It was found that erythrose-4-phosphate (E4P), a precursor to aromatic amino acid biosynthesis, is produced by the non-oxidative portion of the hexose monophosphate pathway, since it lacks 6-phosphogluconate dehydrogenase. All genes leading from E4P plus phosphoenolpyruvate to tryptophan are present in the genome. Nevertheless, these genes are not organized in an operon, as in E. coli, indicating that other mechanisms are involved in expression. The sequencing data also indicated the presence and organization of an operon for violacein biosynthesis. Three of the four gene products of this operon presented similarity with nucleotide-dependent monooxygenases and one with a limiting enzyme polyketide synthase. As previously suggested, genes encoding proteins involved in quorum sensing control by N-hexanoyl-homoserine-lactone, an autoinducer signal molecule, are present in the bacterial genome. These data should help guide strategies to increase violacein biosynthesis, a potentially useful molecule.
Genet Mol Res 2004 Mar 31
PMID:Genetic analysis of violacein biosynthesis by Chromobacterium violaceum. 1510 Sep 90

Phenazine antibiotic production in the biological control bacterium Pseudomonas aureofaciens 30-84 is regulated in part via the PhzR/PhzI N-acyl homoserine lactone (AHL) system. Previous work showed that a subpopulation of the wheat rhizosphere community positively affected phenazine gene expression in strain 30-84 via AHL signals (E. A. Pierson, D. W. Wood, J. A. Cannon, F. M. Blachere, and L. S. Pierson III, Mol. Plant-Microbe Interact. 11:1078-1084, 1998). In the present work, a second subpopulation, one that negatively affected phenazine gene expression, was identified from this rhizosphere community. Strain 30-84 grown in conditioned medium (CM) from several strains produced lower levels of phenazines (1.5- to 9.3-fold) than control when grown in CM from the strain 30-84I(1)/I(2). Growth of the phzB::lacZ reporter strain 30-84Z in this CM resulted in decreased lacZ expression (4.3- to 9.2-fold) compared to growth of the control strain in CM, indicating that inhibition of phzB occurred at the level of gene expression. Preliminary chemical and biological characterizations suggested that these signals, unlike other identified negative signals, were not extractable in ethyl acetate. Introduction of extra copies of phzR and phzI, but not phzI alone, in trans into strain 30-84Z reduced the negative effect on phzB::lacZ expression. The presence of negative-signal-producing strains in a mixture with strain 30-84 reduced strain 30-84's ability to inhibit the take-all disease pathogen in vitro. Together, the results from the previous work on the positive-signal subpopulation and the present work on the negative-signal subpopulation suggest that cross-communication among members of the rhizosphere community and strain 30-84 may control secondary metabolite production and pathogen inhibition.
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PMID:Negative cross-communication among wheat rhizosphere bacteria: effect on antibiotic production by the biological control bacterium Pseudomonas aureofaciens 30-84. 1512 73

Pseudomonas syringae forms large cell aggregates that are more stress tolerant than solitary cells during epiphytic growth on plants. The differential survival of aggregates on leaves suggests that epiphytic fitness traits may be controlled in a density-dependent manner via cell-cell signaling. We investigated this hypothesis in P. syringae B728a. Synthesis of N-acyl-homoserine lactone (AHL), 3-oxo-hexanoyl homoserine lactone, and the expression of the gene encoding AHL synthase ahlI were maximal at high cell concentrations. The expression of the AHL regulator ahlR, in contrast, was similar at all cell concentrations. A screen of Tn5 mutants revealed that P. syringae B728a requires a novel transcriptional activator for AHL production. This regulator, which belongs to the TetR family, was also required for epiphytic fitness and has been designated AefR (for AHL and epiphytic fitness regulator). The expression of ahlI was greatly reduced in both aefR- and gacA- mutants and was completely restored in either mutant after addition of exogenous AHL. In contrast, the expression of aefR was not reduced in either gacS- or gacA- mutants. Thus, AefR appears to positively regulate AHL production independently of the regulators GacS/GacA and also controls traits in P. syringae B728a that are required for epiphytic colonization.
Mol Plant Microbe Interact 2004 May
PMID:Regulation of AHL production and its contribution to epiphytic fitness in Pseudomonas syringae. 1514 56

A unique signal degradation system has recently been discovered in Agrobacterium tumefaciens. Upon entering stationary phase, A. tumefaciens terminates quorum sensing-dependent Ti-plasmid conjugation by degradation of acyl homoserine lactone (AHL) quormone via the enzyme AttM (AHL-lactonase). attM, together with attK and attL, constitute one transcriptional unit subjected to the control of a common promoter. AttJ, the other member of the signal degradation system, is an IclR-like negative transcriptional factor, which tightly represses the expression of AttM at the early stage of bacterial growth. In this study, we found that this quormone degradation system is activated by either carbon or nitrogen starvation. Quormone degradation was significantly delayed when bacterial culture was supplemented with extra carbon or nitrogen source in the nutrient-limited minimal medium before the onset of stationary phase. To identify the signalling pathway and regulatory mechanisms that mediate quormone degradation, we constructed a reporter strain A6(attKLM::lacZ) in which the promoterless lacZ was transcriptionally fused to the attKLM promoter. Transposon mutagenesis of strain A6(attKLM::lacZ) led to identification of the relA gene, which encodes the stress alarmone (p)ppGpp synthetase. Tn5 knock-out of relA abolished the stationary phase-dependent expression of attM. We concluded that the A. tumefaciens quormone degradation system is coupled to and regulated by the generic (p)ppGpp stress response machinery.
Mol Microbiol 2004 Jun
PMID:The quormone degradation system of Agrobacterium tumefaciens is regulated by starvation signal and stress alarmone (p)ppGpp. 1516 41

Cell-cell communication via the production and detection of chemical signal molecules has been the focus of a great deal of research over the past decade. One class of chemical signals widely used by proteobacteria consists of N-acyl-homoserine lactones, which are synthesized by proteins related to LuxI of Vibrio fischeri and are detected by proteins related to the V. fischeri LuxR protein. A related marine bacterium, Vibrio harveyi, communicates using two chemical signals, one of which, autoinducer-2 (AI-2), is a furanone borate diester that is synthesized by the LuxS protein and detected by a periplasmic protein called LuxP. Evidence from a number of laboratories suggests that AI-2 may be used as a signal by diverse groups of bacteria, and might permit intergeneric signalling. These two families of signalling systems have been studied from the perspectives of physiology, ecology, biochemistry, and more recently, structural biology. Here, we review the biochemistry and structural biology of both acyl-homoserine-lactone-dependent and AI-2-dependent signalling systems.
Mol Microbiol 2004 Aug
PMID:Chemical communication in proteobacteria: biochemical and structural studies of signal synthases and receptors required for intercellular signalling. 1525 90

Many gram-negative bacteria employ N-acylhomoserine lactones (AHL) to regulate diverse physiological processes in concert with cell population density (quorum sensing [QS]). In the plant pathogen Erwinia carotovora, the AHL synthesized via the carI/expI genes are responsible for regulating the production of secreted plant cell wall-degrading exoenzymes and the antibiotic carbapen-3-em carboxylic acid. We have previously shown that targeting the product of an AHL synthase gene (yenI) from Yersinia enterocolitica to the chloroplasts of transgenic tobacco plants caused the synthesis in planta of the cognate AHL signaling molecules N-(3-oxohexanoyl)-L-homoserine lactone (3-oxo-C6-HSL) and N-hexanoylhomoserine lactone (C6-HSL), which in turn, were able to complement a carI-QS mutant. In the present study, we demonstrate that transgenic potato plants containing the yenI gene are also able to express AHL and that the presence and level of these AHL in the plant increases susceptibility to infection by E. carotovora. Susceptibility is further affected by both the bacterial level and the plant tissue under investigation.
Mol Plant Microbe Interact 2004 Aug
PMID:Potato plants genetically modified to produce N-acylhomoserine lactones increase susceptibility to soft rot erwiniae. 1530 9

Pseudomonas aeruginosa and other bacterial pathogens express one or more homologous extracellular phospholipases C (PLC) that are secreted through the inner membrane via the twin arginine translocase (TAT) pathway. Analysis of TAT mutants of P. aeruginosa uncovered a previously unidentified extracellular PLC that is secreted via the Sec pathway (PlcB). Whereas all presently known PLCs of P. aeruginosa (PlcH, PlcN and PlcB) hydrolyse phosphatidylcholine (PC), only PlcB is active on phosphatidylethanolamine (PE). plcB candidates were identified based on deductions made from bioinformatics data and extant DNA microarray data. Among these candidates, a gene (PA0026) required for the expression of an extracellular PE-PLC was identified. The protein encoded by PA0026 has limited, but significant similarity, over a short region (approximately 60aa of 328), to a class of zinc-dependent prokaryotic PLCs. A conserved His residue of PlcB (His216) that is required for coordinate binding of zinc in this class of PLCs was mutated. Analysis of this mutant established that the protein encoded by PA0026 is PlcB. Three in-dependent recently published reports indicate that homoserine lactone-mediated quorum sensing regulates the expression of PA0026 (i.e. plcB). PlcB, but not PlcH or PlcN, is required for directed twitching motility up a gradient of certain kinds of phospholipids. This response shows specificity for the fatty acid moiety of the phospholipid.
Mol Microbiol 2004 Aug
PMID:A novel extracellular phospholipase C of Pseudomonas aeruginosa is required for phospholipid chemotaxis. 1530 13

The LasI/LasR quorum-sensing system plays a pivotal role in virulence gene regulation of the opportunistic human pathogen, Pseudomonas aeruginosa. Here we report the crystal structure of the acyl-homoserine lactone (AHL) synthase LasI that produces 3-oxo-C12-AHL from the substrates 3-oxo-C12-acyl-carrier protein (acyl-ACP) and S-adenosyl-L-methionine. The LasI six-stranded beta sheet platform, buttressed by three alpha helices, forms a V-shaped substrate-binding cleft that leads to a tunnel passing through the enzyme that can accommodate the acyl-chain of acyl-ACP. This tunnel places no apparent restriction on acyl-chain length, in contrast to a restrictive hydrophobic pocket seen in the AHL-synthase EsaI. Interactions of essential conserved N-terminal residues, Arg23, Phe27 and Trp33, suggest that the N-terminus forms an enclosed substrate-binding pocket for S-adenosyl-L-methionine. Analysis of AHL-synthase surface residues identified a binding site for acyl-ACP, a role that was supported by in vivo reporter assay analysis of the mutated residues, including Arg154 and Lys150. This structure and the novel explanation of AHL-synthase acyl-chain-length selectivity promise to guide the design of Pseudomonas aeruginosa-specific quorum-sensing inhibitors as antibacterial agents.
Mol Microbiol 2004 Aug
PMID:Structure of the Pseudomonas aeruginosa acyl-homoserinelactone synthase LasI. 1530 17

Agrobacterium tumefaciens C58 communicates using N-acyl-homoserine lactones (acyl-HSL) and contains two lactonase-encoding genes, attM and aiiB, the products of which are capable of inactivating the acyl-HSL signal. In A. tumefaciens A6, the expression of the attKLM operon is controlled by the transcriptional repressor encoded by an adjacent gene, attJ. An attJ::Tn5 mutant does not accumulate acyl-HSL because of the constitutive expression of the lactonase AttM, the activity of which inactivates acyl-HSL. In this work, the attKLM operon of A. tumefaciens C58 was shown to be involved in an assimilative pathway of gamma-butyrolactone (GBL), gamma-hydroxybutyrate (GHB), and succinate semialdehyde (SSA), in which AttM and AttL are key enzymes for GBL and GHB assimilation. The expression of the attKLM promoter was activated in the presence of GBL, GHB, and SSA. Under these conditions, A. tumefaciens C58 did not accumulate the acyl-HSL that it naturally synthesizes, and also became able to inactivate exogenous acyl-HSL signals. Therefore, in A. tumefaciens C58, the assimilative pathway of gamma-butyrolactone interferes with the acyl-HSL signaling.
Mol Plant Microbe Interact 2004 Sep
PMID:The assimilation of gamma-butyrolactone in Agrobacterium tumefaciens C58 interferes with the accumulation of the N-acyl-homoserine lactone signal. 1538 85

TraM, an 11.2 kDa antiactivator, modulates the acyl-homoserine lactone-mediated autoinduction of Ti plasmid conjugative transfer by interacting directly with TraR, the quorum-sensing transcriptional activator. Most antiactivators and antisigma factors examined to date act in dimer form. However, whether, and if so, how TraM dimerizes is unknown. Analyses based on a genetic assay using fusions of TraM to the lambda cI DNA binding domain, and biochemical assays using chemical crosslinking and gel filtration chromatography showed that TraM forms homodimers. Although SDS-PAGE studies suggested that the lone cysteine residue at position 71 was involved in interprotomer disulfide-bridging in TraM, altering Cys-71 to a serine did not significantly affect dimerization or the antiactivator activity of this mutant protein when expressed at wild-type levels in vivo. Analysis of N-terminal, C-terminal, and internal deletion mutants of TraM identified two regions of the protein involved in dimerization; one located within a segment between residues 20 and 50, and the other located to a segment between residues 67 and 96. Both regions are required for formation of fully stable dimers. Analysis of the activity of these deletion mutants in vivo, and their ability to bind TraR and to disrupt TraR-DNA complexes in vitro, suggests that while the internal segment of the protein is required for dimerization, determinants located at the far C-terminus and beginning at between residues 10 and 20 at the N-terminus play a role in TraR binding and antiactivator function. When co-expressed with lambda cI'::TraR fusions, wild-type TraM mediated quormone-independent dimerization of the transcriptional activator, suggesting that dimers of TraM can multimerize TraR.
Mol Microbiol 2004 Sep
PMID:Dimerization properties of TraM, the antiactivator that modulates TraR-mediated quorum-dependent expression of the Ti plasmid tra genes. 1538 23


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