Published on January 22, 2009
Charla bacteriana Q.F.B Felipe Riveroll Aguirre
E. Coli en ausencia de atrayentes o repelentes
Abbreviations:W , CheW; A, CheA; P, phosphoryl group; Z, CheZ; Pi, inorganic phosphate; R, CheR; B, CheB; SAM, S-adenosylmethionine; CH3OH, methanol.
A higher-order array of chemoreceptor-sensing domains may exist in equilibrium between an ordered, tense state (T) and a disordered, relaxed state (R). Top-down view from the periplasm. Aspartate, for example,would have a slightly higher affinity for the R state. A similar relationship between a T state and R state could also exist for the signaling domains in the cytoplasm. While receptor–signaling complexes would remain in the presence of ligand, direct contacts or the distances between CheA andCheWbinding sites in the cytoplasm would be perturbed.
Two-component and one-component signal transduction. (a) A prototypical two-component signal transduction system contains input (colored red) and output (colored yellow) domains in two different proteins that communicate via a His-Asp phosphotransfer. A one-component system is a protein that contains input and output domains but lacks His-Asp phosphotransfer domains (colored grey and white). (b) Examples of two-component and one-component systems that use the same type of input and output domains. Experimentally studied proteins are identified by name, whereas proteins predicted from genome sequences are identified by their GenBank ID number: FixJ (GenBank accession number 120202), FixL (GenBank accession number 120208), NtrB (GenBank accession number 128594), NtrC (GenBank accession number 417388), RocR (GenBank accession number 34395962), TraR (GenBank accession number 17743220). Organism abbreviations: A. tumefaciens, Agrobacterium tumefaciens; B. anthracis, Bacillus anthracis; B. japonicum, Bradyrhizobium japonicum; B. pertussis, Bordetella pertussis; B. subtilis, Bacillus subtilis; C. violaceum, Chromobacterium violaceum; E. coli, Escherichia coli; N. punctiforme, Nostoc punctiforme; P. putida, Pseudomonas putida; S. meliloti, Sinorhizobium meliloti; S. oneidensis, Shewanella oneidensis; T. maritima, Thermotoga maritima.
Dependence of the number of one-component and two-component signal transduction systems on the genome size. The plot is in a double logarithmic scale. One hundred forty-five genomes were ranked by size and split into 16 size classes. Each point indicates the average number of genes for one-component or two component signal-transduction systems in the respective class.
… Lexico Bacteriano
Two general mechanisms of microbial quorum sensing. (a) Signal detection by a cytosolic transcription factor, represented by the AHL-type quorum-sensing system. The signals produced by a LuxI-type protein (I ) accumulate in intercellular environment, transport into cytosol, bind to LuxR-type transcription factors (R), and initiate expression of the target genes (indicated by dashed lines). (b) Signal detection by a two-component sensor and response regulator pair, represented by the AIP-type quorum-sensing system. Precursor peptides (PP) are modified and the resulting AIP signals exported by an ABC transporter (T). The signals are detected by the sensor histidine kinase (S), transduced to the cognate response regulator (RR) by phosphorylation relay (P), which modulates the target gene expression.
The Vibrio fischeri LuxI/LuxR quorum sensing circuit. There are five luciferase structural genes ( luxCDABE ) and two regulatory genes ( luxR and luxI ) required for quorum sensing – controlled light emission in V. fischeri . The genes are arranged in two adjacent but divergently transcribed units. luxR is transcribed to the left, and the luxICDABE operon is transcribed to the right. The LuxI protein (square) is responsible for synthesis of the HSL autoinducer N -(3-oxohexanoyl)-homoserine lactone (hexagons). As the cell-population density increases, the concentration of the autoinducer increases both intra- and extracellularly. At a critical autoinducer concentration, the LuxR protein (circle) binds the autoinducer. The LuxR-autoinducer complex binds at the luxICDABE promoter and activates transcription of this operon. This action results in an exponential increase in autoinducer synthesis via the increase in transcription of luxI and an exponential increase in light production via the increase in transcription of luxCDABE . The LuxR-autoinducer complex also binds at the luxR promoter, but in this case the complex represses the transcription of luxR . This negative action compensates for the positive action at the luxICDABE promoter. The oval represents a bacterial cell.
Model of AI-2-Regulated LuxPQ Receptor Activity (A and B) Structures are displayed from ‘‘above,’’ looking toward the bacterial inner membrane (A), or from the ‘‘side’’ (B). The relative disposition of the two AI-2-free LuxPQ receptors (left panels) is unknown, but the orientation shown here assumes that the transmembrane and histidine kinase domains contain four-helix bundles, as observed crystallographically for the transmembrane domain of Natronomonas pharaonis NpHtrII protein ( Moukhametzianov et al., 2006 ) and for several histidine kinase domains (reviewed in Tomomori et al., 2003 ). At low cell density (left panels), in the absence of AI-2, we propose that the LuxQ cytoplasmic dimerization domains form symmetrical four-helix bundles, favoring autophosphorylation in trans. The phosphate is subsequently transferred from His492 to a conserved aspartate (Asp785) in the receiver domain, ultimately resulting in the formation of LuxO-phosphate (see Figure 1 ). At high cell density (right panels), AI-2 binding induces asymmetric interaction of the LuxPQ periplasmic regions, causing changes in the transmembrane helices (indicated schematically; red arrow). These changes are transmitted to the LuxQ cytoplasmic domains, inhibiting kinase activity. In the kinase-off state, the receiver domain hydrolyzes its aspartyl-phosphate into inorganic phosphate (Pi), thus reversing the flow of phosphate along the signal transduction pathway and reducing the concentration of LuxO-phosphate.
Bacterial Communication over Distances (A) Quorum sensing. Quorum-sensing bacteria produce and respond to the extracellular accumulation of signal molecules called autoinducers (depicted as green spheres). (B) Mixed messaging. Membrane vesicles traffic the Pseudomonas aeruginosa quinolone signal (PQS; depicted as green triangles) between cells. PQS facilitates group behavior when delivered to other P. aeruginosa cells, but other quinolones (X), also contained in the vesicles, are antibiotics that kill other bacterial species. (C) Quorum quenching. Quorum-sensing bacteria are vulnerable to a variety of quorum-quenching mechanisms such as the enzymatic inactivation of autoinducers or the presence of autoinducer antagonists (yellow spheres), molecules with structure similar to autoinducers that prevent autoinducer detection and response. (D) Conversations across kingdoms. Enterococcus faecalis produces a cytolysin composed of two subunits. The small cytolysin subunit acts as an autoinducer that monitors the environment for other E. faecalis cells. The large cytolysin subunit monitors the vicinity for eukaryotic host cells. Left: If no host cells are present, the two subunits form an inactive complex, and production of the subunits is held at a low basal level. Right: If host cells are present, the large subunit binds to the surface of the host cells, leaving the small subunit free to induce high-level production of both subunits, which together bind to target cells and cause them to lyse.
Figure 3. Intimate Bacterial Conversations (A) Reciprocal C signaling in Myxococcus xanthus. The surface-displayed C signal protein interacts with a hypothetical receptor on an adjacent cell to transmit a signal that promotes fruiting-body formation. (B) Crisscross signaling in Bacillus subtilis. Sporulation takes place in a two-compartment sporangium in which three intercellular signals, the secreted signaling proteins SpoIIR (IIR) and SpoIVB (IVB) and an unknown signal (?), are exchanged back and forth. For simplicity, the illustration does not convey that the second and third signaling events take place after the smaller cell has been engulfed by the larger cell. (C) Contact-dependent inhibition in Escherichia coli. Cells producing the proteins CdiAB form aggregates with, and inhibit the growth of, cells lacking the genes for the growth-inhibiting proteins.
The B. subtilis phosphorelay. On the binding of a signal ligand, a sporulation sensor kinase autophosphorylates, and the phosphoryl group is subsequently shuttled from the sensor kinase to Spo0A, the master sporulation regulator. Phosphoryl groups can be drained from the relay by Rap or Spo0E phosphatases, and the sensor kinase KinA is inhibited by anti kinases.
DNA recognition by Spo0A.
SOS and/or competence as general stress responses in B. subtilis and S. pneumoniae. Repair following SOS induction or through genetic transformation is not indicated. Stress A, DNA-damaging agents or antibiotics-induced stress; stress B, other stress conditions.
What cells sense. The ‘information’ that cells obtain from autoinducer sensing is a result of the interaction of autoinducer production with environmental conditions and the density and distribution of the producing cells. a | The autoinducer concentration in a well-mixed liquid, or any other hábitat without limitations to mass transfer and without clustering of cells, measures the cell density. A sufficiently high cell density leads to upregulation of gene expression. b | The concentration of autoinducer produced by a single cell measures the degree to which the mass transfer of this autoinducer is reduced by solids, gas bubbles, or other abiotic or biotic obstacles to mass transfer, and by the composition of the matrix affecting diffusivity and flow velocity. A sufficiently confined space leads to upregulation of gene expression. The cube indicates the volume of the confined space. c | The autoinducer concentration in the absence of mass-transfer constraints measures the degree of clustering of a given number of cells. A sufficiently aggregated spatial distribution of cells leads to upregulation of gene expression. A red or yellow background indicates low or high autoinducer concentration, respectively; bacteria in cyan are not induced, and bacteria in red are induced.
The rhizosphere as an example of a complex habitat. The rhizosphere is characterized by a spatially structured environment that is subject to temporal fluctuations, high biodiversity and a distinct pattern of spatial distribution of individual members of the various bacterial species present. Clear rods represent individuals of a bacterial species producing a certain autoinducer and other, potentially interfering species are indicated by red and green coloured rods. Members of a species are more or less scattered and can occur as single cells or in microcolonies, which are often clusters of cells originating from a single ancestor (clones).
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