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Design principles of the bacterial quorum sensing gene networks

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Abstract Bacterial quorum sensing (QS) has attracted much interest as the manifestation of collective behavior in prokaryotic organisms once considered strictly solitary. Significant amount of genetic, biochemical, and structural data which, has been accumulated in studies on QS in many species allows us to map properties of specific molecules and their interactions on the observed population‐wide bacterial behavior. The present review attempts to give a systems biology perspective on the structure of genetic regulatory networks that control QS and considers functional implications of a variety of design principles that recur in the organization of these networks across species Copyright © 2009 John Wiley & Sons, Inc. This article is categorized under: Models of Systems Properties and Processes > Cellular Models

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Genomic organization and mutual regulation within the QSN core module. (a) Class A luxR/luxI module of Gram(–) bacteria. (b) Class B luxR/luxI module. (c) Representative example of Gram(+) QSN core module. Transcription regulator AgrA activates transcription of the agr operon. agrD and agrB encode AI peptide precursor and peptide extruding enzyme respectively. Two strands of DNA are represented by separate lines. Open boxes symbolize cis‐regulatory elements. Regulatory interactions are shown by hammerheads (negative) and arrows (positive). Interactions found only in a subset of characterized systems are drawn by dashed line.

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Behavior of a hypothetical luxR/luxI type QSN. (a) Network layout. Notations are the same as in Figure 2. (b) Stationary states of the network at three values of extracellular AI: a—‐0 nM; b—‐12 nM; c—‐24 nM. (c) After removal of the QSTR dimerization from the network layout, only one stationary state remains. (Same values of extracellular AI.) Filled circles and squares represent stable states of the network. Open circle denotes an unstable state of the bistable QSN. (Reaction rate values are adopted from Ref. 121).

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Integration of quorum sensing with other sensory inputs. (a) Boolean AND logical circuit in the regulatory network of P. stewartii. (b) Boolean OR in E. caratovora. RNA species, except RsmB, are omitted for brevity. Notations are the same as in Figure 2.

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Regulation of QSNs by mutual inhibition. (a) RNA–RNA switch in the QSN of V. harveyi. (b) RNA–RNA switch in the QSN of S. aureus. The QS positive feedback loop of agr operon is shown schematically. (c) Protein–protein mutual inhibition operates as a switch in A. tumefaciens QSN. AAI is Agrobacterium AI. S symbolizes substrates of AI synthase TraI. (d) Typical ultrasensitive behavior for expression of the QSTR and its inhibitory regulator schematically represented as a function of the extracellular concentration of AI. Small open boxes represent complex formation. Ø symbolizes eventual degradation of the inhibitory complex. RNA species are shown as rectangles and proteins as ovals. Translation of mRNA species is shown by open arrowheads.

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