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1.
FIG. 1

FIG. 1. From: Bacterial Quorum Sensing in Pathogenic Relationships.

Quorum sensing in gram-negative organisms involves two regulatory components: the transcriptional activator protein (R protein) and the AI molecule produced by the autoinducer synthase. Accumulation of AI occurs in a cell-density-dependent manner until a threshold level is reached. At this time the AI binds to and activates the R protein, which in turn induces gene expression. The R protein consists of two domains: the N terminus of the protein that interacts with AI and the C terminus that is involved in DNA binding. Typically, gram-negative AI molecules are N-acyl-HSLs; however, other types of signal molecules do exist.

Teresa R. de Kievit, et al. Infect Immun. 2000 September;68(9):4839-4849.
2.
FIG. 2

FIG. 2. From: Bacterial Quorum Sensing in Pathogenic Relationships.

The quorum-sensing circuitry of P. aeruginosa is illustrated. Expression of the lasR gene is subject to at least two levels of control: the global regulators Vfr and GacA (1, 97) and the las quorum-sensing system, which regulates expression of both lasR and lasI. The latter creates an autoinduction feedback loop. Regulation of the rhl system is similar to las in that GacA affects expression of rhlR (97), and the rhlR and rhlI genes are controlled to some degree by the las system. Interestingly, the las quorum-sensing system was shown to elicit an additional level of control over the rhl system; the las signal molecule, 3-oxo-C12-HSL, can act posttranslationally to block RhlR activation by C4-HSL. The las and rhl quorum-sensing systems regulate expression of numerous genes that contribute to the virulence of P. aeruginosa. In addition, the las signal molecule, 3-oxo-C12-HSL, is required for biofilm differentiation and exhibits immunomodulatory activity.

Teresa R. de Kievit, et al. Infect Immun. 2000 September;68(9):4839-4849.
3.
FIG. 3

FIG. 3. From: Bacterial Quorum Sensing in Pathogenic Relationships.

Quorum sensing in S. aureus is depicted. The agr locus consists of two divergent transcriptional units. One operon (agrBDCA) encodes the proteins responsible for generating and sensing the peptide signal molecule (AgrD), and the other encodes δ-hemolysin and RNAIII. The signal molecule is an octapeptide cleaved from the middle of the agrD gene product and is exported from the cell via the membrane-associated AgrB protein. Once the extracellular concentration of AgrD reaches a threshold level, it stimulates autophosphorylation of the transmembrane AgrC protein, which in turn phosphorylates the response regulator (AgrA). In its phosphorylated state, AgrA activates expression of RNAIII. The rise in the level of regulatory RNAIII leads to increased secretion of numerous factors, reduced expression of specific surface proteins, and induced expression of the agrBDCA operon. SarA is a DNA-binding protein, encoded by the sar locus, that upregulates expression of both agr operons.

Teresa R. de Kievit, et al. Infect Immun. 2000 September;68(9):4839-4849.

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