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Chapter 20 : More than Just a Quorum: Integration of Stress and Other Environmental Cues in Acyl-Homoserine Lactone Signaling
Category: Microbial Genetics and Molecular Biology
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Many bacteria employ chemical communication to coordinate group behaviors, a process that has been termed quorum sensing (QS). Although many different classes of bacteria use QS, the mechanism that has emerged as common in gram-negative proteobacteria is based on acyl-homoserine lactone (acyl-HSL) signal molecules. QS is an important field of study because of its important regulatory roles in pathogenic and environmental bacteria. Acyl-homoserine lactone (Acyl-HSL) QS controls virulence in many plant and animal pathogens such as Pseudomonas aeruginosa, Burkholderia cepacia, and Agrobacterium tumefaciens. This chapter focuses on the integration of acyl-HSL QS and such stress responses. In P. aeruginosa, QS controls the expression of numerous virulence factors such as extracellular enzymes (LasB elastase, LasA protease, alkaline protease), secondary metabolites (pyocyanin, hydrogen cyanide, pyoverdin), and toxins (exotoxin A). Microarray studies revealed that the las and rhl systems together control the expression of more than 300 genes. The majority of the associated genes are predicted to encode secreted factors and secretion machinery, confirming the notion that the core function of las QS is to control the expression of extracellular factors. Indeed, a recent population analysis of concurrently isolated P. aeruginosa from individual Cystic fibrosis (CF) lung infections revealed great heterogeneity of QS phenotypes and genotypes within as well as among patients. This finding suggests that a single selective mechanism, whether of social or nonsocial nature, is unlikely to explain the emergence of QS variants during CF infection.
Structures of selected homoserine lactone and alkylquinolone signals.
Signal integration in the acyl-HSL and AQ-QS systems of P. aeruginosa. The three QS systems las, rhl, and AQ are highlighted. The small circles associated with each system refer to the signal molecules 3OC12-HSL (dark gray), C4-HSL (light gray), and HHQ and PQS (white and black, respectively). Solid arrows and T bars indicate positive and negative regulation, respectively. Dotted lines indicate indirect regulation. For simplicity, only protein components are shown and regulatory effects on individual components within a QS system are generally not distinguished. LasI generates 3OC12-HSL that is bound by LasR. RsaL is activated by LasR-3OC12-HSL and acts to maintain 3OC12-HSL homeostasis by negatively regulating lasI. LasR-3OC12-HSL activates lasI, numerous target genes as well as the rhl and AQ systems. RhlR binds C4-HSL generated by RhlI and activates further rhlI expression and negatively regulates the AQ-dependent QS system. Both acyl-HSL QS systems are controlled at both the transcriptional and posttranscriptional level by regulators including VqsR; GidA; and a regulatory pathway involving the two-component system GacAS, the RNA-binding protein RsmA, and the small RNA RsmZ. Vfr, a cAMP-binding protein, responds to increased cAMP production during calcium limitation and positively regulates LasR. An orphan LuxR homolog, QscR, negatively regulates acyl-HSL QS and controls other genes in response to endogenous and perhaps exogenous acyl-HSLs. In the AQ system, products of pqsABCD generate HHQ, which is converted to PQS by another protein, PqsH. The LysR-type regulator PqsR responds to either HHQ or PQS. AQ biosynthesis is positively regulated by PqsR. Expression of AQ-PqsR-dependent target genes can also be mediated through another component, PqsE, which is not shown. The response regulator PhoB regulates transcription of pqsR during phosphate-limiting conditions. Under iron-limiting conditions, Fur indirectly affects PQS production through de-repression of two small RNAs. Starvation and stress conditions lead to induction of the stringent response and RpoS that directly and indirectly control QS gene expression.
Examples of signal integration in acyl-HSL QS systems