Pseudomonas aeruginosa is an opportunistic human pathogen that infects immunocompromised individuals such as those suffering from burns or the genetic disorder cystic fibrosis. This organism utilizes a cell-cell communication mechanism known as quorum sensing (QS) to coordinate virulence gene expression and biofilm formation. It has three interconnected QS systems, namely las, rhl and pqs. Each system is comprised of autoinducer synthesis genes, lasI, rhlI, and pqsABCDH, and the cognate regulatory genes, lasR, rhlR, and pqsR, respectively. Here, we primarily focused on understanding the regulatory mechanisms of QS, which we investigated at two levels. First, we sought to identify additional activators that regulate QS at the level of the las and rhl systems, and second, we investigated the regulation of downstream genes, particularly biofilm exopolysaccharide genes, by QS. For the first approach, we employed a mutagenesis screen to identify global QS activators. We screened a non-redundant transposon library for mutants deficient in QS-dependent phenotypes. We identified a novel regulator, GidA, a glucose-inhibited cell division protein, that selectively controls QS gene expression posttranscriptionally via RhlR-dependent and –independent pathways. For the second part, we established a regulatory link between QS and Pel exopolysaccharide. We showed that the las system represses Pel and modulates colony biofilm structure through the pqs pathway. LasR mediated colony rugosity via 4-hydroxy-2-alkylquinolines in a PqsR-independent manner, ascribing a novel function to this class of signaling molecules in P. aeruginosa. Taken together, our study highlights the complexity of QS, which involves integration of various regulatory pathways to control downstream processes in response to different environmental conditions.
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