Stringent response activates quorum sensing and modulates cell density-dependent gene expression in Pseudomonas aeruginosa

During nutrient starvation, Escherichia coli elicits a stringent response i nvolving the ribosome-associated protein RelA. Activation of RelA results i n a global change in the cellular metabolism including enhanced expression of the stationary-phase sigma factor RpoS. In the human pathogen Pseudomona s aeruginosa, a complex quorum-sensing circuitry, linked to RpoS expression , is required for cell density-dependent production of many secreted virule nce factors, including LasB elastase. Quorum sensing relies on the activati on of specific transcriptional regulators (LasR and RhIR) by their correspo nding autoinducers (3-oxo-C-12-homoserine lactone [HSL] and C-4-HSL), which function as intercellular signals. We found that overexpression of relA ac tivated the expression of rpoS in P. aeruginosa and led to premature, cell density-independent LasB elastase production. We therefore investigated the effects of the stringent response on quorum sensing. Both lasR and rhlR ge ne expression and autoinducer synthesis were prematurely activated during t he stringent response induced by overexpression of relA. Premature expressi on of lasR and rhIR was also observed when relA was overexpressed in a PAO1 rpoS mutant. The stringent response induced by the amino acid analogue ser ine hydroxamate (SHX) also led to premature production of the 3-oxo-C-12-HS L autoinducer. This response to SHX was absent in a PAO1 relA mutant. These findings suggest that the stringent response can activate the two quorum-s ensing systems of P. aeruginosa independently of cell density.