Expression of Pseudomonas aeruginosa exoS is controlled by quorum sensing and RpoS

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Abstract
InPseudomonas aeruginosa, virulence determinants and biofilm formation are coordinated via a hierarchical quorum sensing cascade, which involves the transcriptional regulators LasR and RhlR and their cognate homoserine lactone activators C12-HSL [N-(3-oxododecanoyl)-l-homoserine lactone] and c4-hsl (n-butanoyl-l-homoserine lactone), which are produced by LasI and RhlI, respectively. The exoenzyme S regulon ofP. aeruginosa, comprises genes for a type III secretion system and for four anti-host effector proteins (ExoS, T, U and Y), which are translocated into host cells. It is a reasonable assumption that this ExoS regulon should be downregulated in the biofilm growth state and thus should also be under the regulatory control of the Las/Rhl system. Therefore, anexoS′-gfpreporter construct was used, and the influence of the Las and Rhl quorum sensing systems and the effect of the stationary-phase sigma factor RpoS on regulation of theexoSgene was examined. Evidence is provided for downregulation ofexoSduring biofilm formation ofP. aeruginosaPAO1. TherhlImutant PDO100 andrhlRmutant PDO111, but not thelasImutant PDO-JP1, showed approximately twofold upregulation of theexoS′-gfpreporter in comparison to PAO1. Upregulation ofexoS′-gfpin the PDO100 mutant could be repressed to normal level by adding C4-HSL autoinducer, indicating a negative regulatory effect of RhlR/C4-HSL onexoSexpression. As RhlR/C4-HSL is also involved in regulation of RpoS, theP. aeruginosa rpoSmutant SS24 was examined and theexoS′-gfpreporter was found to be fivefold upregulated in comparison to PAO1. For the first time evidence is reported for a regulatory cascade linking RhlR/RhlI and RpoS with the expression of the anti-host effector ExoS, part of the exoenzyme S regulon. Moreover, these data suggest that the exoenzyme S regulon may be downregulated inP. aeruginosabiofilms.