The multi-output incoherent feedforward loop constituted by the transcriptional regulators LasR and RsaL confers robustness to a subset of quorum sensing genes in Pseudomonas aeruginosa
Quorum sensing (QS) is an intercellular communication system which controls virulence-related phenotypes in the human pathogen Pseudomonas aeruginosa. LasR is the QS receptor protein which responds to the signal molecule N-(3-oxododecanoyl)homoserine lactone (3OC12-HSL) and promotes signal production by increasing the transcription of the 3OC12-HSL synthase gene, lasI. LasR also activates the expression of other genes, including rsaL, coding for the RsaL protein which acts as a transcriptional repressor of lasI. Direct gene activation and RsaL-mediated gene repression, both exerted by LasR on the expression of the output gene lasI, generate a regulatory network motif known as type 1 incoherent feedforward loop (IFFL-1) that governs 3OC12-HSL production. Beside lasI, RsaL directly represses a set of LasR-activated genes; hence, the IFFL-1 generated by LasR and RsaL is a multi-output IFFL-1. Here we demonstrate that the multi-output IFFL-1 constituted by LasR and RsaL confers robustness with respect to fluctuations in the levels of LasR to the phenotypes controlled by both these transcriptional regulators (e.g. 3OC12-HSL synthesis and pyocyanin production). On the contrary, other virulence-related phenotypes controlled by LasR but not by RsaL (e.g. elastase and proteases production) are sensitive to changes in LasR levels. As a whole, the multi output IFFL-1 generated by LasR and RsaL splits the QS regulon in two distinct sub-regulons with different robustness with respect to LasR fluctuations. This emerging regulatory property enhances the phenotypic plasticity of P. aeruginosa, thus contributing to its adaptation to changing environments.