LasR Might Act as an Intermediate in Overproduction of Phenazines in the Absence of RpoS in Pseudomonas aeruginosa

As an opportunistic bacterial pathogen, Pseudomonas aeruginosa PAO1 contains two phenazine-producing gene operons, phzA1B1C1D1E1F1G1 (phz1) and phzA2B2C2D2E2F2G2 (phz2), each of which is independently capable of encoding all enzymes for biosynthesizing phenazines, including phenazine-1-carboxylic acid and its derivatives. Other previous study reported that the RpoS-deficient mutant SS24 overproduced pyocyanin, a derivative of phenazine-1-carboxylic acid. However, it is not known how RpoS mediates the expression of two phz operons and regulates pyocyanin biosynthesis in detail. In this study, with deletion of the rpoS gene in the $PA{\Delta}phz1$ mutant and the $PA{\Delta}phz2$ mutant respectively, we demonstrated that RpoS exerted opposite regulatory roles on the expression of the phz1and phz2 operons. We also confirmed that the phz1 operon played a critical role and especially biosynthesized much more phenazines than the phz2 operon when the rpoS gene was knocked out in P. aeruginosa. By constructing the translational reporter fusion vector lasR'-'lacZ and the chromosomal fusion mutant $PA{\Delta}lasR::lacZ$, we verified that RpoS deficiency caused increased expression of lasR, a transcription regulator gene in a first quorum sensing system (las) that activates overexpression of the phz1 operon, suggesting that in the absence of RpoS, LasR might act as an intermediate in overproduction of phenazine biosynthesis mediated by the phz1 operon in P. aeruginosa.