Role of GacA, LasI, RhlI, ppk, PsrA, vfr and ClpXP in the regulation of the stationary-phase sigma factor rpoS/RpoS in Pseudomonas

RpoS is the stationary phase sigma factor responsible for increased transcription of a set of genes when bacterial cells enter stationary phase and under stress conditions. In Escherichia coli, RpoS expression is modulated at the level of transcription, translation, and post-translational stability whereas in Pseudomonas, previous studies have implicated four genetic loci (psrA, gacA, lasI and rhlI) involved in rpoS transcription. In this report, the transcription, translation and proteins profiles of rpoS/RpoS were analyzed in response to growth phase of knockout genomic mutants in the P. aeruginosa transcriptional regulatory loci psrA, gacA, vfr, and in the las and rhl quorum-sensing systems. Gene expression and protein profiles were also analyzed in the ppk genomic mutant. This gene is responsible for the biosynthesis of polyphosphate, an alarmone involved in the regulation of RpoS accumulation in E. coli. Finally, the role of the ClpXP protease in RpoS regulation was also studied; in E. coli, this protease has been shown to rapidly degrade RpoS during exponential growth. These studies confirm the significant role of PsrA in rpoS transcription during the late-exponential and stationary growth phases, the probable role of Vfr in transcriptional repression during exponential phase, and the function of the ClpXP protease in RpoS degradation during exponential phase. GacA/GacS, the quorum-sensing systems, and the polyphosphate alarmone molecule were not significant in rpoS/RpoS regulation. These results demonstrate important similarities and differences with the regulation of this sigma factor in E. coli and in Pseudomonas.