Regulation of the type III secretion system in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic bacterial pathogen which primarily infects patients with cystic fibrosis (CF), severe burns, or immunosuppression. P. aeruginosa possesses a type III secretion system (TTSS) which injects effector proteins into host cells, resulting in cell rounding, lifting, and death by necrosis or apoptosis. By screening a transposon insertional mutant library of a wild-type strain PAK, mutation in the mucA or prtR gene was found to cause repression of the TTSS.; Mutation in the mucA gene causes alginate overproduction, resulting in a mucoid phenotype. Comparison of global gene expression profiles of the mucA mutant and wild-type PAK under TTSS inducing condition confirmed the down regulation of TTSS genes and up regulation of genes involved in the alginate biosynthesis. Further analysis indicated that the repression of the TTSS in the mucA mutant was AlgU and AlgR dependent. Overexpression of the algR gene inhibited type III gene expression.; PrtR is an inhibitor of prtN, which encodes a transcriptional activator for pyocin synthesis genes. In P. aeruginosa, pyocin synthesis is activated when PrtR is degraded during the SOS response. Treatment of a wild-type P. aeruginosa strain with mitomycin C, a DNA-damaging agent resulted in the inhibition of TTSS activation. A prtR/prtN double mutant had the same TTSS defect as the prtR mutant, and complementation by a prtR gene but not by a prtN gene restored the TTSS function. Also, overexpression of the prtN gene in wild-type PAK had no effect on the TTSS; thus PrtN is not involved in the repression of the TTSS. To identify the PrtR-regulated TTSS repressor, another round of Tn mutagenesis was performed in the background of a prtR/prtN double mutant. Insertion in a small gene, designated ptrB, restored the normal TTSS activity. Expression of ptrB is specifically repressed by PrtR, and mitomycin C-mediated suppression of the TTSS is abolished in a ptrB mutant strain. Therefore, PtrB is a newly discovered TTSS repressor that regulates the TTSS under the stress of DNA damage.; My study revealed new regulatory relationship between MucA, PrtR and the TTSS, and indicated that the TTSS might be repressed under environmental stresses.