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Regulation of pyocin genes in Pseudomonas aeruginosa by constitutively repressive PrtR

Killgore, Creed
Pyocins are bacteriophage tail-like complexes that are produced by Pseudomonas aeruginosa under the SOS response of the cell. These molecules are used for intraspecies competition by targeting and killing susceptible Pseudomonas strains. The pathway of pyocin production begins with the protein RecA which is activated by the SOS response. Its function is to cleave the pyocin gene repressor, PrtR, which allows for the activation of the gene activator, PrtN. The repressor inhibits pyocin production by binding to the promoter of the gene encoding the activator, prohibiting the expression of pyocin genes. Notably, pyocin production can also be induced independently of the SOS response in strains that are deficient in the gene xerC. Additionally, the xerC deleted mutant significantly increases the number of pyocin-producing cells compared to a wild-type strain. In my lab research, we reveal the limitations of the repressive ability of PrtR as well as the potential existence of other targets of PrtR. We constructed a mutant version of PrtR in a xerC deficient strain, specifically PrtRS162A, which is an uncleavable repressor. Surprisingly, the uncleavable repressor is found to be insufficient in completely blocking pyocin gene expression in a pyocin-overproducing strain, but it is able to block the production of functional pyocins. Expression of at least some pyocin-encoding genes, including our reporter (hol), suggests that the pyocin-producing strain can somehow bypass the repressive activity of PrtR. However, the strain containing PrtRS162A did not produce any functional pyocins, suggesting that one or more genes required for pyocin function are not expressed. This study suggests that PrtR has novel targets within the pyocin gene cluster in addition to its previously known target of PrtN expression.