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Pseudomonas aeruginosa PAO1 lon mutants have phenotypes of deficiencies in cell division, swarming, twitching, and biofilm formation as well as a phenotype of ciprofloxacin supersusceptibility. In this study, we demonstrated that a lon mutant was also supersensitive to the DNA-damaging agent UV light. To understand the influence of lon in causing these phenotypes, global gene expression was characterized by performing microarrays on the lon mutant and the PAO1 wild type grown in the presence of subinhibitory concentrations of ciprofloxacin. This revealed major differences in the expression of genes involved in the SOS response and DNA repair. Real-time quantitative PCR confirmed that these genes were highly upregulated upon ciprofloxacin exposure in the wild type but were significantly less induced in the lon mutant, indicating that Lon modulates the SOS response and consequentially ciprofloxacin susceptibility. As the known Lon target SulA is a member of the SOS response regulon, the influence of mutating or overexpressing this gene, and the negative regulator of the SOS response, LexA, was examined. Overexpression of lexA had no effect on the Lon-related phenotypes, but sulA overexpression recapitulated certain lon mutant phenotypes, including altered motility and cell division, indicating that Lon regulates these phenotypes through SulA. However, sulA overexpression did not affect ciprofloxacin susceptibility or biofilm formation, indicating that these properties were independently determined. Lon protease was also demonstrated to strongly influence RecA protein accumulation in the presence of ciprofloxacin. A model of DNA repair involving the Lon protease is proposed.

Involvement of the Lon Protease in the SOS Response Triggered by Ciprofloxacin in Pseudomonas aeruginosa PAO1