Expression of the Pho regulon negatively regulates biofilm formation by Pseudomonas aureofaciens PA147-2

We report the isolation of insertional mutations to the pstC and pstA genes of the phosphate-specific transport (pst) operon that results in loss of biofilm formation by Pseudomonas aureofaciens PA147-2. Consistent with the known roles of the Pst system in Escherichia coli and Pseudomonas aeruginosa, both P. aureofaciens pst mutants were demonstrated to have defects in inorganic phosphate (P-i) transport and repression of Pho regulon expression. Subsequently, biofilm formation by the wild type was shown to require a threshold concentration of extracellular P-i. The two-component regulatory pair PhoR/PhoB is responsible for upregulation of Pho regulon expression in response to P-i-limiting environments. By generating phoR mutants that were unable to express the Pho regulon, we were able to restore biofilm formation by A aureofaciens in P-i-limiting conditions. This result suggests that gene(s) within the Pho regulon act to regulate biofilm formation negatively in low-P-i environments, and that phoR mutations uncouple PA147-2 from such regulatory constraints. Furthermore, the inability of psi mutants to repress Pho regulon expression accounts for their inability to form biofilms in non-limiting P-i environments. Preliminary evidence suggests that the st system is also required for antifungal activity by PA147-2. During phenotypic analysis of pst mutants, we also uncovered novelties in relation to P-i assimilation and Pho regulon control in P. aureofaciens.