Identification and characterization of two chemotactic transducers for inorganic phosphate in Pseudomonas aeruginosa

Two chemotactic transducers for inorganic phosphate (P-i), designated CtpH and CtpL, have been identified in Pseudomonas aeruginosa. The corresponding genes (ctpH and ctpL) were inactivated by inserting kanamycin and tetracycline resistance gene cassettes into the wild-type genes in the P. aeruginosa PAO1 genome. Computer-assisted capillary assays showed that the ctpH single mutant failed to exhibit P-i taxis when the concentration of P-i in the capillary was higher than 5 mM. Conversely, the ctpL single mutant could not respond to P-i at the concentration of 0.01 mM. The ctpH ctpL double mutant was defective in P-i taxis at any concentration ranging from 0.01 to 10 mM. To investigate regulation of P-i taxis, the ctpH and ctpL genes were also disrupted individually in the P. aeruginosa phoU and phoB single mutants, The ctpH phoU and ctpH phoB double mutants were defective in P-i taxis, regardless of whether the cells were starved for P-i. The ctpL phoU double mutant was constitutive for P-i taxis, whereas the ctpL phoB double mutant was induced by P-i limitation far P-i taxis. The region upstream of ctpL, but not ctpH, contained a putative pho box sequence. Expression of ctpL::lacZ was induced by P-i limitation in PAO1, while it was constitutive in the phoU mutant. In contrast, the phoB mutant showed only background levels of ctpL::lacZ expression. These results showed that ctpL is involved in the pho regulon genes in P. aeruginosa, The ctpH phoU mutant, which failed to exhibit P-i taxis, was constitutive for ctpL::lacZ expression, suggesting that the P-i detection by CtpL requires PhoU. Like PAO1, the phoB and phoU single mutants were constitutive for expression of ctpH::lacZ. Thus, the evidence that the ctpL phoU mutant, but not the ctpL phoB mutant and PAO1, was constitutive for P-i taxis raised the possibility that PhoU exerts a negative control on P-i detection by CtpH at the posttranscriptional level.