Molecular analysis of phosphate taxis in bacteria.

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a) We isolated P.aeruginosa PAO1 and E.cloacae IFO3320 mutants which constitutively produced alkaline phospgatase (AP). Genetic analysis of these mutants revealed that mutations located in the pst operon. P.aeruginosa mutant strains showed chemotactic response to P_i under conditions of P_i excess. These results indicate that Pst negatively controls the expression of P_i taxis in P.aeruginosa. E.cloacae AP constitutive mutants did not show chemotactic response to P_i even under conditions of P_i limitations. These results suggest that the E.cloacae Pst system is required for P_i chemoreception. b) Since there is no convenient method for selecting P_i defective mutants, we decided to clone a gene encoding a MCP for P_i taxis by using the pctA gene, which encodes a MCP for amino acids, as probe. Two MCP genes (termed pctB and pctC) were found in the pctA-flanking region. Genetic analysis revealed that PctB and PctC were involved in chemotaxis to seven and two L-amino acids, respectively. The P.aeruginosa knock-out mutant of pctA,pctB and pctC showed P_i taxis, suggestig that three MCPs were not involved in P_i taxis. We have isolated seven more DNA fragments hybridized with the pctA gene. Investigation of these fragments is under way.