The pho regulon of Shigella flexneri

Growth of Escherichia coli K-12 in low-phosphate conditions results in the induction of the synthesis of many proteins, including the outer membrane porin PhoE, alkaline phosphatase, and the Pst system for the transport of phosphate (P_i). This response is controlled by a two-component regulatory system of which PhoB and PhoR are the response-regulator and the sensor/kinase, respectively. When Shigella flexneri was starved for P_i, neither PhoE nor alkaline phosphatase was produced. However, induction of the synthesis of the PstS protein was observed, indicating that S. flexneri contains a functional PhoB/PhoR regulatory system. Consistent with this notion, the introduction of the B. coli phoA gene in S. flexneri resulted in the induction of alkaline phosphatase synthesis under phosphate limitation. However, introduction of phoE on a plasmid did not lead to the expression of PhoE protein, indicating that S. flexneri PhoB does not recognize the phoE promoter region. The phoB gene was cloned and sequenced and in the deduced amino acid sequence two deviations from that of E. coli PhoB were detected. Site-directed mutagenesis revealed that one of these deviations, i.e. Leu-172, which is Arg in E. coli PhoB, is responsible for the lack of expression of the PhoE protein in S. flexneri.