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-com ponent regulatory system of which PhoB and PhoR are the response-regul ator and the sensor/kinase, respectively. When Shigella flexneri was s tarved for P-i, neither PhoE nor alkaline phosphatase was produced, Ho wever, induction of the synthesis of the PstS protein was observed, in dicating that S. flexneri contains a functional PhoB/PhoR regulatory s ystem. Consistent with this notion, the introduction of the E. coli ph oA gene in S. flexneri resulted in the induction of alkaline phosphata se synthesis under phosphate limitation, However, introduction of phoE on a plasmid did not lead to the expression of PhoE protein, indicati ng that S. flexneri PhoB does not recognize the phoE promoter region, The phoB gene was cloned and sequenced and in the deduced amino acid s equence two deviations from that of E. coli PhoB were detected. Site-d irected mutagenesis revealed that one of these deviations, i.e. Leu-17 2, which is Arg in E. coli PhoB, is responsible for the lack of expres sion of the PhoE protein in S. flexneri.