Molecular aspects of phosphate transport in Escherichia coli

Summary Escherichia coli transports inorganic phosphate (Pi) by the low‐affinity transport system, Pit. When the level of the external Pi is lower than 20μM, another transport system, Pst, is induced with a Kt of 0.25μM. An outer‐membrane porin, PhoE, with a Km of about 1μM is also induced. The outer membrane allows the intake of organic phosphates which are degraded to Pi by phosphatases in the periplasm. The Pi‐binding protein will capture the free Pi produced in the periplasm and direct it to the transmembrane channel of the cytoplasmic membrane. The channel consists of two proteins, PstA and PstC, which have six and five transmembrane helices, respectively. On the cytoplasmic side of the membrane the channel is linked to the PstB protein, which carries a nucleotide (probably ATP)‐binding site. PstB probably provides the energy required by the channel to free Pi. The Pst system has two functions in E. coli: (i) the transport of Pi, and (ii) the negative regulation of the phosphate regulon (a complex of 20 proteins mostly related to organic phosphate transport). It is remarkable that these two functions are not related, since the repressibility of the regulon depends on the integ