Two systems for the uptake of inorganic phosphate (P i ) in Escherichia coli, PitA and Pst, have been described. A revertant of a pitA pstS double mutant that could grow on P i was isolated. We demonstrate that the expression of a new P i transporter, PitB, is activated in this strain by a gene amplification event.Transport of inorganic phosphate (P i ) across the cytoplasmic membrane of Escherichia coli is mediated by the PitA protein and Pst system. PitA, which transports metal phosphates (28) and is constitutively expressed (17), is driven by the proton motive force (PMF) (18, 28). The Pst system, which transports P i at the expense of ATP (6, 9), is composed of a periplasmic P i -binding protein (PstS), two integral membrane proteins (PstC and PstA), and an ATP-binding protein (PstB) (24). The genes encoding these four proteins constitute an operon (24), together with phoU, which encodes a protein not required for P i transport (23). Under P i limitation, the expression of the Pst system, which is produced at a basal level under P i -replete conditions, is further induced. Like, for example, phoA, which encodes the periplasmic enzyme alkaline phosphatase (26), the pst-phoU operon is part of the pho regulon, which is under the control of a two-component regulatory system consisting of the proteins PhoB and PhoR (29). Furthermore, the Pst system appears to be involved in regulation, since mutations in the genes of the pst-phoU operon generally result in constitutive expression of the pho regulon (29-31). The exact mechanism by which the Pst system controls the expression of the pho regulon is not known. To study the role of PstS in regulation, we attempted to isolate mutants with mutations in the membrane components of the Pst system that can transport P i in the absence of the PstS protein. In one of the mutants obtained, a new P i transporter, PitB, appeared to be expressed.Construction of a pstS pitA double mutant. A pstS pitA double-mutant strain is expected to behave as an organic phosphate auxotroph (22), but previously described pstS pitA double mutants, such as strain C86, appear to take up P i and to grow on P i as the sole source of phosphate (reference 30 and data not shown). Western blotting revealed that the PstS protein was produced in this strain, although at much lower levels than in its parental strain, K10, grown under P i limitation (data not shown). To characterize the pstS mutation in strain C86, a DNA fragment was amplified by PCR. The amplified fragment was considerably larger than the expected 1.4 kb, which was found when strains MC4100 and K10 were analyzed (Fig. 1A). An enlarged PCR fragment was also found in another pitA pstS strain, C78 (Fig. 1A). Sequencing of the PCR fragment from strain C86 revealed the presence of an IS2 element in the promoter region of pstS (Fig. 1B), whereas no other mutation was found in the pstS gene or the other genes of the pst-phoU operon. Hence, strain C86 and probably also strain C78 contain a Pst system, which is expressed at a lower level due to the ...