Use of New Methods for Construction of Tightly Regulated Arabinose and Rhamnose Promoter Fusions in Studies of the Escherichia coli Phosphate Regulon

Escherichia coli genes regulated by environmental inorganic phosphate (P(i)) levels form the phosphate (Pho) regulon. This regulation requires seven proteins, whose synthesis is under autogenous control, including response regulator PhoB, its partner, histidine sensor kinase PhoR, all four components of the P(i)-specific transport (Pst) system (PstA, PstB, PstC, and PstS), and a protein of unknown function called PhoU. Here we examined the effects of uncoupling PhoB synthesis and PhoR synthesis from their normal controls by placing each under the tight control of the arabinose-regulated P(araB) promoter or the rhamnose-regulated P(rhaB) promoter. To do this, we made allele replacement plasmids that may be generally useful for construction of P(araB) or P(rhaB) fusions and for recombination of them onto the E. coli chromosome at the araCBAD or rhaRSBAD locus, respectively. Using strains carrying such single-copy fusions, we showed that a P(rhaB) fusion is more tightly regulated than a P(araB) fusion in that a P(rhaB)-phoR(+) fusion but not a P(araB)-phoR(+) fusion shows a null phenotype in the absence of its specific inducer. Yet in the absence of induction, both P(araB)-phoB(+) and P(rhaB)-phoB(+) fusions exhibit a null phenotype. These data indicate that less PhoR than PhoB is required for transcriptional activation of the Pho regulon, which is consistent with their respective modes of action. We also used these fusions to study PhoU. Previously, we had constructed strains with precise ΔphoU mutations. However, we unexpectedly found that such ΔphoU mutants have a severe growth defect (P. M. Steed and B. L. Wanner, J. Bacteriol. 175:6797–6809, 1993). They also readily give rise to compensatory mutants with lesions in phoB, phoR, or a pst gene, making their study particularly difficult. Here we found that, by using P(araB)-phoB(+), P(rhaB)-phoB(+), or P(rhaB)-phoR(+) fusions, we were able to overcome the extremely deleterious growth defect of a Pst(+) ΔphoU mutant. The growth defect is apparently a consequence of high-level Pst synthesis resulting from autogenous control of PhoB and PhoR synthesis in the absence of PhoU.