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Category: Microbial Genetics and Molecular Biology
Signal Transduction and Cross Regulation in the Escherichia coli Phosphate Regulon by PhoR, CreC, and Acetyl Phosphate, Page 1 of 2
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This chapter focuses on the control of the Pho regulon by the signal transduction pathways involving the Pi sensor PhoR, the catabolite regulatory sensor CreC, and acetyl phosphate. It is also poorly understood whether cross regulation by CreC or by acetyl phosphate has a bonafide role in Pho regulon control under certain conditions in normal cells. Therefore, some speculations are provided about the nature of the Pi signal transduction pathway and about the roles of CreC and acetyl phosphate in Pho regulon control. The chapter discusses signal transduction pathways of the Pho regulon. A Pi repression complex may contain all components of the Pst system, PhoU, and PhoR, because all these are required for Pi repression. By testing effects due to ackA and pta mutations, it was shown that activation of the Pho regulon in the absence of both PhoR and CreC requires acetyl phosphate synthesis. Evolutionarily related proteins share sequence similarities at the protein level with other members of the same family. Therefore, sensors are probably structurally and functionally similar to other sensors, and response regulators are probably structurally and functionally similar to other response regulators. The primary control of the Pho regulon involves a signal transduction pathway responsive to the extracellular Pi level.
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Alternative repression complexes for signal transduction by Pi limitation. Pi control may involve Pi binding only to PstS or to both PstS and a regulatory site, which may be on a different Pst component or on PhoR. A hypothetical regulatory site is indicated by a question mark in parentheses. In repression complex I, the PhoR repressor form is shown as a monomer, which is intended to imply any lower oligomeric form. In repression complex II, the PhoR repressor form is shown as a dimer, which is intended to imply any higher oligomeric form. These complexes are compatible with alternative mechanisms for inter-conversion of the PhoR repressor and activator forms as described in the text.
Alternative repression complexes for signal transduction by Pi limitation. Pi control may involve Pi binding only to PstS or to both PstS and a regulatory site, which may be on a different Pst component or on PhoR. A hypothetical regulatory site is indicated by a question mark in parentheses. In repression complex I, the PhoR repressor form is shown as a monomer, which is intended to imply any lower oligomeric form. In repression complex II, the PhoR repressor form is shown as a dimer, which is intended to imply any higher oligomeric form. These complexes are compatible with alternative mechanisms for inter-conversion of the PhoR repressor and activator forms as described in the text.
Mechanisms of detection of environmental Pi.The stoichiometric and regulatory site mechanisms are illustrated. In accordance with either mechanism, the PhoR repressor form may be associated with a repression complex when Pi is in excess. The stoichiometric mechanism may lead to the release of PhoR from those complexes to which no Pi is bound under conditions of Pi limitation. The regulatory site mechanism may lead to a conformational change in PhoR (without its release from a repression complex) due to absence of Pi occupancy of the regulatory site. PhoR is shown as a monomer solely for the purpose of illustration. No particular oligomeric form of PhoR is implied.
Mechanisms of detection of environmental Pi.The stoichiometric and regulatory site mechanisms are illustrated. In accordance with either mechanism, the PhoR repressor form may be associated with a repression complex when Pi is in excess. The stoichiometric mechanism may lead to the release of PhoR from those complexes to which no Pi is bound under conditions of Pi limitation. The regulatory site mechanism may lead to a conformational change in PhoR (without its release from a repression complex) due to absence of Pi occupancy of the regulatory site. PhoR is shown as a monomer solely for the purpose of illustration. No particular oligomeric form of PhoR is implied.
Mechanisms for interconversion of PhoRR and PhoRA by environmental Pi. Association-dissociation and conformational change mechanisms are illustrated. PhoRR may prevail when the environmental Pi level is in excess (an environmental Pi level greater than about 4 µM); and PhoRA may prevail under conditions of Pi limitation (an environmental Pi level less than about 4 µM). PhoRR may be a phospho-PhoB (PhoB-P) phosphatase, and PhoRA is a phosphoryl transferase and PhoB kinase. Accordingly, PhoRA may autophosphorylate on a histidine residue and transfer this phosphoryl group to an aspartate residue on PhoB. Although PhoRR and PhoRA are depicted in association with PhoB and PhoB-P, respectively, these associations are likely to be transient. No particular oligomeric form of PhoR is implied, except that PhoRA is probably a dimer or higher oligomer. This is because the mechanism of auto-phosphorylation for the PhoR homologs EnvZ and CheA involves phosphorylation of one subunit by the other subunit ( Yang and Inouye, 1991 , 1993 ; Swanson et al., 1993 ; Wolfe and Stewart, 1993 ). The oligomeric structure(s) of PhoR is unknown.
Mechanisms for interconversion of PhoRR and PhoRA by environmental Pi. Association-dissociation and conformational change mechanisms are illustrated. PhoRR may prevail when the environmental Pi level is in excess (an environmental Pi level greater than about 4 µM); and PhoRA may prevail under conditions of Pi limitation (an environmental Pi level less than about 4 µM). PhoRR may be a phospho-PhoB (PhoB-P) phosphatase, and PhoRA is a phosphoryl transferase and PhoB kinase. Accordingly, PhoRA may autophosphorylate on a histidine residue and transfer this phosphoryl group to an aspartate residue on PhoB. Although PhoRR and PhoRA are depicted in association with PhoB and PhoB-P, respectively, these associations are likely to be transient. No particular oligomeric form of PhoR is implied, except that PhoRA is probably a dimer or higher oligomer. This is because the mechanism of auto-phosphorylation for the PhoR homologs EnvZ and CheA involves phosphorylation of one subunit by the other subunit ( Yang and Inouye, 1991 , 1993 ; Swanson et al., 1993 ; Wolfe and Stewart, 1993 ). The oligomeric structure(s) of PhoR is unknown.
Pta-AckA pathway for ATP synthesis.
Pta-AckA pathway for ATP synthesis.
Pathways for Pi incorporation into ATP. AtpIBEFHAGDC, ATP synthase; Gap, glyceraldehyde-3-phosphate dehydrogenase; Pgk, phosphoglycerate kinase; PMF, proton motive force; SucCD, succinyl coenzyme A synthetase.
Pathways for Pi incorporation into ATP. AtpIBEFHAGDC, ATP synthase; Gap, glyceraldehyde-3-phosphate dehydrogenase; Pgk, phosphoglycerate kinase; PMF, proton motive force; SucCD, succinyl coenzyme A synthetase.
Multiple controls of PhoB phosphorylation by PhoR, CreC, and acetyl phosphate. A signal for the extracellular (ext) Pi level controls the Pho regulon via the Pst system, PhoU, and the Pi sensor PhoR. A signal for an unknown catabolite (which may be an intermediate of a central pathway of carbon, energy, and Pi metabolism) controls the Pho regulon via a central pathway and the catabolite sensor CreC. A signal for ATP synthesis controls the Pho regulon via the Pta-AckA pathway and acetyl phosphate. PhoR, CreC, and acetyl phosphate, in turn, activate the regulator PhoB by phosphorylation. PhoB has been shown to be directly phosphorylated by acetyl phosphate in vitro (Hiratsu et al., personal communication). Nevertheless, it has not been established whether acetyl phosphate acts directly on PhoB as a phosphoryl donor or indirectly via an unknown sensor in vivo ( Wanner and Wilmes-Riesenberg, 1992 ). The solid arrow symbolizes signal transduction by Pi limitation. The dashed arrows symbolize signal transduction due to (proposed) cross regulation. Adapted from Wanner (1992) .
Multiple controls of PhoB phosphorylation by PhoR, CreC, and acetyl phosphate. A signal for the extracellular (ext) Pi level controls the Pho regulon via the Pst system, PhoU, and the Pi sensor PhoR. A signal for an unknown catabolite (which may be an intermediate of a central pathway of carbon, energy, and Pi metabolism) controls the Pho regulon via a central pathway and the catabolite sensor CreC. A signal for ATP synthesis controls the Pho regulon via the Pta-AckA pathway and acetyl phosphate. PhoR, CreC, and acetyl phosphate, in turn, activate the regulator PhoB by phosphorylation. PhoB has been shown to be directly phosphorylated by acetyl phosphate in vitro (Hiratsu et al., personal communication). Nevertheless, it has not been established whether acetyl phosphate acts directly on PhoB as a phosphoryl donor or indirectly via an unknown sensor in vivo ( Wanner and Wilmes-Riesenberg, 1992 ). The solid arrow symbolizes signal transduction by Pi limitation. The dashed arrows symbolize signal transduction due to (proposed) cross regulation. Adapted from Wanner (1992) .
Effects of Pi limitation and catabolites on Pho regulon control
Effects of Pi limitation and catabolites on Pho regulon control
Genes involved in Pi-independent controls of the Pho regulon
Genes involved in Pi-independent controls of the Pho regulon
Pi-regulated genes of the E. coli Pho regulonª
Pi-regulated genes of the E. coli Pho regulonª