Effect of polyphosphate metabolism on the Escherichia coli phosphate-starvation response

A previously developed dynamic model of the Escherichia coli Pho regulon wa s extended to investigate the effect of polyphosphate synthesis and degrada tion on this control system. Differential equations for ATP and polyphospha te were formulated, and the model was applied to the growth of cells contai ning the ppk and ppx genes under control of separate, inducible promoters. In agreement with recent experimental observations, the degradation of poly phosphate by PPX during a period of phosphate limitation could repress the phosphate-starvation response. This is attributed to the release of phospha te from the cell into the periplasm, where it can be detected by the extern al phosphate sensor. A segregated model was then developed to account for d ifferences in KI, the dissociation constant for the repression complex, amo ng cells of the population. Since KI is the key parameter in determining wh ether the Pho response is induced or repressed at a particular surface phos phate concentration, this permitted the induction of some cells while other s remained repressed. The induction profiles resulting from the population- averaged values more closely matched experimental results than did those wi th the nonsegregated model.