Inorganic polyphosphate regulates responses of Escherichia coli to nutritional stringencies, environmental stresses and survival in the stationary phase.

The molecular mechanisms responsible for polyP accumulation in E. coli remain largely obscure. Based on the available data, a tentative model is proposed (Fig. 1; Ault-Riché et al. 1998). Inhibition by (p)ppGpp of PPX interrupts the dynamic balance between the synthesis of polyP by PPK and its hydrolysis by PPX, accounting for polyP accumulation. However, mutants lacking PhoB, the response regulator of the Pho regulon, fail to accumulate polyP even in the face of high levels of (p)ppGpp. Clearly, PhoB is required in some undefined manner. With regard to osmotic stress, the pathway to polyP accumulation is also distinct from the one identified with the activation of envZ and the associated changes in membrane functions. A tentative scheme attempting to describe the metabolic turnover of polyP is given in Fig. 4. [figure: see text] In adaptations to stress, cells must coordinate major changes in the rates of transcription, translation, and replication as well as make choices in the genes expressed (Kolter et al. 1993). PolyP could provide activated phosphates or coordinate an adaptive response by binding metals and/or specific proteins. Accumulation of polyP in E. coli and other organisms is commonly assumed to provide a reservoir of energy convertible to ATP. This seems implausible because of the turnover of ATP which consumes only a fraction of a second (Chapman and Atkinson 1977). Thus, other functions for polyP need to be considered, among them a regulatory role. PolyP, even at very low levels, is essential in E. coli for adaptations in stationary phase and for survival (Rao and Kornberg 1996). As a polyanionic polymer, polyP has chemical similarities to DNA and RNA in interactions with basic domains of proteins. Further investigation of the cellular location of polyP, its state of metabolic availability and identification of its binding partners are needed. In view of the ubiquity of polyP in eukaryotic cells (including dynamic turnover in the nuclei of some mammalian cells), studies similar to those undertaken in E. coli may reveal comparable functions.