Subsystem: High affinity phosphate transporter and control of PHO regulon

This subsystem's description is:

Inorganic phosphate (Pi), the preferred source of phosphorus, is often limiting in the environment. Multiple genes induced under Pi limiting conditions comprise the PHO operon. These genes are coregulated by extra-cellular phosphate and are likely all involved in phosphorus assimilation (Wanner, 1993).

The signal transduction pathway required for the regulation of the PHO regulon has been analyzed in great detail in E. coli and related bacteria (Wanner, 1993, Wanner, 1996; van Veen, 1997; White, 2000). Its components required for regulation are: (i) PstS, a periplasmic Pi binding protein, (ii) PstACB – high-affinity inner membrane Pi uptake ABC transporter, (iii) PhoU (iv) histidine kinase PhoR, response regulator PhoB. Repression of PHO regulon under conditions of Pi excess (above 4 microM) requires the PstSABC system, PhoU, and PhoR. Induction of PHO regulon under Pi-limiting conditions requires PhoR and PhoB.

Model for the PHO regulation according to (Wanner, 1993, Wanner, 1996):
When Pi is available in access, the Pi-PstS in the periplasm binds to the PstABC transporter in the membrane. A “repressor” complex forms between Pi-PstS, PstABC, PhoU, and PhoR. PhoR within the complex acts a phosphatase and maintains PhoB in the dephosphorylated state. When Pi becomes limiting, PhoR is released from the repressor complex, autophosphorylates, and then phosphorylates PhoB. Phosphorylated PhoB is a positive transcription regulator for the PHO regulon.

In GramPositive microorganisms, regulatory circuit(s) induced by limiting Pi is less understood. For example, in B. subtilis at least three global regulatory systems are responsible for changes in gene expression upon phosphate deprivation (Paul et al., 2004). One set of genes is controlled either positively or negatively by the PhoP-PhoR two-component regulators, genes referred to as the Pho regulon genes (for review, Hulett, 2002). Other genes that are induced upon phosphate limitation are dependent on SigB, an alternative stress sigma factor. A third class of genes is expressed under phosphate-limiting growth conditions that are independent of either SigB or PhoP-PhoR (Antelmann, et al., 2000). The regulatory coordination between these three sets of genes is unclear (Paul et al., 2004).

Pho regulon genes are the most extensively studied set of phosphate-regulated genes in B. subtilis. A high-affinity Pi transport system (PstS system) is directly induced for the uptake of inorganic phosphate, while a family of alkaline phosphatases, PhoA, PhoB, and PhoD, are secreted whose activity may function to supply the decreasing Pi pool. Anionic cell wall polymer turnover (Archibald et al., 1993) is controlled by PhoP-PhoR, as phosphorylated PhoP (PhoP~P) directly represses tag genes that are required for synthesis of the high-phosphate anionic polymer, teichoic acid, and activates the tua genes responsible for synthesis of a non-phosphate-containing polymer, teichuronic acid, under phosphate-limiting conditions. Hence, B. subtilis “carries its phosphate reserve on its back”, as teichoic acid is turned over as the teichuronic acid replaces it. The secreted phosphodiesterases and phosphomonoesterases, PhoD, PhoB, and PhoA, are believed to have a role in the teichoic acid degradation, providing an additional phosphate supply for uptake via the PstS high-affinity transport system (Paul et al., 2004).

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