One of the most important achievements of bacteria is its capability to adapt to the changing conditions of the environment. The competition for nutrients with other microorganisms, especially in the soil, where nutritional conditions are more variable, has led bacteria to evolve a plethora of mechanisms to rapidly fine-tune the requirements of the cell. One of the essential nutrients that are normally found in low concentrations in nature is inorganic phosphate (Pi). Bacteria, as well as other organisms, have developed several systems to cope for the scarcity of this nutrient. To date, the unique mechanism responding to Pi starvation known in detail is the Pho regulon, which is normally controlled by a two component system and constitutes one of the most sensible and efficient regulatory mechanisms in bacteria. Many new members of the Pho regulon have emerged in the last years in several bacteria; however, there are still many unknown questions regarding the activation and function of the whole system. This review describes the most important findings of the last three decades in relation to Pi regulation in bacteria, including: the PHO box, the Pi signaling pathway and the Pi starvation response. The role of the Pho regulon in nutritional regulation cross-talk, secondary metabolite production, and pathogenesis is discussed in detail.

Phosphate limitation in Streptomyces and in other bacteria triggers expression changes of a large number of genes. This response is mediated by the two-component PhoR-PhoP system. A Streptomyces coelicolor DeltaphoP mutant (lacking phoP) has been obtained by gene replacement. A genome-wide analysis of the primary response to phosphate limitation using transcriptomic and proteomic studies has been made in the parental S. coelicolor M145 and in the DeltaphoP mutant strains. Statistical analysis of the contrasts between the four sets of data generated (two strains under two phosphate conditions) allowed the classification of all genes into 12 types of profiles. The primary response to phosphate limitation involves upregulation of genes encoding scavenging enzymes needed to obtain phosphate from different phosphorylated organic compounds and overexpression of the high-affinity phosphate transport system pstSCAB. Clear interactions have been found between phosphate metabolism and expression of nitrogen-regulated genes and between phosphate and nitrate respiration genes. PhoP-dependent repressions of antibiotic biosynthesis and of the morphological differentiation genes correlated with the observed DeltaphoP mutant phenotype. Bioinformatic analysis of the presence of PHO boxes (PhoP-binding sequences) in the upstream regions of PhoP-controlled genes were validated by binding of PhoP, as shown by electrophoretic mobility shift assays.

Bacterial growth requires equilibrated concentration of C, N and P sources. This work shows a phosphate control over the nitrogen metabolism in the model actinomycete Streptomyces coelicolor. Phosphate control of metabolism in Streptomyces is exerted by the two component system PhoR-PhoP. The response regulator PhoP binds to well-known PHO boxes composed of direct repeat units (DRus). PhoP binds to the glnR promoter, encoding the major nitrogen regulator as shown by EMSA studies, but not to the glnRII promoter under identical experimental conditions. PhoP also binds to the promoters of glnA and glnII encoding two glutamine synthetases, and to the promoter of the amtB-glnK-glnD operon, encoding an ammonium transporter and two putative nitrogen sensing/regulatory proteins. Footprinting analyses revealed that the PhoP-binding sequence overlaps the GlnR boxes in both glnA and glnII. ‘Information theory’ quantitative analyses of base conservation allowed us to establish the structure of the PhoP-binding regions in the glnR, glnA, glnII and amtB genes. Expression studies using luxAB as reporter showed that PhoP represses the above mentioned nitrogen metabolism genes. A mutant deleted in PhoP showed increased expression of the nitrogen metabolism genes. The possible conservation of phosphate control over nitrogen metabolism in other microorganisms is discussed.

SummaryThe regulatory proteins AfsR and PhoP control expression of the biosynthesis of actinorhodin and undecylprodigiosin in Streptomyces coelicolor. Electrophoretic mobility shift assays showed that PhoP