Molecular Mechanisms of Phosphate Sensing, Transport and Signalling in Streptomyces and Related Actinobacteria

Phosphorous, in the form of phosphate, is a key element in the nutrition of all living beings. In nature, it is present in the form of phosphate salts, organophosphates, and phosphonates. Bacteria transport inorganic phosphate by the high affinity phosphate transport system PstSCAB, and the low affinity PitH transporters. The PstSCAB system consists of four components. PstS is the phosphate binding protein and discriminates between arsenate and phosphate. In the species, the PstS protein, attached to the outer side of the cell membrane, is glycosylated and released as a soluble protein that lacks its phosphate binding ability. Transport of phosphate by the PstSCAB system is drastically regulated by the inorganic phosphate concentration and mediated by binding of phosphorylated PhoP to the promoter of the PstSCAB operon. In an additional high affinity transport system, PhnCDE, is also under PhoP regulation. Additionally, have a duplicated low affinity phosphate transport system encoded by the genes In this system phosphate is transported as a metal-phosphate complex in simport with protons. Expression of , but not that of in is regulated by PhoP. Interestingly, in many species, three gene clusters (for a polyphosphate kinase), are linked in a supercluster formed by nine genes related to phosphate metabolism. Glycerol-3-phosphate may be transported by the actinobacteria that contains a gene cluster for glycerol-3-P uptake, but the cluster is not present in genomes. Sugar phosphates and nucleotides are used as phosphate source by the species, but there is no evidence of the gene involved in the transport of sugar phosphates. Sugar phosphates and nucleotides are dephosphorylated by extracellular phosphatases and nucleotidases. An isolated gene for a hexose phosphate antiporter is present in several pathogenic corynebacteria, such as , but not in non-pathogenic ones. Phosphonates are molecules that contains phosphate linked covalently to a carbon atom through a very stable C-P bond. Their utilization requires the genes for phosphonates/phosphate transport and genes for degradation, including those for the subunits of the C-P lyase. Strains of the and genera were reported to degrade simple phosphonates, but bioinformatic analysis reveals that whole sets of genes for putative phosphonate degradation are present only in three species and a few species. Genes encoding the C-P lyase subunits occur in several species associated with plant roots or with mangroves, but not in the laboratory model species; however, the genes that encode phosphonates/phosphate transport systems are frequent in species, suggesting that these genes, in the absence of C-P lyase genes, might be used as surrogate phosphate transporters. In summary, and related actinobacteria seem to be less versatile in phosphate transport systems than Enterobacteria.