Regulation of Intracellular Toxic Metals and Other Cations by Hydrolysis of Polyphosphate

Heavy metal tolerance in a number of microorganisms has been correlated with the presence of long-chain polymers of inorganic phosphate called polyphosphate. It has been proposed that the polyphosphate sequesters the metals, thereby reducing their effective intracellular concentration. However, recent evidence indicates that it is not only the amount of stored polyphosphate that is important for heavy metal tolerance but also the ability to degrade polyphosphate to orthophosphate. It is proposed that, in the presence of heavy metals, polyphosphate is degraded to orthophosphate by polyphosphatase and that the metal phosphates are transported out of the cell by the inorganic phosphate transport (PIT) system. Evidence supporting this hypothesis is presented.