Substrate specificity of the two phosphate transport systems of Acinetobacter johnsonii 210A in relation to phosphate speciation in its aquatic environment

In natural waters and domestic waste waters in which divalent metal ions are present in excess of Pi, H2PO4-, HPO4(2-), and MeHPO4 prevail at pH values physiological for Acinetobacter johnsonii 210A (pH 5.5-8.0). In view of the ability of this organism to extensively accumulate Pi and divalent cations in cytoplasmic polyphosphate granules, the substrate specificity of its two Pi transport systems was studied. The constitutive, proton motive force-driven Pi carrier, previously shown to be dependent on divalent cations, plays a major role in the divalent cation and Pi flux by translocating MeHPO4 rather than Pi. This notion is confirmed by the observation that divalent cations are cotransported with Pi in a 1:1 stoichiometry in proteoliposomes containing reconstituted Pi carrier protein. In contrast, the Pi repressible, periplasmic binding protein-dependent Pi transport system mediates the uptake of H2PO4- and HPO4(2-). Pi uptake, but not MeHPO4 uptake, was stimulated in cells under Pi limitation, and the periplasmic Pi-binding protein has affinity for H2PO4- and HPO4(2-), but not for MeHPO4. When operating in concert, both systems enable A. johnsonii 210A to efficiently acquire Pi from its habitat through uptake of the predominant Pi species.