The biochemical characterization of two phosphate transport systems in Phytomonas serpens

Inorganic phosphate (Pi) is an essential nutrient for all organisms because it is required for a variety of biochemical processes, such as signal transduction and the synthesis of phosphate-containing bio-molecules. Assays of P-32(i) uptake performed in the absence or in the presence of Na+ indicated the existence of a Na+-dependent and a Na+-independent P-i transporter in Phytomonas serpens. Phylogenetic analysis of two hypothetical protein sequences of Phytomonas (EM1) showed similarities to the high-affinity P-i transporters of Saccharomyces cerevisiae: Pho84, a Na+-independent P-i transporter, and Pho89, a Na+-dependent P-i transporter. Plasma membrane depolarization by FCCP, an H+ ionophore, strongly decreased P-i uptake via both Na+-independent and Na+-dependent carriers, indicating that a membrane potential is essential for P-i influx. In addition, the furosemide-sensitive Na+-pump activity in the cells grown in low P-i conditions was found to be higher than the activity detected in the plasma membrane of cells cultivated at high P-i concentration, suggesting that the up -regulation of the Na+-ATPase pump could be related to the increase of P-i uptake by the Pho89p Na+:P-i symporter. Here we characterize for the first time two inorganic phosphate transporters powered by Na+ and H+ gradients and activated by low P-i availability in the phytopathogen P. serpens. (C) 2016 Elsevier Inc. All rights reserved.