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 biomolecules. Assays of (32)Pi uptake performed in the absence or in the presence of Na(+) indicated the existence of a Na(+)-dependent and a Na(+)-independent Pi transporter in Phytomonas serpens. Phylogenetic analysis of two hypothetical protein sequences of Phytomonas (EM1) showed similarities to the high-affinity Pi transporters of Saccharomyces cerevisiae: Pho84, a Na(+)-independent Pi transporter, and Pho89, a Na(+)-dependent Pi transporter. Plasma membrane depolarization by FCCP, an H(+) ionophore, strongly decreased Pi uptake via both Na(+)-independent and Na(+)-dependent carriers, indicating that a membrane potential is essential for Pi influx. In addition, the furosemide-sensitive Na(+)-pump activity in the cells grown in low Pi conditions was found to be higher than the activity detected in the plasma membrane of cells cultivated at high Pi concentration, suggesting that the up-regulation of the Na(+)-ATPase pump could be related to the increase of Pi uptake by the Pho89p Na(+):Pi symporter. Here we characterize for the first time two inorganic phosphate transporters powered by Na(+) and H(+) gradients and activated by low Pi availability in the phytopathogen P. serpens.