The biochemical characterization of two phosphate transport systems in Phytomonas serpens.

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