A NEW CLASS OF PLASTIDIC PHOSPHATE TRANSLOCATORS - A PUTATIVE LINK BETWEEN PRIMARY AND SECONDARY METABOLISM BY THE PHOSPHOENOLPYRUVATE PHOSPHATE ANTIPORTER/

We have purified a plastidic phosphate transport protein from maize en dosperm membranes and cloned and sequenced the corresponding cDNAs fro m maize endosperm, maize roots, cauliflower buds, tobacco leaves, and Arabidopsis leaves. All of these cDNAs exhibit high homology to each o ther but only similar to 30% identity to the known chloroplast triose phosphate/phosphate translocators, The corresponding genes are express ed in both photosynthetically active tissues and in nongreen tissues, although transcripts were more abundant in nongreen tissues. Expressio n of the coding region in transformed yeast cells end subsequent trans port measurements of the purified recombinant translocator showed that the protein mediates transport of inorganic phosphate in exchange wit h C3 compounds phosphorylated at C-atom 2, particularly phosphoenolpyr uvate, which is required inside the plastids for the synthesis of, for example, aromatic amino acids. This plastidic phosphate transporter i s thus different in structure and function from the known triose phosp hate/phosphate translocator. We propose that plastids contain various phosphate translocators with overlapping substrate specificities to en sure an efficient supply of plastids with a single substrate, even in the presence of other phosphorylated metabolites.