A new class of plastidic phosphate translocators: a putative link between primary and secondary metabolism by the phosphoenolpyruvate/phosphate antiporter. The Plant Cell 9

We have purified a plastidic phosphate transport protein f rom maize endosperm membranes and cloned and se-quenced the corresponding cDNAs from maize endosperm, , maize roots, cauliflower buds, tobacco leaves, and Arabidopsis leaves. All of these cDNAs exhibit high homology taeach other but only-30 % identity to the known chlo-roplast triose phosphate/phosphate translocators. The corresporrding genes are expressed in both photosynthetically active tissues and in nongreen tissues, although transcripts were more abundant in nongreen tissues. Expression of the coding region in transformed yeast cells and subsequent transport measurements of the purified recombinant translocator showed that the protein mediates transport of inorganic phosphate in exchange with C3 compounds phosphorylated at C-atom 2, particularly phosphoenolpyruvate, which is required inside the plastids for the synthesis of, for example, aromatic amino acids. This plastidic phosphate transporter is thus different in structure and function from the known triose phosphate/phosphate translocator. We prsp,ose that plastids contain various phosphate translo-cators with overlapping substrate specificities to ensure an effici6nt supplyiof plastids with a single substrate, even in the presence of other phosphorylated metabolites