Immunological similarity between a cyanobacterial enzyme and a nuclear DNA-encoded plastid-specific isozyme from spinach

The immunochemical properties of the plastid and cytosolic isozymes of phosphoglucose isomerase (glucosephosphate isomerase; D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9) in spinach (Spinacia oleracea ) and the single phosphoglucose isomerase enzyme from the cyanobacteriumSynechococcus sp. were compared by an application of the enzyme-linked immunosorbent assay. Utilizing antibodies made in rabbits against subunits of purified plastid and cytosolic phosphoglucose isomerase isozymes from spinach, we demonstrate that the plastid isozyme is immunochemically more similar to the cyanobacterial enzyme than to the spinach cytosolic counterpart. The antiserum to plastid phosphoglucose isomerase crossreacted strongly with plastid phosphoglucose isomerases from other flowering plants. The antiserum to cytosolic phosphoglucose isomerase crossreacted with other plant cytosolic phosphoglucose isomerase isozymes. The results are consistent with the hypothesis [Weeden, N. F. (1981)J. Mol. Evol. 17, 133-139] that the nuclear gene specifying plastid phosphoglucose isomerase was derived from a prokaryote after the incorporation of a plastid-like symbiont into the ancestral plant cell.