Subunits A and B of chloroplast glyceraldehyde-3-phosphate dehydrogenase are synthesized as higher molecular weight precursors when polyadenylylated mRNA from angiosperm seedlings is translated in vitro by wheat germ ribosomes. The in vivo levels of mRNA coding for these precursors are strongly light dependent, and the increase in translational activity stimulated by continuous white light, relative to dark-grown seedlings, is at least 5-to 10-fold for the seven plant species investigated. As opposed to this, light does not seem to change mRNA levels coding for cytosolic glyceraldehyde-3-phosphate dehydrogenase, and the polypeptides synthesized in vitro have the same size as the authentic subunits. In addition, precursors of the chloroplast enzyme were identified for 12 different angiosperm species and compared with their respective subunits synthesized in vivo. The patterns of the in vitro and in vivo products correlate in several major characteristics. They both display a remarkable interspecific heterogeneity with respect to size and number of polypeptides. The peptide extensions of the enzyme precursors calculated from these data vary between 4,000 and 12,000 daltons and seem to fall into three major size classes. The present data demonstrate that chloroplast glyceraldehyde-3-phosphate dehydrogenase, like its cytosolic counterpart, is encoded in the nucleus. Yet, the two dehydrogenases are controlled differently at both the ontogenetic and phylogenetic levels. They follow separate biosynthetic pathways with respect to light regulation, post-translational processing, and transport and also exhibit different evolutionary rates. The fast evolutionary change observed for the chloroplast enzyme contrasts sharply with the conservative structure and sequence of the cytosolic enzyme.Chloroplast and cytosolic glyceraldehyde-3-phosphate dehydrogenases (GAPDHs) of higher plants are marker proteins for important aspects of chloroplast evolution and biogenesis. Although descendants of a common phylogenetic ancestor (1, 2), they differ in several structural features. The cytosolic NADspecific dehydrogenase is a single homotetramer like the corresponding enzymes from other sources. Chloroplast NADPdependent dehydrogenase is composed of two major isoenzymes A2B2 (isoenzyme 1) and A4 (isoenzyme 2); subunits A and B are distinguished by slightly different molecular weights (A s B) (3). The primary structures ofsubunits A and B are similar but differ considerably from the subunit structure of the cytosolic enzyme (1). The two dehydrogenases also differ in their evolutionary rates, as shown by a recent electrophoretic survey of the enzymes from 12 different angiosperm species (4). Whereas the cytosolic enzyme is an extremely conservative protein, the chloroplast enzyme seems to change rapidly during evolution.This differential control of the two dehydrogenases is not restricted to their evolutionary rates but also can be observed at the ontogenetic level. It has long been known that formation of the chloroplast dehydro...