Structural diversity and differential light control of mRNAs coding for angiosperm glyceraldehyde-3-phosphate dehydrogenases

Abstract

Subunits A and B of chloroplast glyceraldehyde-3-phosphate dehydrogenase are synthesized as higher MW precursors when polyadenylated 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 plant species [Sinapis alba, tomato, cucumber, mustard, pea, bean, maize, Sorghum, rye, wheat, oat and barley] 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. Precursors of the chloroplast enzyme were identified for the 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 4000 and 12,000 daltons and seem to fall into 3 major size classes. Chloroplast glyceraldehyde-3-phosphate dehydrogenase, like its cytosolic counterpart, is encoded in the nucleus. The 2 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.