Light regulation of the synthesis of the large subunit of ribulose-1,5-bisphosphate carboxylase in peas: Evidence for translational control.

The specific activity of ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) increases 30- to 50-fold when dark-grown pea seedlings are shifted into the light. The large subunit (LS) of this multimeric protein is known to be synthesized in the chloroplast, but plastids from dark-grown cells contain relatively low levels of LS. However, despite the low level of LS synthesis in the plastids of dark-grown plants, these organelles contain significant levels of LS mRNA. Hybridization studies showed that the amount of LS mRNA increased about 3-fold, relative to total plant RNA, when dark-grown plants were illuminated. This increase in LS mRNA can be accounted for by a similar increase in chloroplast genome copy number. It was found that the amount of translatable LS mRNA per mug of plastid RNA is similar when isolated from either dark-grown plants or dark-grown plants subjected to light. These results suggest that although light can increase the level of LS mRNA by increasing the copy number of this gene, the primary regulation of LS synthesis by light in pea chloroplasts is at the level of translation.