Regulation of gene expression of ribulose bisphosphate carboxylase in greening pea leaves.

Ribulose bisphosphate carboxylase/oxygenase (RuBisCO) is composed of two subunits, the chloroplastcoded large subunit (LS) and the nuclear-coded small subunit (SS). The effects of different light doses on the levels of the large subunit gene (rbcL), the rbcL and the rbcS mRNAs, and the rate of synthesis of RuBisCO were followed during greening. The rbcL gene dosage changed in response to light whereas the level of the rbcL mRNA changed independently of the gene dosage. This suggests that the expression of the rbcL gene is transcriptionally regulated and that the change in gene dosage only partially contributes to the increase in the mRNA. It appeared that the RuBisCO synthesis rate was proportional to the rbcS mRNA level rather than rbcL mRNA level. These results, taken together with the earlier observations of many researchers, suggest that RuBisCO synthesis in greening pea leaves is controlled primarily at the level of transcription of both genes and is fine-tuned at the post-transcriptional level in chloroplasts, so that the amount of LS is almost stoichiometric to that of SS.