Activation of Rubisco controls CO(2) assimilation in light: a perspective on its discovery.

CO(2) fixation during photosynthesis is regulated by the activity of ribulose bisphosphate carboxylase (Rubisco). This conclusion became more apparent to me after CO(2)-fixation experiments using isolated spinach chloroplasts and protoplasts, purified Rubisco enzyme, and intact leaves. Ribulose bisphosphate (RuBP) pools and activation of Rubisco were measured and compared to (14)CO(2) fixation in light. The rates of (14)CO (2) assimilation best followed the changes in Rubisco activation under moderate to high light intensities. RuBP pool sizes regulated (14) (2) assimilation only in very high CO(2) levels, low light and in darkness. Activation of Rubisco involves two separate processes: carbamylation of the protein and removal of inhibitors blocking carbamylation or blocking RuBP binding to carbamylated sites before reaction with CO(2) or O(2).