The Arabidopsis BELL1 and KNOX TALE Homeodomain Proteins Interact through a Domain Conserved between Plants and Animals

Abstract

Previous studies of photosynthetic carbon fixation in the marine alga Gonyaulax have shown that the reaction rates in vivo vary threefold between day and night but that the in vitro activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which catalyzes the rate-limiting step in this process, remains constant. Using protein gel blotting, we confirm that Rubisco protein levels are constant over time. We present simultaneous measurements of the rhythms of CO₂ fixation and O₂ evolution and show that the two rhythms are ∼6 hr out of phase. We further show that the distribution of Rubisco within chloroplasts varies as a function of circadian time and that this rhythm in Rubisco distribution correlates with the CO₂ fixation rhythm. At times of high carbon fixation, Rubisco is found in pyrenoids, regions of the chloroplasts located near the cell center, and is separated from most of the light-harvesting protein PCP (for peridinin–chlorophyll a–protein), which is found in cortical regions of the plastids. We propose that the rhythm in Rubisco distribution is causally related to the rhythm in carbon fixation and suggest that several mechanisms involving enzyme sequestration could account for the increase in the efficiency of carbon fixation.