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Metabolism and Enzymology

A Novel Role for Light in the Activation of Ribulosebisphosphate Carboxylase/Oxygenase

Department of Agronomy, University of Illinois, Urbana, Illinois 61801, Agricultural Research Service, U.S. Department of Agriculture, Urbana, Illinois 61801

Light stimulated the activation of ribulosebisphosphate carboxylase/oxygenase (rubisco) in a buffered lysed chloroplast system in the presence of saturating concentrations of ATP. This indicates a role for light in the rubisco activase activation system in addition to the previously identified requirement for the synthesis of ATP. Rubisco activation was nearly as great at low irradiance (10 micromoles of photons per square meter per second) as at high irradiance (1000 micromoles of photons per square meter per second). Light stimulation of activation occurred at both low bicarbonate (equivalent to air levels of CO₂) and high bicarbonate (10 mM) concentrations. Light activation was inhibited by DCMU and glyoxylate. Methyl viologen did not inhibit light activation, and dithiothreitol did not stimulate activation in the dark, indicating that the ferredoxin/thioredoxin system was not involved. Following a transition of the lysed chloroplasts from light to dark, the light-dependent increase in activation ceased immediately. The experiments were conducted with chloroplasts from spinach (Spinacea oleracea L.), a species which was previously shown not to contain the endogenous inhibitor of rubisco, 2-carboxyarabinitol 1-phosphate. Assays of total rubisco activity in the light and dark confirmed the absence of such a tight binding inhibitor of activity. The observations reported here cannot be explained by current hypotheses of the role of light in rubisco activation and demonstrate that in addition to providing ATP needed for rubisco activase activity, at least one other light-dependent reaction is required for regulating the activation state of rubisco in vivo.

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