THE ABSENCE OF RUBISCO ACTIVASE ACTIVITY IN TOTAL WHEAT LEAF EXTRACTSIS RECOVERED IN THE PURIFIED PROTEIN

When desalted extracts of soluble protein from dark-adapted wheat leav es were assayed for ribulose-1,5-bisphosphate carboxylase/oxygenase (R ubisco) activase activity in the presence of 1 mM ATP and an ATP-regen erating system, very little ATP-dependent activation of RuBP-inactivat ed Rubisco was found. In extracts from light-adapted leaves a very sim ilar pattern of Rubisco activation was observed except that the overal l level of Rubisco activity was much lower than in the extracts from d ark-adapted leaves, These features were apparent both at low (120 mu g per ml) and high (640 mu g per ml) protein concentrations. We were un able to demonstrate Rubisco activase activity in crude leaf extracts. Consequently, in order to establish that Rubisco activase was present in wheat leaf extracts the wheat leaf protein was purified to homogene ity. The identity of the protein was confirmed with antibodies to the spinach enzyme, ATPase activity and activase-mediated release of the i nhibitor, carboxyarabinitol-1-phosphate (CA1P) from the tertiary Rubis co complex. The pure wheat Rubisco activase relieved the CA1P-induced inhibition of Rubisco activity. Rubisco activase had no significant ef fect on the affinity of wheat Rubisco for the substrate, ribulose-1,5- bisphosphate (RuBP).