Factors Affecting the Activation State and the Level of Total Activity of Ribulose Bisphosphate Carboxylase in Tobacco Protoplasts

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Abstract
The relationship between the activation state and the level of total activatable activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) was examined in tobacco protoplasts. When darkened protoplasts were illuminated, both activation and total activity increased, but at different rates; the t½ were 2.3 and 6.7 minutes, respectively. The light response of rubisco activation state and total activity, measured after 15 minutes of illumination, were similar but their responses to light transitions and photosynthetic inhibitors were different. When irradiance was reduced from saturating to subsaturating, deactivation of rubisco in protoplasts was immediate, whereas there was little change in total activity during the first 20 minutes following the transition. The light-induced increases in activation state and total activity were inhibited by nigericin, but activation was more sensitive exhibiting a response similar to that of photosynthesis. Treatment of tobacco protoplasts and leaves with methyl viologen at limiting irradiance increased rubisco activation, but inhibited the light-induced increase in total activity. These results indicate that light activation of rubisco is mechanistically distinct from the light-dependent changes in total activity in tobacco, a species containing carboxyarabinitol 1-phosphate, an endogenous inhibitor of total rubisco activity.