Decrease in carbamylation of rubisco by high CO2 concentration is due to decrease of rubisco activase in kidney bean

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Decrease in rubisco activation at high CO2 concentration was caused by decrease in carbamylation of rubisco (Rohet al., 1996). However, it is unclear whether decrease in carbamylation rate at high CO2 concentration is due to decrease in activity itself or content of rubisco activase. To clarify this ambiguity, investigation was performed to determine effects of CO2 concentration on rubisco activase with kidney bean (Phaseolus vulgaris L.) leaves grown at normal CO2 (350 ppm) and high CO2 (650 ppm) concentration. The analysis of Western blotting showed that the 50 and 14.5 kl) polypeptides were identified immunochemically as the large and small subunits of rubisco in the preparation, respectively. For the 14.5 kD small subunit, the degree of intensity at high CO2 concentration was similar to that at normal CO2 concentration. For the 50 kD large sububit, however, the intensity of a band at high CO, concentration was significantly higher than that at normal CO2 concentration, indicating that only the large subunit is affected by high CO2 concentration. The analysis of Western immunoblotting showed two major polypeptides at 46 and 42 kD which were identified as rubisco activase subunits. The intensities of two bands were shown to be higher at normal CO2 than high CO2 concentration. These data indicate that decrease of carbamylation resulting from increase of CO2 concentration was caused by rubisco activase. Finally, by employing ATP hydrolysis assay and ELISA, we also observed a significant decrease in both activity and content of rubisco activase as CO2 concentration was raised from normal to high CO2 concentration. These results suggest that decrease in rubisco carbamylation at high CO2 concentration is caused by activity itself and/or content of rubisco activase.