Regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase by carbamylation and 2-carboxyarabinitol 1-phosphate in tobacco: Insights from studies of antisense plants containing reduced amounts of rubisco activase

The regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity by 2-carboxyarabinitol l-phosphate (CA1P) was investigated using gas-exchange analysis of antisense tobacco (Nicotiana tabacum) plants conta ining reduced levels of Rubisco activase. When an increase in light flux fr om darkness to 1200 mu mol quanta m(-2) s(-1) was followed, the slow increa se in CO2 assimilation by antisense leaves contained two phases: one repres ented the activation of the noncarbamylated form of Rubisco, which was desc ribed previously, and the other represented the activation of the CA1P-inhi bited form of Rubisco. We present evidence supporting this conclusion, incl uding the observation that this second phase, like CA1P, is only present fo llowing darkness or very low light flux. In addition, the second phase of C O2 assimilation was correlated with leaf CA1P content. When this novel phas e was resolved from the CO2 assimilation trace, most of it was found to hav e kinetics similar to the activation of the noncarbamylated form of Rubisco . Additionally, kinetics of the novel phase indicated that the activation o f the CA1P-inhibited form of Rubisco proceeds faster than the degradation o f CA1P by CA1P phosphatase. These results may be significant with respect t o current models of the regulation of Rubisco activity by Rubisco activase.