REDUCTION OF RIBULOSE-BISPHOSPHATE CARBOXYLASE ACTIVASE LEVELS IN TOBACCO (NICOTIANA-TABACUM) BY ANTISENSE RNA REDUCES RIBULOSE-BISPHOSPHATE CARBOXYLASE CARBAMYLATION AND IMPAIRS PHOTOSYNTHESIS

The in vivo activity of ribulose-1-5-bisphosphate carboxylase/oxygenas e (Rubisco) is modulated in response to light intensity by carbamylati on of the active site and by the binding of sugar phosphate inhibitors such as 2'-carboxyarabinitol-1-phosphate (CA1P). These changes are in fluenced by the regulatory protein Rubisco activase, which facilitates the release of sugar phosphates from Rubisco's catalytic site. Activa se levels in Nicotiana tabacum were reduced by transformation with an antisense gene directed against the mRNA for Rubisco activase. Activas e-deficient plants were photosynthetically impaired, and their Rubisco carbamylation levels declined upon illumination. Such plants needed h igh CO2 concentrations to sustain reasonable growth rates, but the lev el of carbamylation was not increased by high CO2. The antisense plant s had, on average, approximately twice as much Rubisco as the control plants. The maximum catalytic turnover rate (k(cat)) of Rubisco decrea ses in darkened tobacco leaves because of the binding of CA1P. The dar k-to-light increase in k(cat) that accompanies CA1P release occurred t o similar extents in antisense and control plants, indicating that nor mal levels of activase were not essential for CA1P release from Rubisc o in the antisense plants. However, CA1P was released in the antisense plants at less than one-quarter of the rate that it was released in t he control plants, indicating a role for activase in accelerating the release of CA1P.