THE RELATIONSHIP BETWEEN CO2-ASSIMILATION RATE, RUBISCO CARBAMYLATIONAND RUBISCO ACTIVASE CONTENT IN ACTIVASE-DEFICIENT TRANSGENIC TOBACCOSUGGESTS A SIMPLE-MODEL OF ACTIVASE ACTION

Transgenic tobacco (Nicotiana tabacum L. cv. W38) plants with an antis ense gene directed against the mRNA of ribulose-1,5-bisphosphate carbo xylase/oxygenase (Rubisco) activase were used to examine the relations hip between CO2-assimilation rate, Rubisco carbamylation and activase content. Plants used were those members of the R(1) progeny of a prima ry transformant with two independent T-DNA inserts that could be grown without CO2 supplementation. These plants had from < 1% to 20% of the activase content of control plants. Severe suppression of activase to amounts below 5% of those present in the controls was required before reductions in CO2-assimilation rate and Rubisco carbamylation were ob served, indicating that one activase tetramer is able to service as ma ny as 200 Rubisco hexadecamers and maintain wild-type carbamylation le vels in vivo. The reduction in CO2-assimilation rate was correlated wi th the reduction in Rubisco carbamylation. The anti-activase plants ha d similar ribulose-1,5-bisphosphate pool sizes but reduced 3-phosphogl ycerate pool sizes compared to those of control plants. Stomatal condu ctance was not affected by reduced activase content or CO2-assimilatio n rate. A mathematical model of activase action is used to explain the observed hyperbolic dependence of Rubisco carbamylation on activase c ontent.