RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE ACTIVASE DEFICIENCY DELAYS SENESCENCE OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE/OXYGENASE BUT PROGRESSIVELY IMPAIRS ITS CATALYSIS DURING TOBACCO LEAF DEVELOPMENT/

Transgenic tobacco (Nicotiana tabacum L. cv W38) plants with an antise nse gene directed against the mRNA of ribulose-1,5-biphosphate carboxy lase/oxygenase (Rubisco) activase grew more slowly than wild-type plan ts in a CO2-enriched atmosphere, but eventually attained the same heig ht and number of leaves. Compared with the wild type, the anti-activas e plants had reduced CO2 assimilation rates, normal contents of chloro phyll and soluble leaf protein, and much higher Rubisco contents, part icularly in older leaves. Activase deficiency greatly delayed the usua l developmental decline in Rubisco content seen in wild-type leaves. T his effect was much less obvious in another transgenic tobacco with an antisense gene directed against chloroplast-located glyceraldehyde-3- phosphate dehydrogenase, which also had reduced photosynthetic rates a nd delayed development. Although Rubisco carbamylation was reduced in the anti-activase plants, the reduction was not sufficient to explain the reduced photosynthetic rate of older anti-activase leaves. Instead , up to a 10-fold reduction in the catalytic turnover rate of carbamyl ated Rubisco in vivo appeared to be the main cause. Slower catalytic t urnover by carbamylated Rubisco was particularly obvious in high-CO2-g rown leaves but was also detectable in air-grown leaves. Rubisco activ ity measured immediately after rapid extraction of anti-activase leave s was not much less than that predicted from its degree of carbamylati on, ruling out slow release of an inhibitor from carbamylated sites as a major cause of the phenomenon. Nor could substrate scarcity or prod uct inhibition account for the impairment. We conclude that activase m ust have a role in vivo, direct or indirect, in promoting the activity of carbamylated Rubisco in addition to its role in promoting carbamyl ation.