Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2000. Rubisco activase is a nuclear-encoded chloroplast protein that is required for the light activation of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in vivo. In most plants examined to date, there are two soforms of Rubisco activase arising from alternative splicing that differ only at the carboxyl terminus. In Arabidopsis the larger isoform has a unique role in the regulation of Rubisco activity. At physiological ratios of ADP/ATP, the 46-kDa isoform has minimal ATP hydrolysis and Rubisco activation activity in comparison with 43-kDa isoform. Analysis of a series of carboxyl-terminal deletion and Ala substitution mutants of the 46-kDa isoform revealed that the presence of Cys residues at positions 411 and 392 was essential to preserve a low ATP hydrolysis and Rubisco activation activity in the presence of ADP. Consequently, incubation of the 46-kDa isoform with DTT and thioredoxin-f increased both activities, whereas incubation with DTT alone or with thioredoxin-m was ineffective. Thioredoxin-f and DTT had no effect on the 43-kDa isoform. However, premixing both soforms before conducting a reduction and oxidation cycle demonstrated that the activity of both soforms could be regulated. Reduction and oxidation also modulated the activity of native activase proteins isolated from either Arabidopsis or spinach, but not tobacco, which only has the smaller isoform. Transgenic plants expressing 46-kDa soforms with Cys-Ala substitution in the carboxyl-terminus unique to this isoform, lost the ability to down-regulate Rubisco activity during a high to low light transition, in contrast to plants expressing only the 46-kDa isoform or wild-type plants. These findings suggest that in plants containing both soforms Rubisco activase regulates the activity of Rubisco in response to light-induced changes in both the ADP/ATP ratio and the redox potential via thioredoxin-f. U of I Only Restricted to the U of I community idenfinitely during batch ingest of legacy ETDs