XVI International Botanical Congess

More detailed studies on the mechanism of light-dark modulation of chloroplast enzymes revealed the cooperation of the thiol/disulfide redox reaction with structural changes which, in some cases, are able to bring about activation even without reduction. Therefore, we have attempted to dissect the various contributions to the process leading to activation of the Calvin-cycle enzymes fructose 1,6-phosphatase (FBPase), phosphoribulokinase (PRK) and NADP-glyceraldehyde 3-phosphate dehydrogenase (NADP-GAPDH) by using mutant proteins with single cysteines substituted for by serine residues. For spinach FBPase we demonstrate evidence for a single disulfide bridge between C155 and C174. An alternative conformation could be the result of a change that can also be induced in the recombinant wildtype enzyme by lack of Mg2+ during purification. For PRK and NADP-GAPDH a model will be discussed involving changes of the aggregation state. The role of the newly discovered chloroplast protein CP12 for such reversible structural changes will be evaluated.