Physiological and biochemical studies on leaf development and senescence : Degradation of Rubisco mediated by reactive oxygen species

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Rubilose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39, Rubiscso) is a bifunctional enzyme which catalyzes two competing reactions, photosynthetic COィイD22ィエD2 assimilation and photorespiratory carbon oxidation in the stroma of the chloroplasts, and is the most abundant protein in leaves. During leaf senescence or under environmental stress conditions, Rubisco is one of the early proteins that are broken down in leaves. However, only limited information is available on the triggering mechanisms that cause Rubisco degradation in plants. Here, we found that the large subunit of Rubisco is directly, fragmented into the 37-kDa and 16-kDa fragments by reactive oxygen species generated in the illuminated lysates of chloroplasts or intact chloroplasts. Analysis of the terminal amino acid residues of both fragments indicated that the fragmentation of the LSU occurs at Gly-329. Purified Rubisco, exposed to a hydroxyl radical generating system, was also cleaved at the same site. Reactive oxygen species generated at its catalytic site by a Fenton-type reaction may trigger the site specific degradation of the LSU in the light.