D1-D2 protein degradation in the chloroplast - Complex light saturation kinetics

The D1 and D2 proteins of the photosystem II (PSII) reaction center are sta ble in the dark, while rapid degradation occurs in the light. Thus far, a q uantitative correlation between degradation and photon fluences has not bee n determined. In Spirodela oligorrhiza, D1-D2 degradation increases with ph oton flux. We find that kinetics for D2 degradation mirror those for D1, ex cept that the actual half-life times of the D2 protein are about three time s larger than those of the D1. The degradation ratio, D2/D1, is fluence ind ependent, supporting the proposal [Jansen, M.A.K., Greenberg, B.M., Edelman , M., Mattoo, A.K. & Gaba, V. (1996), Photochem. Photobiol. 63, 814-817] th at degradation of the two proteins is coupled. It is commonly conceived tha t D1 degradation is predominantly associated with photon fluences that are supersaturating for photosynthesis. We now show that a fluence as low as 5 mu mol.m(-2.)s(-1) elicited a reaction constituting > 25 % of the total deg radation response, while > 90 % of the degradation potential was attained a t intensities below saturation for photosynthesis (approximate to 750 mu mo l.m(-2) .s(-1)). Thus, in intact plants, DI degradation is overwhelmingly a ssociated with fluences limiting for photosynthesis. D1 degradation increas es with photon flux in a complex, multiphasic manner. Four phases were unco vered over the fluence range from 0-1600 mu mol m(-2).s(-1). The multiphasi c saturation kinetics underscore that the D1 and D2 degradation response is complex, and emanates from more than one parameter. The physiological proc esses associated with each phase remain to be determined.