EFFECT OF PHOTOSYSTEM-II INHIBITOR K-15 ON PHOTOCHEMICAL-REACTIONS OFTHE ISOLATED D1 D2 CYTOCHROME B559 COMPLEX/

Effect of a highly efficient inhibitor of Photosystem II (PS II), K-15 oxy-bis-(trifluoromethyl)methyl)-2,6-dinitrophenyl hydrazone methyl k etone), was investigated using the D1/D2/cytochrome b559 reaction cent re (RC) complex. A novel approach for photoaccumulating reduced pheoph ytin (Pheo(-)) in the absence of the strong reducing agent, sodium dit hionite, was demonstrated which involved illumination in the presence of TMPD (from 5 to 100 mu M) under anaerobic conditions. The addition of K-15 at concentrations of 0.5 mu M and 2 mu M resulted in approx. 5 0% and near 100%, respectively, inhibition of this photoreaction, whil e subsequent additions of dithionite eliminated the inhibitory effect of K-15. Methyl viologen induced similar inhibition at much higher con centrations (>1 mM). Moreover, K-15 efficiently quenched the 'variable ' part of chlorophyll fluorescence (which is the recombination lumines cence of the pair P-680(+) Pheo(-)). A 50% inhibition was induced by 5 mu M K-15 and the effect was maximal in the range 20 to 200 mu M. Pho tooxidation of P-680 in the presence of 0.1 mM silicomolybdate was als o efficiently inhibited by K-15 (50% inhibition at 15 mu M). The data are consistent with the idea put forward earlier (Klimov et al. 1992) that the inhibitory effect of K-15 is based on facilitating a rapid re combination between Pheo(-) and P-680(+) (or Z(+)) via its redox prope rties. The inhibitor can be useful for suppressing PS II reactions in isolated RCs of PS II which are resistant to all traditional inhibitor s, like diuron, and probably functions by substituting for Q(A) missin g in the preparation. At a concentration of 0.5-50 mu M K-15 considera bly increased both the rate and extent of cytochrome b559 photoreducti on in the presence, as well as in the absence, of 5 mM MnCl2. Conseque ntly it is suggested that K-15 also serves as a mediator for electron transfer from Pheo(-) to cytochrome b559.