Restoration of the high-potential form of cytochrome b-559 by electron transport reactions through Photosystem II in Tris-treated Photosystem II membranes

Abstract

The conversion of cytochrome (Cyt) b-559 from the low-potential (LP) to the high-potential (HP) form under conditions for photoactivation of O 2 evolution (reconstitution of the Mn cluster) was investigated using Photosystem II (PS II) membranes that had been depleted of the Mn cluster by treatment with Tris. Illumination of the PS II membranes with continuous or flashing light in the presence of 0.1 mM Mn 2+ reactivated O 2 evolution and increased the level of the HP form of Cyt b-559 with a concomitant decrease in the level of the LP form. When illumination was achieved with flashing light, the restoration of the HP form occurred after two flashes, while reactivation of O 2 evolution required more than six flashes. It was also found that the HP form could be restored when the PS II membranes were illuminated in the presence of artificial electron donors instead of Mn 2+. NH 2OH (10–100 μM), 1,5-diphenylcarbazide (50–100 μM) and semicarbazide (0.5–1 mM) were effective in restoring the HP form. These observations suggest that, under photoactivation conditions, not the reconstitution of the Mn cluster but electron donation by Mn 2+ to PS II is responsible for the restoration of the HP form. The restoration of the HP form by illumination in the presence of Mn 2+ was not affected by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) but it was completely suppressed by artificial electron acceptors which bind to the Q B site and reoxidize Q A −, namely, 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone (DBMIB) and 2,6-dichloro-1,4-benzoquinone. These results suggest that some redox reaction(s) at the acceptor side of PS II, which probably involves Q A −, occurs during the course of the restoration of the HP form.