2Energy Sciences, National Renewable Energy Laboratory, Golden, CO 80401, USA; E-mail: email@example.com
* To whom correspondence should be addressed.
Received September 8, 2011; Revision received October 10, 2011
A “decoupling effect” (light-induced electron transport without O2 evolution) was observed in Ca-depleted photosystem II (PSII(–Ca)) membranes, which lack PsbP and PsbQ (Semin et al. (2008) Photosynth. Res., 98, 235-249). Here PsbO-depleted PSII (PSII(–PsbO)) membranes (which also lack PsbP and PsbQ) were used to examine effects of PsbO on the decoupling. PSII(–PsbO) membranes do not reduce the acceptor 2,6-dichlorophenolindophenol (DCIP), in contrast to PSII(–Ca) membranes. To understand why DCIP reduction is lost, we studied light effects on the Mn content of PSII(–PsbO) samples and found that when they are first illuminated, Mn cations are rapidly released from the Mn cluster. Addition of an electron acceptor to PSII(–PsbO) samples accelerates the process. No effect of light was found on the Mn cluster in PSII(–Ca) membranes. Our results demonstrate that: (a) the oxidant, which directly oxidizes an as yet undefined substrate in PSII(–Ca) membranes, is the Mn cluster (not the YZ radical or P680+); (b) light causes rapid release of Mn cations from the Mn cluster in PSII(–PsbO) membranes, and the mechanism is discussed; and (c) rapid degradation of the Mn cluster under illumination is significant for understanding the lack of functional activity in some PSII(–PsbO) samples reported by others.
KEY WORDS: photosystem II, oxygen-evolving complex, manganese, PsbO extrinsic protein