Antimycin A inhibits cytochrome b559-mediated cyclic electron flow within photosystem II
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The light reactions of photosynthesis are known to comprise both linear and cyclic electron flow in order to convert light energy into chemical energy in the form of NADPH and ATP. Antimycin A (AA) has been proposed as an inhibitor of ferredoxin-dependent cyclic electron flow around photosystem I (CEF-PSI) in photosynthesis research. However, its precise inhibitory mechanism and target site had not been elucidated yet. Here we show that AA inhibits the cyclic (alternative) electron flow via cytochrome b559 (Cyt b559) within photosystem II (CEF-PSII). When AA was applied to thylakoid membranes isolated from spinach leaves, the high potential form of Cyt b559, which was reduced in the dark, was transformed into the lower potential forms and readily oxidized by molecular oxygen. In the absence of AA, the reduced Cyt b559 was oxidized by P680+ upon light illumination and re-reduced in the dark, mainly by the electron from the QB site on the acceptor side of PSII. In contrast, AA suppressed the oxidation of Cyt b559 and induced its reduction under the illumination. This inhibition of Cyt b559 oxidation by AA enhanced photoinhibition of PSII. Based on the above results, we propose caution regarding the use of AA for evaluating CEF-PSI per se and concurrently propose that AA provides for new insights into, and interpretations of, the physiological importance of Cyt b559, rather than that of CEF-PSI in photosynthetic organisms.
KeywordsCytochrome b559 Photosystem II Antimycin A Photoinhibition Cyclic electron flow within photosystem II
PS II-enriched membranes prepared as in Berthold et al. (1981)
Cyclic electron flow
- Fe(III) CN
This work was supported by the Core Research for Environmental Science and Technology (Grant No. AL65D21010 to C.M. and K.I.), and MEXT/JSPS Grants-in-Aid for Scientific Research(C) No. 16K07690 to K.I. and for Scientific Research on Innovative Areas 16H06554 to K.I. We also thank Mgr. Kateřina Perútková from Photon Systems Instruments for the lecture on the TL 500 operation. We also thank Editage (Cactus Communications Inc., http://www.editage.jp/) for editing our manuscript.
DT conceived original research plans. DT, KI, and CM designed the experiments; DT performed most of the work; KI, TN, and CM provided suggestions for the experiments and the interpretation of data; DT and KI drafted and completed the manuscript with valuable suggestions from all the authors.
Compliance with ethical standards
Conflict of interest
The authors have no conflicts of interest to declare.
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