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Plant and Cell Physiology Advance Access published online on June 27, 2007
Plant and Cell Physiology, doi:10.1093/pcp/pcm078
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© The Author 2007. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Binding affinity of Chl b to the Chl a-binding sites in PS1 core complexes

Isamu Ikegami, Someyo Satoh and Masayuki Aoki

Faculty of Pharmaceutical Sciences, Teikyo University, Sagamiko, Kanagawa 229-0195, Japan

Correspondence should be addressed to: Dr. Isamu Ikegami. Faculty of Pharmaceutical Sciences, Teikyo University, Sagamiko, Kanagawa 229-0195, JAPAN. Tel: +81-(0)42-685-3773, FAX: +81-(0)42-685-2713, E-mail: i-ike{at}pharm.teikyo-u.ac.jp


  Abstract

Most chlorophyll a (Chl a) in PSI complexes was removed without any loss of P700 by ether treatment, yielding antenna-depleted P700-Chl a-protein complexes (CP1) with a Chl a/P700 ratio of 12. On addition of about 60 Chl b (/P700) with phosphatidylglycerol, about 20 Chl b (/P700) were bound to the complexes. The ratio of the bound Chl b to the added Chl b was about one-third, irrespective of the amount of Chl b added. The same partition ratio was obtained on reconstitution with Chl a, suggesting that the binding affinity of Chl b to the Chl a-binding sites is similar to that of Chl a. The relative quantum efficiency of P700 photooxidation, determined by the increase in its initial rate, increased in proportion to the increase in number of bound Chl b. The degree of the increase was the same as expected if the bound Chl b had the same antenna activity as the bound Chl a. The bound Chl b emitted fluorescence with a peak at 660 nm and its yield was as high as the Chl a remaining in the complexes. However, the excitation spectrum of the Chl a fluorescence, detected at 680 nm, was almost the same as the absorption spectrum of the Chl b-bound complexes, indicating efficient energy transfer of the bound Chl b to Chl a. These results suggest that Chl b primarily occupies the Chl a-binding sites close to the reaction center region, acting as an efficient antenna for P700.

Keywords: Antenna pigments - Chlorophyll - Fluorescence - P700 - Photosystem I - Reconstitution of antenna


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