Regulation of electron transport in photosystem-II fragments by magnesium ions

Honoring Bacon Ke at 100: a legend among the many luminaries and a highly collaborative scientist in photosynthesis research

Bacon Ke, who did pioneering research on the primary photochemistry of photosynthesis, was born in China on July 26, 1920, and currently, he is living in a senior home in San Francisco, California, and is a centenarian. To us, this is a very happy and unique occasion to honor him. After providing a brief account of his life, and a glimpse of his research in photosynthesis, we present here "messages" for Bacon Ke@ 100 from: Robert Alfano (USA), Charles Arntzen (USA), Sandor Demeter (Hungary), Richard A. Dilley (USA), John Golbeck (USA), Isamu Ikegami (Japan), Ting-Yun Kuang (China), Richard Malkin (USA), Hualing Mi (China), Teruo Ogawa (Japan), Yasusi Yamamoto (Japan), and Xin-Guang Zhu (China).

Membrane-surface electric properties of triton-fractionated spinach subchloroplast fragments

Surface charge density of subchloroplast fragments fractionated from spinach by Triton X-100 treatment was estimated from cation-induced quenching of chlorophyll fluorescence, with the premise that the fluorescence yield is dependent on the surface electric potential of the preparations. Application of the Gouy-Chapman theory of diffuse double layer to the subchloroplast preparations, or treating the surface of the preparations under electric charge regulation conditions yielded a result suggesting the Photosystem II reaction-center preparation (TSF-IIa) to be more negatively charged than the Photosystem I reaction-center preparation (TSF-I). Isoelectric points of the subchloroplast fragments were determined by measuring 90 degrees light scattering and more directly by gel isoelectric focusing. Isoelectric points of TSF-I and -IIa were estimated to be 4.8 and 4.0 from light-scattering experiments, and 4.5 and 4.1 from gel electrophoresis, respectively. The TSF-II preparation that contains both a light-harvesting complex and the reaction-center (core) complex showed a small cation-induced quenching of chlorophyll fluorescence. This fluorescence quenching may be ascribed mostly to the regulation of energy transfer in the preparation (Yamamoto, Y. and Ke, B. (1980) Biochim, Biophys. Acta 592, 296-302). Furthermore, the TSF-II preparation showed a broad and indefinite peak in light scattering in the pH range 3-8, suggesting that the complex probably carries a small amount of charge in this pH range. The physiological role of the membrane surface charge of the subchloroplast preparations in membrane structure and cation regulated processes in chloroplast is discussed.

Regulation of excitation energy distribution in photosystem-II fragments by magnesium ions

Fluorescence characteristics of Photosystem-II subchloroplasts (TSF-II and TSF-IIa) fractionated by Triton X-100 treatment were studied in relation to cation-induced regulation of excitation-energy distribution within subchloroplast fragments. Absorption spectra and fluorescence-emission spectra at 77 K showed that TSF-II contains the light-harvesting chlorophyll-protein complex in addition to the reaction-center complex, which is present alone in TSF-IIa. Mg2+ increased the ratio of F695nm to F685nm in the fluorescence-emission spectrum of TSF-II particles at 77 K, but had no effect on TSF-IIa particles. Mg2+ also induced a quenching of chlorophyll fluorescence at room temperature in TSF-II, an effect that was insensitive to the presence of DCMU. The DCMU-insensitive fluorescence quenching was not observed in the TSF-IIa preparation. These results suggest an existence of cation-induced regulation of excitation-energy transfer in TSF-II preparations. Presence of antenna chlorophyll molecules alone does not seem to be sufficient for observing energy-transfer regulation by cations in Photosystem-II preparations.