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Time-Resolved Detection of Conformational Changes in Oat Phytochrome A: Time-Dependent Diffusion

Takeshi Eitoku ^*, Xristo Zarate , Gennady V. Kozhukh , Jeong-Il Kim   , Pill-Soon Song   ^¶ and Masahide Terazima ^*

^* Department of Chemistry, Graduate School of Science, Kyoto University, Japan; Kumho Life & Environmental Science Laboratory, Gwangju, Korea; Department of Biotechnology (BK21 program), College of Agriculture and Life Sciences, Chonnam National University, Gwangju, Korea; Environmental Biotechnology National Core Research Center, Gyeongsang National University, Jinju, Korea; and ^¶ Faculty of Biotechnology, College of Applied Life Sciences, Cheju University, Jeju, Korea

Correspondence: Address reprint requests to M. Terazima, Tel.: 81-75-753-4026; E-mail: mterazima{at}kuchem.kyoto-u.ac.jp.

Conformational changes in oat phytochrome A (phy) in solution after photoexcitation of the red-absorbing form (Pr) were studied in time-domain by the pulsed laser-induced transient grating technique. It was found that the diffusion coefficient (D) of far-red-absorbing form (Pfr) of large phy (1.3 x 10^–11 m² s^–1) is markedly reduced compared with that of Pr (5.8 x 10^–11 m² s^–1). This large reduction indicates that the conformation of Pfr is significantly changed from that of Pr, so that the intermolecular interaction with water molecules increases. This change completes within 1 ms after the photoexcitation. On the other hand, D of Pr of intact phy (4.1 x 10^–11 m² s^–1) first decreases upon photoexcitation to 0.89 x 10^–11 m² s^–1 within 1 ms and then gradually increases with a time constant of 100 ms to the value of Pfr, 1.7 x 10^–11 m² s^–1. This slower phase suggests that the conformation of the N-terminal region changes with 100 ms to decrease the intermolecular interaction with water after a global change in the large phy region. The increase of D was interpreted in terms of -helix formation in the Pfr form from the random coil structure in the Pr form.

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