Dynamic Inhomogeneity in the Photodynamics of Cyanobacterial Phytochrome Cph1

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• 作者：Peter W. Kim ; Nathan C. Rockwell ; Shelley S. Martin ; J. Clark Lagarias ; Delmar S. Larsen
• 刊名：Biochemistry
• 出版年：2014
• 出版时间：May 6, 2014
• 年：2014
• 卷：53
• 期：17
• 页码：2818-2826
• 全文大小：484K
• 年卷期：v.53,no.17(May 6, 2014)
• ISSN：1520-4995

文摘

Phytochromes are widespread red/far-red photosensory proteins well known as critical regulators of photomorphogenesis in plants. It is often assumed that natural selection would have optimized the light sensing efficiency of phytochromes to minimize nonproductive photochemical deexcitation pathways. Surprisingly, the quantum efficiency for the forward P_r-to-P_fr photoconversion of phytochromes seldom exceeds 15%, a value very much lower than that of animal rhodopsins. Exploiting ultrafast excitation wavelength- and temperature-dependent transient absorption spectroscopy, we resolve multiple pathways within the ultrafast photodynamics of the N-terminal PAS-GAF-PHY photosensory core module of cyanobacterial phytochrome Cph1 (termed Cph1螖) that are primarily responsible for the overall low quantum efficiency. This inhomogeneity primarily reflects a long-lived fluorescent subpopulation that exists in equilibrium with a spectrally distinct, photoactive subpopulation. The fluorescent subpopulation is favored at elevated temperatures, resulting in anomalous excited-state dynamics (slower kinetics at higher temperatures). The spectral and kinetic behavior of the fluorescent subpopulation strongly resembles that of the photochemically compromised and highly fluorescent Y₁₇₆H variant of Cph1螖. We present an integrated, heterogeneous model for Cph1螖 that is based on the observed transient and static spectroscopic signals. Understanding the molecular basis for this dynamic inhomogeneity holds potential for rational design of efficient phytochrome-based fluorescent and photoswitchable probes.