PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

How plants see blue light: The photochemistry of phototropin. John Kennis^*,1, Sean Crosson², Magdalena Gauden¹, Maxime Alexandre¹, Ivo van Stokkum¹, Klaas Hellingwerf³, Keith Moffat², Rienk van Grondelle¹, ¹ Department of Biophysics, Amsterdam, The Netherlands² Department of Biochemistry and Molecular Biology, Chicago, IL, USA³ Swammerdam Institute for Life Sciences, Amsterdam, The Netherlands

ABSTRACT- Plants regulate and optimize photosynthetic activity not only in their photosynthetic apparatus, but also by light-directed movement of their stems, leaves and chloroplasts. The phototropins are an important class of plant blue-light photoreceptors that control many of these responses including phototropism (i.e., the growing of plants towards light sources), light-directed chloroplast movement, light-induced stomatal opening and solar tracking. They are serine/threonine kinases that undergo autophosphorylation in response to absorption of blue light. The loci for photochemistry in this class of photoreceptors are two flavin-binding light, oxygen, or voltage (LOV) domains. Each LOV domain consists of approximately 100 amino acids and undergoes a photocycle in which absorption of a blue photon leads to the formation of a long-lived covalent bond between the flavin and a highly conserved cysteine residue. This state, which lives for minutes and shows a pronounced absorption band in the near-UV, is believed to be the signaling state of the protein. Here, we present a spectroscopic study of the photochemical events in the LOV domain after blue or near-UV light absorption. We find that the the flavin triplet state is the reactive species from which the photoreaction occurs and show that it is the primary photoproduct in the LOV photocycle, generated at 60% efficiency. No spectroscopically distinguishable intermediates precede the flavin triplet on the femtosecond to nanosecond timescale, indicating that it is formed directly via intersystem crossing (ISC) from the singlet state. Upon absorption of a near-UV photon by the photoaccumulated signaling state of the LOV domain, the covalent bond between the flavin and the conserved cysteine is broken and the blue-light sensitive ground state is regained on an ultrafast timescale of 100 ps. We thus demonstrate that the LOV domain is a reversible photochromic switch, which can be activated by blue light, and deactivated by near-UV light.

KEY WORDS: photomovement, phototropin, blue-light photoreceptor, spectroscopy