Two ground state isoforms and a chromophore D -ring photoflip triggering extensive intramolecular changes in a canonical phytochrome

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Abstract
Phytochrome photoreceptors mediate light responses in plants and in many microorganisms. Here we report studies using 1 H– 13 C magic-angle spinning NMR spectroscopy of the sensor module of cyanobacterial phytochrome Cph1. Two isoforms of the red-light absorbing Pr ground state are identified. Conclusive evidence that photoisomerization occurs at the C15-methine bridge leading to a β -facial disposition of the ring D is presented. In the far-red-light absorbing Pfr state, strong hydrogen-bonding interactions of the D -ring carbonyl group to Tyr-263 and of N24 to Asp-207 hold the chromophore in a tensed conformation. Signaling is triggered when Asp-207 is released from its salt bridge to Arg-472, probably inducing conformational changes in the tongue region. A second signal route is initiated by partner swapping of the B -ring propionate between Arg-254 and Arg-222.

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