Photoinduced Orientation Change of the Dimer Structure of the Pr‑I State of Cph1Δ2

Phytochromes are red and far-red light sensor proteins found in many organisms. Photoisomerization of a chromophore triggers subsequent reactions leading to conformational changes that are essential for signaling. Conformational changes of the N-terminal sensor domain of cyanobacterium phytochrome 1 from <i>Synechocystis</i> sp. PCC6803 (Cph1Δ2) were studied in this report primarily by the transient grating (TG) method in the time domain. Although the reaction kinetics monitored by ultraviolet–visible absorption changes were insensitive to variations in pH, it was found for the first time that the diffusion coefficient and molecular volume monitored by the TG method are significantly dependent on pH. This pH sensitivity indicates that the conformational changes of Cph1Δ2 during the reaction are modulated by pH and was explained consistently by two contributions from two isomers (Pr-I and Pr-II) possessing different protonation states of His260. Thus, His260 is responsible for the conformational heterogeneity of Pr, as discussed previously, and this protonation difference leads to different conformational changes of the sensor domain of Cph1 during the reaction. The photoreaction dynamics of the Pr-I state was studied in detail, and a significant diffusion change was observed, which was attributed to a change in the orientation (quaternary structure) of the dimer with a time constant of 400 μs. Furthermore, it was revealed that this reorientation of the dimer was induced by a conformational change in the tongue region. New site-directed mutations to change the dimer–monomer equilibrium were reported