LAUSR

Phytochromes (Phys) are photoreceptor proteins that sense red/far-red light in plants, fungi, and bacteria. The proteins consist of a light-sensing photosensory module and a signaling output module, which is typically a histidine kinase (HK) domain in bacteriophytochromes. Although the time-resolved detection of the HK domain is essential for obtaining insights into the reaction mechanism of photoactivation, it has been very difficult to detect the change. Here, the reaction of Cph1, one of the Phys found in the cyanobacterium Synechocystis sp. PCC6803, was studied using time-resolved translational diffusion detection. It was found that the kinetics of the HK domain movement of the Cph1 dimer could be monitored successfully. The diffusion coefficient of the Cph1 dimer decreases significantly with a time constant similar to that of the final step of the reaction monitored by the transient absorption method (780 ms), whereas the monomer does not exhibit this change. We attribute this change to the closed-to-open type of conformational change in the HK domain of the Cph1 dimer without the secondary structure change. The fact that the rate is similar to that from the transient absorption method suggests that the proton uptake at His260 is the rate-determining step of the conformational change.