The photochemical reaction dynamics of a light oxygen voltage (LOV) domain from the blue light sensor protein FKF1 (flavin binding Kelch repeat F box) was studied by means of the pulsed laser induced transient grating method The observed absorption spectral changes upon photoexcitation were similar to the spectral changes observed for typical LOV domain proteins (e g, phototropins) The adduct formation took place with a time constant of 6 As After this reaction, a significant conformational change with a time constant of 6 ms was observed as a change in the diffusion coefficient An FKF1 LOV mutant without the conserved loop connecting helices E and F which is present only in the FKF1/LOV Kelch protein 2/ZEITLUPE family did not show these slow phase dynamics This result indicates that the conformational change in the loop region represents a major change in the FKF1 LOV photoreaction