PapB is a short BLUF protein from the purple bacterium Rhodopseudomonas palustris, and it has the function of regulating the phosphodiesterase activity of its partner protein PapA. Because PapA is an enzymatic protein containing an EAL domain, which hydrolyzes a cyclic- dimeric-GMP, the PapB-PapA complex regulates, for example, biofilm formation in a light-dependent manner. The reaction dynamics of PapB, as well as dynamics of signal transduction in the PapB-PapA complex, were studied mostly by time-resolved transient grating and CD methods. It was found that although PapB exists as a monomer in both dark and light states, the diffusion coefficient slightly increases and partial molar volume changes upon light irradiation with time constants of 80 mu s, 3.2 ms, and 24 ms. The last conformational change leads to a signaling state that is attributed to the a-helix unfolding in the C-terminal region of the BLUF domain. Interestingly, when PapA is added to a PapB solution to form a PapB-PapA complex, the diffusion coefficient decreases significantly, with a time constant of 26 ms, which is similar to the rate of the last conformational change of PapB. It was found that this change is due to the conformational change of PapA, and the light signal from PapB is transmitted through the conformational change of PapB. Compared with another BLUF protein, BlrP1, which possesses the EAL effector domain and requires photoexcitation of two monomer units of the dimer for a major conformational change, the conformational change of the PapB-PapA complex is induced by excitation of one monomer PapB among the heterotetramer.