The recently discovered photo-activated adenylyl cyclase (mPAC from Microcoleus chthonoplastes) is the first PAC that owes a light-, oxygen- and voltage-sensitive (LOV) domain for blue-light sensing. The photoreaction of the mPAC receptor was studied by time-resolved UV/vis and light-induced Fourier transform infrared (FTIR) absorption difference spectroscopy. The photocycle comprises of the typical triplet state LOV715and the thio-adduct state LOV390. While the adduct state decays with a time constant of 8 s, the lifetime of the triplet state is with 656 ns significantly shorter than in all other reported LOV domains. The light-induced FTIR difference spectrum shows the typical bands of the LOV390and LOV450intermediates. The negative S-H stretching vibration at 2573 cm-1is asymmetric suggesting two rotamer configurations of the protonated side chain of C194. A positive band at 3632 cm-1is observed, which is assigned to an internal water molecule. In contrast to other LOV domains, mPAC exhibits a second positive feature at 3674 cm-1which is due to the O-H stretch of a second intrinsic water molecule and the side chain of Y476. We conclude that the latter might be involved in the dimerization of the cyclase domain which is crucial for ATP binding.