Illumination accelerates the decay of the O-intermediate of pharaonis phoborhodopsin (sensory rhodopsin II)

pharaonis phoborhodopsin (ppR, also called pharaonis sensory rhodopsin II [psRII]) is a member of the archaeal rhodopsin family and acts as a repellent phototaxis receptor of Natronobacterium pharaonis. Upon illumination, ppR is excited and undergoes a linear cyclic photoreaction, namely, a photocycle that constitutes photointermediates such as M- and O-intermediates (ppR_M and ppR_O, respectively). Under a constant background illumination (>600 nm) that irradiates ppR_O, the decay rate of the flash-induced ppR_O increased with an increase in the background light intensity, indicating the photoreactivity of ppR_O. Azide did not influence the light-accelerated ppR_O decay, but the time required for the cycle to be completed became shortened in an azide concentration-dependent manner because of acceleration of ppR_M decay. Hence, the turnover rate of photocycling increased appreciably in the presence of both the background illumination and the azide. The observation reported previously (Schmies, G. et al. 2000, Biophys. J. 78:967-976) is discussed in connection with the present observations.