Association of pharaonis Phoborhodopsin with its Cognate Transducer Decreases the Photo-Dependent Reactivity by Water-Soluble Reagents of Azide and Hydroxylamine By

pharaonis phoborhodopsin (ppR; also pharaonis sensory rhodopsin II, psRII) is a receptor of the negative phototaxis of Natronobacterium pharaonis. In bacterial membrane, ppR forms a complex with its transducer pHtrII, and this complex transmits the light signal to the sensory system in the cytoplasm. In the present work, the truncated transducer, t-Htr was used which interacts with ppR [Sudo et al. Photochem. Photobiol. in press]. Two water-soluble reagents, hydroxylamine and azide reacted both to the transducer-free ppR and to the complex ppR/t-Htr (the complex between ppR and its truncated transducer). In the dark, the bleaching rates caused by hydroxylamine were not significantly changed between transducer-free ppR and ppR/t-Htr, or that of the free ppR was a little slower. Illumination accelerated the bleach rates because the reaction occurs selectively at the M-intermediate, but the rate of the complex was about 7.4-fold slower than that of the transducer-free ppR. Azide accelerated the M-decay, and its reaction rate of ppR/t-Htr was about 4.6-fold slower than the free ppR. These findings suggest that the transducer binding decreases the water accessibility around the chromophore at the M-intermediate. Its implication is discussed.