PHOTOEQUILIBRIUM IN THE PRIMARY STEPS OF THE PHOTORECEPTORS PHYTOCHROME-A AND PHOTOACTIVE YELLOW PROTEIN

The photochromic equilibria between the ground state and the first int ermediate of the native photoreceptors phytochrome A (phyA) of oat and photoactive yellow protein (PYP) of Ectothiorhodospira halophila have been studied by laser-induced optoacoustic spectroscopy, employing ph oton densities sufficiently high to reach saturation and to establish the photoequilibria between the red-light-absorbing phyA form, P-r, an d the first intermediate, I-700, and between the PYP ground state, pG, and the first intermediate, pR. The parameters for the photoequilibri a P-r reversible arrow I-700 and pG reversible arrow pR were determine d by the fluence saturation curves of the structural volume change, De lta V-r, analyzed with model functions taking photoselection into acco unt. The quantum yield of the photoreversion, Phi(1700 --> Pr) = 0.22 +/- 0.12, proved to be ca 1.4 times larger than the known quantum yiel d of the forward photoreaction (assuming Phi(Pr --> 1700) = Phi(Pr --> Pfr)). This suggests that the chromophore-binding protein domain stru ctures of P-r and I-700 are quite similar. in contrast, the photorever sion quantum yield in PYP for pR --> pG is small (Phi(pR --> pG) = 0.0 7) compared with the known value for the thermal forward process to th e signaling state pB (Phi(pG --> pR) greater than or equal to Phi(pG - -> pB) = 0.35). This is tentatively attributed to a main conformationa l change associated with the pG --> pR phototransformation. The result s of this study emphasize the need of considering photoequilibria in p hotoreceptors when working with high-fluence laser pulses.