Kinetics of and intermediates in a photocycle branching reaction of the photoactive yellow protein from Ectothiorhodospira halophila

We have studied the kinetics of the blue light-induced branching reaction i n the photocycle of photoactive yellow protein (PYP) from Ectothiorhodospir a halophila, by nanosecond time-resolved UV/Vis spectroscopy. As compared t o the parallel dark recovery reaction of the presumed blue-shifted signalin g state pB, the light-induced branching reaction showed a 1000-fold higher rate, In addition, a new intermediate was detected in this branching pathwa y, which, compared to pB, showed a larger extinction coefficient and a blue -shifted absorption maximum. This substantiates the conclusion that isomeri zation of the chromophore is the rate-controlling step in the thermal photo cycle reactions of PYP and implies that absorption of a blue photon leads t o cis --> trans isomerization of the 4-hydroxy-cinnamyl chromophore of PYP in its pB state. (C) 1999 Federation of European Biochemical Societies.