INTERACTION OF BACTERIORHODOPSIN MOLECULES IN THE PURPLE MEMBRANE PHOTOCYCLE

Dependence of the bacteriorhodopsin (BR) photocycle on exciting flash intensity was studied in purple membranes from Halobacterium salinariu m wild type and mutant D96N. A high-intensity flash elicited concurren tly two photocycles, one due to a single excited BR molecule and the o ther involving two or three excited BRs in the trimer. Both photocycle s had the same sequence M --> N --> BR, the latter step coupled with p roton uptake from the aqueous milieu. However, excitation of two or th ree BRs in the trimer slightly slowed down the M --> N step, induced t he N --> M back reaction, and accelerated the N --> BR step.