DELTA-PSI-MEDIATED SIGNALING IN THE BACTERIORHODOPSIN-DEPENDENT PHOTORESPONSE

It has been shown previously that the proton-pumping activity of bacte riorhodopsin from Halobacterium salinarium can transmit an attractant signal to the bacterial flagella upon an increase in light intensity o ver a wide range of wavelengths. Here, we studied the effect of blue l ight on phototactic responses by the mutant strain Pho81-B4, which lac ks both sensory rhodopsins hut has the ability to synthesize bacterior hodopsin, Under conditions in which bacteriorhodopsin was largely accu mulated as the M(412) bacteriorhodopsin photo-cycle intermediate, halo bacterial cells responded to blue light as a repellent. This response was pronounced when the membrane electric potential level mas high in the presence of arginine, active oxygen consumption, or high backgroun d long-wavelength light intensity but was inhibited by an uncoupler of oxidative phosphorylation (carbonyl cyanide 3-chlorophenylhydrazone) and was inverted in a background of low long-wavelength light intensit y. The response to changes in the intensity of blue light under high b ackground light was asymmetric, since removal of blue light did not pr oduce an expected suppression of reversals. Addition of ammonium aceta te, which is known to reduce the pH gradient changes across the membra ne, did not inhibit the repellent effect of blue light, while the disc harge of the membrane electric potential by tetraphenylphosphonium ion s inhibited this sensory reaction. We conclude that the primary signal from bacteriorhodopsin to the sensory pathway involves changes in mem brane potential.