Two possible roles of bacteriorhodopsin; a comparative study of strains of Halobacterium halobium differing in pigmentation.

The light-induced changes in pH and ATP level were compared for cell suspensions between strains of Halobacterium halobium differing in pigmentation after growth under the same conditions. Upon illumination, red cells which contained no detectable amount of bacteriorhodopsin showed only a pH increase, which, in the case of purple cells containing bacteriorhodopsin, was followed by a spontaneous pH decrease during illumination. Pre-incubation of cells at 75° for 5 min depressed the pH increase in both cells. Pre-illumination of cells with hydroxylamine depressed the pH decrease in purple cells. Whenever the pH increase was observed, the cellular ATP level increased. The presence of a bacteriorhodopsin different from that in the purple membrane is postulated.