Light-induced reaction of halorhodopsin prepared under low salt conditions.

1. Membrane fraction containing halorhodopsin was prepared from the lysate of a mutant strain of Halobacterium halobium, Y1, which is defective in bacteriorhodopsin synthesis. 2. Irradiation of the membrane with red light at 0 degrees C decreased the absorbance intensities over the whole range of 500--600 nm and a new absorption peak appeared at about 400 nm. This process was reversed either by irradiation with blue light or simply by incubation in the dark. The wavelength at which the red light-induced absorbance decrease became maximum was 566 nm in the absence of NaCl and moved to 576 nm upon addition of NaCl. The midpoint for the conversion was at about 0.1 M NaCl. 3. Even though the membrane containing halorhodopsin was prepared in the absence of NaCl, this pigment was photochemically active in the sense that it could form a bathochromic photoproduct, a batho-intermediate, at -196 degrees C. 4. Retinal isomer composition during irradiation with red light at 0 degrees C was determined. Unirradiated halorhodopsin had predominantly all-trans retinal. With the increase of the blue-shifted product on irradiation, 13-cis content increased.