THE SENSORY PHOTORECEPTORS OF Halobacterium halobium REVISITED: ACTION SPECTRA and INFLUENCE OF BACKGROUND LIGHT*

Action spectra of the light‐dependent behavior of Halobacterium and the effect of background light have been measured with regard to the current hypothesis of Spudich and Bogomolni [Nature312,509–513 (1984)], which proposes sensory rhodopsin I (sRI587) to be the receptor for long‐wavelength light, and its photoproduct S373 to be the receptor for UV light. The action spectrum shows three maxima for attractant responses (prolonged swimming intervals) at 565, 590, and 610 nm, and two maxima for repellent responses (shortened intervals) at 370 and 480 nm. The latter is assigned to sensory rhodopsin II (P‐480). All peaks are red‐shifted after substitution of the endogeneous retinal by 3, 4‐dehydroretinal. The peaks at 590 and 610 nm are suppressed by long‐wavelength background light. Ultraviolet background light converts all attractant peaks into repellent peaks. The response at 370 nm is strongly activated by visible background light, the maximal effect occurring with 510 nm. The activated state declines with a half‐life of about 1.2 s. In a growing culture, full sensitivity to UV and blue light is restored about 10 h earlier than sensitivity to long‐wavelength light. Some of the results cannot easily be explained by the sRI587/S373 hypothesis. Explanations for the three maxima in the long‐wavelength range and for the maximal activation of the UV response by 510 nm light are discussed.