Sensory transduction in Halobacterium halobium: retinal protein pigment controls UV-induced behavioral response.

Both photosystems, PS 370 and PS 565, controlling behavioral responses in Halobacterium halobium [E. Hildebrand and N. Dencher, Nature 2 5 7 ,4 6 — 48 (1975)] are reversibly inhibited when bacteria are grown in the presence o f 1 m M nicotine which is known to block biosynthesis o f reti­ nal. Photobehavior can be restored within some minutes to hours by adding retinal to nicotinetreated bacteria, PS 370 thereby reappearing earlier than PS 565. The reconstitution rate depends on the concentration and on the kind o f retinal isomers applied. A\\-trans retinal is most effective. PS 370 becomes fully sensitive if reconstituted in the presence o f nicotine. This rules out the possi­ bility that the alkaloid may directly inhibit steps o f signal transmission following photoreception. The action spectrum o f PS 370 regenerated with retinal alone o f H. h., strain RjLg (a mutant deficient in carotenoids), fails to show all secondary peaks around 450 nm which in strain Rj occur besides the prominent maximum at 370 nm. Addition o f carotenoids (mainly a-bacterioruberin) to reconstituted cells o f R,L3 restores the sensitivity in that spectral region. Carotenoids or flavin solely added to nicotine-treated bacteria cannot restore photobehavior. We conclude that the active pigment o f PS 370, which mediates the photophobic response to increase o f light intensity (step-up response), represents a retinal protein com plex and that carote­ noids participate in photoreceptor function as accessory pigments. The biochemical relation o f the UV-absorbing retinal protein complex to bacteriorhodopsin is discussed.