Sensory Transduction in Halobacterium halobium: Retinal Protein Pigment Controls UV-Induced Behavioral Response

Abstract

Both photosystems, PS 370 and PS 565, controlling behavioral responses in H. halobium are reversibly inhibited when bacteria are grown in the presence of 1 mM nicotine which is known to block biosynthesis of retinal. Photobehavior can be restored within some minutes to hours by adding retinal to nicotine-treated bacteria, PS 370 thereby reappearing earlier than PS 565. The reconstitution rate depends on the concentration and on the kind of retinal isomers applied. All-trans retinal is most effective. PS 370 becomes fully sensitive if reconstituted in the presence of nicotine. This rules out the possibility that the alkaloid may directly inhibit steps of signal transmission following photoreception. The action spectrum of PS 370 regenerated with retinal alone of H. halobium, strain R1L3 (a mutant deficient in carotenoids), fails to show all secondary peaks around 450 nm which in strain R1 occur besides the prominent maximum at 370 nm. Addition of carotenoids (mainly .alpha.-bacterioruberin) to reconstituted cells of R1L3 restores the sensitivity in that spectral region. Carotenoids or flavine solely added to nicotine-treated bacteria cannot restore photobehavior. Apparently, the active pigment of PS 370, which mediates the photophobic response to increase of light intensity (step-up response), represents a retinal protein complex and carotenoids participate in photoreceptor function as accessory pigments. The biochemical relation of the UV-absorbing retinal protein complex to bacteriorhodopsin is discussed.