Two photosystems controlling behavioural responses of Halobacterium halobium

Abstract

Halobacterium halobium contains a retinal–protein complex, bacteriorhodopsin, which is the only protein of the so-called purple membrane forming distinct patches within the surface membrane^1,2. Bacteriorhodopsin acts as a photoreceptor molecule and is chemically similar to the visual pigment rhodopsin^1,3. On illumination bacteriorhodopsin undergoes a fast cyclic photoreaction with at least four intermediates occurring after microseconds and milliseconds^4–6. Various experiments indicate that bacteriorhodopsin functions as a light-driven proton pump which builds up a proton gradient across the cell membrane and is used for ATP synthesis^7,8. Thus its primary function, different from that of rhodopsin in the eye, seems to be energy transformation⁷. Moreover, H. halobium shows light dependent motor responses, and so we supposed that bacteriorhodopsin also has a sensory function. To test this possibility we obtained action spectra for the light-induced behavioural responses from H. halobium. We found two photosystems, one of which shows that bacteriorhodopsin is involved in the light-controlled motor response.