Photosensory retinal pigments in Halobacterium halobium

In Halobacterium halobium, nicotine is known to block the synthesis of retinal. Cells grown in the presence of nicotine do not show any photophobic response. Addition of retinal1 or retinal2 restored the photophobic responses to light-increase in the UV and to light-decrease in the green-yellow part of the spectrum. The action spectra of the two retinal2-photosystems were red-shifted by 15--20 nm, compared with the corresponding retinal1 systems. We conclude that each of the two photosystems PS 370 and PS 565, has its own photosensory pigment with retinal as the chromophoric group.