PHOTOSENSORY BEHAVIOR OF A BACTERIORHODOPSIN-DEFICIENT MUTANT, ET-15, OF Halobacterium halobium

– Halobacterium halobium , strain ET-15, which does not contain detectable amounts of bacteriorhodopsin (BR) shows behavioral responses to UV and yellow-green light. Attractant stimuli. i.e. light-increases in the yellow-green range or light-decreases in the UV, suppress the spontaneous reversals of the swimming direction for a certain time. Repellent stimuli, i.e. light-decreases in the yellow-green range or light-increases in the UV, elicit an additional reversal response after a few seconds. Action spectra of both sensory photosystems, PS 370 and PS 565, were measured with attractant as well as with repellent stimuli. As in BR-containing cells, maximal sensitivity was always found at 370 nm for the UV-system and at 565 nm for the long-wavelength system. Fluence-response curves at 370 and 565 nm obtained with strain ET-15 and with a BR-containing strain show that the sensitivity of both photosystems is not reduced in the absence of BR. It is concluded that BR is required neither for PS 565 nor for PS 370. Instead retinal-containing pigments different from BR have to be assumed to mediate photosensory behavior.     

HALORHODOPSIN and PHOTOSENSORY BEHAVIOR IN Halobacterium halobium MUTANT STRAIN L-33

Halobacterium halobium , strain L-33, which is deficient in bacteriorhodopsin (BR) but synthesizes increased amounts of halorhodopsin (HR), shows behavioral responses upon changes in fluence rate with visible light or with UV light. The observations support the earlier report (Schimz et al. , 1982). that BR is not essential for photosensing in H. halobium. In the UV-range, changes in light intensity elicit the maximal response at λ= 370 nm. In the visible range, changes in light intensity show the maximal response at Δ= 565 nm and a secondary peak at Δ= 590 nm. The latter corresponds to the absorption maximum of HR (Δ_max= 588 nm). This light-energy converting retinal pigment of H. halobium thus appears to contribute to photosensory behavior.     

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

Abstract– 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 [Nature 312 ,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.     

BACTERIORHODOPSIN AND THE SENSORY PIGMENT OF THE PHOTOSYSTEM 565 IN HALOBACTERIUM HALOBIUM

Abstract Blocking in vivo synthesis of retinal by addition of nicotine to the culture medium leads to the loss of photobehavior in Halobacterium halobium. Addition of rrans -retinol or frans-retinol₂ (3,4-dehy-droretinol) restores the responses to light decreases in the green-yellow spectral range. Action spectra of the reconstituted retinal- and retinal₂-photosystem show maximal sensitivity at 565 and 580 nm, respectively. Addition of retinol or retinol₂ also restores the formation of bacteriorhodopsin (BR) or bacteriorhodopsin₂ (BR₂= 3,4-dehydroretinal-bacterio-opsin complex). The absorption spectra of BR and BR₂, measured in isolated membranes, as well as in living bacteria, show maxima at 568 nm (BR) and at about 600 nm (BR₂), respectively. Comparison of the action spectrum of the retinal2-containing sensory photosystem with the absorption spectrum of BR₂ suggests that a retinal pigment different from BR is responsible for the photosensory behavior to green-yellow light.     

CONSECUTIVE FORMATION OF SENSORY PHOTOSYSTEMS IN GROWING Halobacterium halobium

Abstract— The sensory photosystems PS 370 and PS 565 of Halobacierium halobium are actively degraded in the early growth phase and later resynthesized. Neither degradation nor resynthesis is correlated to the rate of cell division. The reappearance of photosensory activity requires de novo synthesis of proteins which are most likely directly involved in the sensory pathway. PS 370 appears earlier than PS 565 and thus may be studied independently of PS 565, before the latter is synthesized, or by blocking the synthesis of PS 565 with puromycin after PS 370 has appeared. The action spectrum of PS 370 alone shows the same maximum as the spectrum obtained when PS 565 is present. Carotenoids, which act as accessory pigments of PS 370, do not shift its activity peak. Also the maximum of PS 565 is not influenced by PS 370. We conclude that the maxima of the action spectra of PS 370 and PS 56.5 truely reflect the absorption maxima of the sensory retinal pigmentsP–370 andP–565.