BIOSYNTHESIS AND ASSEMBLY OF PURPLE MEMBRANE OF HALOBACTERIUM HALOBIUM S9: LIGHT STIMULATION OF BACTERIORHODOPSIN FORMATION

Abstract

Abstract— The effect of light on purple membrane biogenesis in Halobacterium halobium S9 strain was investigated. When bacteria were grown in the dark, the 570nm absorption due to bacteriorhodopsin increased more slowly than under illumination, but eventually after longer incubation, reached the same level as that seen in the illuminated culture.Analysis of membrane fractions by sucrose density gradient centrifugation revealed that two different membrane fractions, containing purple and brown membrane could be detected in the exponential growth phase. Another fraction whose density was higher than that of purple membrane, disappeared concomitantly with the increase in purple membrane and brown membrane, indicating that it may be related to purple membrane formation.HPLC analysis of membrane proteins showed that there was no significant difference in de novo synthesis of bacterio–opsin between dark and illuminated cultures. This led us to conclude that light stimulated retinal binding to bacterio–opsin and/or retinal biosynthesis rather than bacterio–opsin synthesis. Bacteriorhodopsin seemed to form the brown membrane fraction first, which then spontaneously reorganized into purple membrane.When incorporated in liposomes, bacteriorhodopsin in brown membrane was found to have rather higher proton pump activity than that in purple membrane. The H+ pumping activity was quite heat labile. This and the CD spectra indicated that bacteriorhodopsin in brown membrane might exist without forming normal timer unit.