A. Deshpande et S. Sonar, Bacterioopsin-triggered retinal biosynthesis is inhibited by bacteriorhodopsin formation in Halobacterium salinarium, J BIOL CHEM, 274(33), 1999, pp. 23535-23540
Factors regulating retinal biosynthesis in halobacteria are not clearly und
erstood. In halobacteria, events leading to the biosynthesis of bacteriorho
dopsin have been proposed to participate in stringent regulation of retinal
biosynthesis, The present study describes a novel approach of in vivo intr
oductions of mRNA and membrane proteins via liposome fusion to test their r
ole in cellular metabolism. Both the bacterioopsin-encoding mRNA and the li
posome-encapsulated bacterioopsin (apoprotein) are independently introduced
in spheroplasts of the purple membrane-negative strain Halobacterium salin
arium that initially contain neither bacterioopsin nor retinal, Isoprenoid
analyses of these cells indicate that the expression/presence of bacterioop
sin triggers retinal biosynthesis from lycopene, and its subsequent binding
to opsin generates bacteriorhodopsin, When bacteriorhodopsin and excess re
tinal were independently introduced into spheroplasts of purple membrane-ne
gative cells, the introduction of bacteriorhodopsin resulted in an accumula
tion of lycopene, indicating an inhibition of retinal biosynthesis, These r
esults provide direct evidence that the formation of bacterioopsin acts as
a trigger for lycopene conversion to beta-carotene in retinal biosynthesis.
The trigger for this event does not lie with either transcription or trans
lation of the bop gene, It is clearly associated with the folded and the me
mbrane-integrated state of bacterioopsin. On the other hand, the trigger si
gnaling inhibition of retinal biosynthesis does not lie with the presence o
f excess retinal but with the correctly folded, retinal-bound form, bacteri
orhodopsin.