Brp and blh are required for synthesis of the retinal cofactor of bacteriorhodopsin in Halobacterium salinarum

Bacteriorhodopsin, the light-driven proton pump of Halobacterium salinarum, consists of the membrane apoprotein bacterioopsin and a covalently bound r etinal cofactor. The mechanism by which retinal is synthesized and bound to bacterioopsin in vivo is unknown. As a step toward identifying cellular fa ctors involved in this process, we constructed an in-frame deletion of brp, a gene implicated in bacteriorhodopsin biogenesis. In the Delta brp strain , bacteriorhodopsin levels are decreased similar to4.0-fold compared with w ild type, whereas bacterioopsin levels are normal. The probable precursor o f retinal, beta -carotene, is increased similar to3.8-fold, whereas retinal is decreased by similar to3.7-fold. These results suggest that brp is invo lved in retinal synthesis. Additional cellular factors may substitute for b rp function in the Delta brp strain because retinal production is not aboli shed. The in-frame deletion of blh, a brp paralog identified by analysis of the Halobacterium sp, NRC-1 genome, reduced bacteriorhodopsin accumulation on solid medium but not in liquid. However, deletion of both brp and blh a bolished bacteriorhodopsin and retinal production in liquid medium, again w ithout affecting bacterioopsin accumulation. The level of beta -carotene in creased similar to5.3-fold. The simplest interpretation of these results is that brp and blh encode similar proteins that catalyze or regulate the con version of beta -carotene to retinal.