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 retinal cofactor. The mechanism by which retinal is synthesized and bound to bacterioopsin in vivo is unknown. As a step toward identifying cellular factors involved in this process, we constructed an in-frame deletion of brp, a gene implicated in bacteriorhodopsin biogenesis. In the Δbrp strain, bacteriorhodopsin levels are decreased ∼4.0-fold compared with wild type, whereas bacterioopsin levels are normal. The probable precursor of retinal, β-carotene, is increased ∼3.8-fold, whereas retinal is decreased by ∼3.7-fold. These results suggest that brp is involved in retinal synthesis. Additional cellular factors may substitute for brp function in the Δbrp strain because retinal production is not abolished. 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 abolished bacteriorhodopsin and retinal production in liquid medium, again without affecting bacterioopsin accumulation. The level of β-carotene increased ∼5.3-fold. The simplest interpretation of these results is that brp and blh encode similar proteins that catalyze or regulate the conversion of β-carotene to retinal.