Light Energy Transducing and Signal Transducing Rhodopsins of Halobacteria

Abstract

Bacteriorhodopsin (bR) is a purple-colored 26 kDa protein with a retinal chromophore and a broad absorption band centered at 570 nm. The molar extinction coefficient is 64,000 M^-1 cm^-1 It forms two-dimensional crystals in the ~5.0 nm-thick cell membrane of Halohaoteviwn halohivm, which are easily isolated and known as the purple membrane (pm). Halobacteria are found in natural salt lakes and salterns; they require molar concentrations of NaCl for growth, and through bR, can use sunlight as an energy source, because, in the light, bR ejects protons from the cell and generates an electrochemical gradient across the cell membrane. For a brief recent review, see Stoeckenius (1985).^*