Action spectrum and quantum efficiency for proton pumping in Halobacterium halobium.

The action spectrum and quantum efficiency (phi H+) for proton ejection from Halobacterium halobium have been determined under conditions chosen to minimize light-triggered proton influx which is usually observed in intact cells. The action spectrum for the carotenoid-containing strain, R1, shows that light energy absorbed by the carotenoids does not contribute to the proton ejection. After correction for shielding by the carotenoids and other cell pigments, the action spectrum closely follows the absorption spectrum of bacteriorhosopsin. Values determined for phi H+ in H. halobium cells and cell envelopes range from 0.4 to 0.7. These values are significantly higher than the currently accepted value for the quantum efficiency for the photoreaction cycle of bacteriorhodopsin in isolated purple membrane, suggesting that at least in intact cells and envelopes more than one proton is pumped during the bacteriorhodopsin photocycle. A new nondestructive assay for bacteriorhopopsin in intact cells and envelopes which also contain other pigments was used in this work.