2ND MECHANISM FOR SODIUM EXTRUSION IN HALOBACTERIUM-HALOBIUM - LIGHT-DRIVEN SODIUM-PUMP

Membrane vesicles from a red mutant of Halobacterium halobium R1 accumulate protons when illuminated causing the pH of the suspension to rise. Sodium is extruded from the vesicles and a membrane potential is formed. This potential and the proton uptake are abolished by valinomycin if K+ is present. In contrast, Na+-efflux is uninhibited by valinomycin even though no membrane potential is detectable and H+ influx does not occur. Bis (hexafluoracetonyl)acetone (1799) stimulates proton uptake but does not abolish membrane potential. We propose that a light-dependent sodium pump is present. Passive proton uptake occurs in response to the electrical gradient created by this light-driven Na+ pump in contrast to the active proton, and passive Na+ flux that occurs in response to the light-driven proton pump described in vesicles of the parent strain of H. halobium R1. © 1979.