PHOTOPHOSPHORYLATION IN ALKALOPHILIC HALOBACTERIAL CELLS CONTAINING HALORHODOPSIN - CHLORIDE-ION CYCLE

Light-driven ATP synthesis is found in cells of the alkalophilic bacte rium Natronobacterium pharaonis containing halorhodopsin but deficient in H+-pumping bacteriorhodopsin. Photophosphorylation occurs with cya nide-inhibited respiratory chain as well as without cyanide in conditi ons with low Cl- concentration in the incubation medium. Increase in C l- concentration from 0.1 to 2.35 M in the incubation medium leads to inhibition of photophosphorylation. Continuous illumination increases membrane Delta Psi if respiration is inhibited by cyanide. This effect is stimulated by DCCD, an ATPase inhibitor. These data can be explain ed if one suggests that halorhodopsin pumps Cl- into the cells whereas Cl- release from the cells through Cl(-)ATP-synthase is coupled with the ATP synthesis (chloride-ion cycle).