BIOENERGETIC FACTORS CONTROLLING IN-VITRO PHOSPHORYLATION OF LHI-ALPHA (B870) POLYPEPTIDES IN MEMBRANES ISOLATED FROM RHODOBACTER-CAPSULATUS

Membranes of Rhodobacter capsulatus strain U43 (pTX35) showed qualitat ively very similar phosphorylation patterns under in vitro and in vivo conditions. In vitro, it was irrelevant whether the phosphate source was orthophosphate or ATP. Inhibitors of electron transport did not in hibit light-harvesting complex I (LHI alpha) (B870) polypeptide phosph orylation, except for o-phenanthroline, which was strongly inhibitory. Redox conditions regulated the amount of protein phosphorylated; exte rnal redox potentials between +200 and +300 mV promoted the reaction. Phosphorylation was inhibited by uncouplers such as carbonyl cyanide n z-chlorophenyl hydrazone and nigericin plus valinomycin plus potassium ions. Inhibitors of the H+-ATPase were also inhibitory when the phosp hate source was [P-32]P-i or [gamma-P-32]ATP. From these results, it w as concluded that an operative reaction center, a coupled membrane, an d external redox potentials higher than +200 mV are required for optim um LHI alpha phosphorylation. We also demonstrated that phosphorylatio n of LHI alpha polypeptide occurs before insertion into the membrane a nd that phosphate is preferentially incorporated into specific domains within the cytoplasmic membrane. Intracytoplasmic membranes, identifi ed here as light membranes, were found to contain a dephosphorylated L HI alpha polypeptide.