Phosphate binding to chromatophores of Rhodospirillum rubrum

Equilibrium dialysis has been used to determine the binding of phosphate to chromatophores of Rhodospirillum rubrum. Assuming a complete exchange of the added 32Pi with endogenous phosphate, the saturation with phosphate retained in any form by chromatophores was reached at about 20 nmoles Pi per mg of bacteriochlorophyll. The retention of phosphate had a pH optimum at pH 6.5 to 6.8. At pH 8.0 only chromatophores which have not been liberated from DNA and RNA show a considerable retention of phosphate. However, illumination of chromatophores prior to dialysis in the presence of ADP leads to a retention of phosphate at pH 8.0 which persists during dark dialysis in the absence of added magnesium.