A Cyanobacterial ATPase Distinct from the Coupling Factor of Photophosphorylation

A particle-bound, Mg2+-dependent ATPase activity is investigated in cell-free extracts o f the cyanobacterium Anabaena variabilis. The enzyme can be clearly distinguished from the cyanobacterialcoupling factor o f photophosphoiylation and from the alkaline phosphatase. It requires low concentrations o f Ca2+ for maximal activity and is inhibited by orz/io-vanadate, indicating that the enzyme may form a phosphorylated intermediate in its catalytic cycle. The distribution o f the ATPase in sucrose density gradients does not follow that o f thylakoids. It is concluded from these characteristics that the enzyme is bound to the plasma membrane. The cytochrome oxidase ac­ tivity o f the extracts appears to be restricted to the thylakoids.