The glutamine synthetase of the cyanobacterium Synechocystis sp. PCC 6803 can be inactivated in vivo by ammonium addition by a new mechanism that involves the binding to the enzyme of an inactivating factor. This binding provokes a different mobility of the inactive enzyme with respect to the active form in non-denaturing PAGE, but not in SDS-PAGE. This modification of glutamine synthetase is for the first time visualized by Western blot analysis of the active and inactive forms. Cross-linking experiments using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) demonstrate the existence of two main complexes of 56 kDa and 67 kDa between the inactivating factor and the glutamine synthetase subunit (53 kDa) in the inactive but not in the active form of glutamine synthetase.
