Two forms of glutamine synthetase in free-living root-nodule bacteria

Cell-free extracts of Rhizobium japonicum 61A76 contain two forms of glutamine synthetase (EC 6.3.1.2) which can be easily separated by isoelectric focusing. The more acid form (pI = 5.4), like the enzyme from E. coli , is stable at 50° and catalyses an ADP-dependent transferase reaction, whose inhibition by excess Mg ++ can be relieved by snake venom phosphodiesterase. The more alkaline form (pI = 6.1) is labile at 50°, and catalyses and ADP-dependent transferase reaction which is strongly inhibited by Mg ++ regardless of phosphodiesterase treatment. We have also observed the two forms of the enzyme in a nitrogenaseless mutant of 61A76, and in other strains of rhizobia, but only the acid form in E. coli W, A. vinelandii OP, and K. pneumoniae M 51A.