Isolation and characterization of glutamine synthetase from the diazotroph Azospirillum Brasilense

Glutamine synthetase was purified from the diazotroph Azospirillum brasilense. The holoenzyme with a M(r) of 630,000 is composed of 12 subunits of M(r) 52,000. A modified subunit of M(r) 53,000 was also found by electrophoresis under denaturing conditions. It is shown that the M(r) 53,000 species is the adenylylated subunit. The apparent K(m) values for glutamate, ATP and ammonia were 2.5 ± 0.3 mM, 200 ± 20 μM and 42 ± 2 μM, respectively. Levels of glutamine synthetase activity in A. brasilense cells varied by a factor of 8 depending on the nitrogen source and its concentration in the growth medium.