Physiological roles of glutamine synthetases I and II in ammonium assimilation in Rhizobium sp. 32H1.

The two glutamine synthetases of Rhizobium sp. 32H1 appear to be structurally and functionally distinct. Glutamine synthetase I was reversibly adenylylated, and its synthesis was repressed only twofold by ammonium. When in the unadenylylated configuration, it was the enzyme which allowed the organism to grow, albeit marginally, on ammonium as a nitrogen source. There is no evidence to suggest that the second enzyme, glutamine synthetase II, is regulated by adenylylation. However, this enzyme was repressed at least 50-fold by even low amounts of ammonium. Glutamine synthetase II does not seem to function in ammonium assimilation but rather in purine biosynthesis.