Symbiotic nitrogen fixation is accompanied by a shift of Rhizobium nitrogen metabolism from ammonium assimilation to ammonium export, which probably involves genetic or metabolic regulation of glutamine synthetase activity. In free-living Rhizobium meliloti glutamine synthetase I (GSI) is regulated post-translationally by reversible adenylylation in response to ammonium addition. Moreover, full expression of the GSI gene glnA requires the transcriptional activator, NtrC. A glnA1 mutant synthesizing a non-adenylylatable GSI produces normal nitrogen-fixing nodules on alfalfa: GSI adenylylation is dispensable for symbiotic nitrogen fixation. This is rationalized by the observation that less GS protein is present in R. meliloti bacteroids than in free-living bacterial cells.
Arcondéguy, T., Huez, I., Fourment, J., & Kahn, D. (1996). Symbiotic nitrogen fixation does not require adenylylation of glutamine synthetase I in Rhizobium meliloti. FEMS Microbiology Letters, 145(1), 33–40. https://doi.org/10.1016/0378-1097(96)00381-3