Differential transcription of the two glutamine synthetase genes of Bradyrhizobium japonicum

Bradyrhizobium japonicum induces the formation of nitrogen-fixing symbiotic root nodules on soybean plants. The B. japonicum genome encodes two isoforms of glutamine synthetase (GS). One form, GSI, encoded by the gene glnA, is similar in structure and activity to the enzyme found in all other bacteria. The second form, GSII, encoded by glnII, is structurally related to the eucaryotic enzyme. Genetic analyses indicate that glnA or glnII alone is sufficient to provide glutamine prototrophy, whereas the double mutation glnA glnII produces glutamine auxotrophy. The glnA gene is transcribed from a single promoter that has a structure most similar to that of the bacterial consensus promoter. The level of transcription of glnA is not specifically affected by nitrogen limitation of growth. The glnII gene is also transcribed from a single promoter; however, this promoter has structural features characteristic of promoters controlled by the nitrogen regulation system. In contrast to glnA, physiological studies indicate that glnII transcription is regulated in response to nitrogen source availability. Under aerobic growth conditions, expression of glnII is induced when growth is limited by nitrogen source depletion as expected for regulation by the nitrogen regulation system.