Bradyrhizobium japonicum ntrBC/glnA and nifA/glnA Mutants: Further Evidence that Separate Regulatory Pathways Govern glnll Expression in Free-Living and Symbiotic Cells. Gregory B. Martin. Michigan State University/Department of Energy Plant Research Laboratory, Genetics Program, Michigan State University, East Lansing 48824-1312 U.S.A. Barry K. Chelm(1,3). (1)Michigan State University/Department of Energy Plant Research Laboratory, and (3)Department of Microbiology, Michigan State University, East Lansing 48824-1312 U.S.A. MPMI 4:254-261. Accepted 18 January 1991. This article is in the public domain and not copryightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991.

Bradyrhizobium japonicum produces two glutamine synthetases that are encoded by the genes glnA and glnII. We have previously hypothesized that two separate regulatory networks involving either ntrC or nifA control glnII expression in B. japonicum (G. B. Martin, K. A. Chapman, and B. K. Chelm, Journal of Bacteriology 170:5452-5459, 1988). Here we test this model by constructing strains of B. japonicum that carry a deletion in glnA and in either the ntrBC operon or nifA. These double mutants were compared with strains carrying single mutations in glnA, ntrBC, or nifA for the ability to utilize various nitrogen sources, nodulate soybeans, and fix atmospheric nitrogen. The ntrBC/glnA mutant was unable to grow on nitrate or ammonia as the sole nitrogen source when grown in aerobic cultures. In addition, it produced fewer nodules per plant than did the wild-type strain or the single mutants and was altered in its ability to fix nitrogen. The nifA/glnA mutant had no nitrogen utilization defects in aerobic, free-living culture, but it produced many fewer nodules on soybeans than did the wild-type strain or a nifA mutant. These observations are interpreted in the context of a model of separate regulatory networks controlling glnII expression in B. japonicum.