Genetic characterization of a mutant of Sinorhizobium fredii strain USDA208 with enhanced competitive ability for nodulation of soybean, Glycine max (L.) Merr.

Sinorhizobium fredii strain USDA208 is a nitrogen‐fixing bacterium that forms nodules on roots of soybean and other legume plants. We previously found that the Tn 5 ‐containing mutant 208T3, which was derived from strain USDA208, is both deficient in production of exopolysaccharides and more competitive than the wild‐type strain in competing against other rhizobia for nodulation of soybean. We now demonstrate that the transposon insertion of the mutant lies in a locus that is highly homologous to a portion of the exo region, which functions in exopolysaccharide biosynthesis by Sinorhizobium meliloti. We sequenced 2906 bp surrounding the insertion site and identified three genes: exoA, exoM , and exoO . The transposon lies within exoM, a glucosyl transferase. A cosmid containing exoHKLAMONP of S. meliloti restores exopolysaccharide production by mutant 208T3 to wild‐type levels. Although exo mutants of S. meliloti are defective in their abilities to form indeterminate nodules, the capacities of mutant 208T3 and its wild‐type parent to form such nodules on five legume species are indistinguishable. Thus the symbiotic function of exopolysaccharide in S. fredii appears to differ fundamentally from that in S. meliloti .