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Sulfur is an essential element for rhizobia,such as sulfated modified Nod factors and nitrogenase.To investigate the role of sulfur metabolism in Rhizobium-Soybean symbiosis,a transponson random insertional mutants library was constructed and sulfur assimilation-related gene was isolated and characterized.A mutant strain 105.26 unable to utilized sulfate was screened from 11000 random insertional mutants of Sinorhizobium fredii WGF03.