In order to find the regulatory genes related to symbiotic nitrogen fixation in rhizobia,homologous recombination was used to knock out 10 up-regulated genes during the symbiotic nitrogen fixation of Lotus japonicus and  Rhizobium MAFF303099.The results showed that the nitrogenase activity of the Δmlr5883 was 40% lower than that of the wild-type MAFF303099. The morphology of infected cells did not change significantly,and replenishing mlr5883 restored nitrogenase activity. It is predicted that mlr5883 encodes an aspartate aminotransferase. The aspartate aminotransferase activity of this MLR5883 was 16.67 U/mg in vitro. The mutation of mlr5883 affected the efficiency of nitrogen fixation,indicating that it may be involved in the metabolic process of supplying carbon sources to plants.