Identification of Bradyrhizobium japonicum malonamidaseE2 in the periplasmic space of soybean nodule bacteroids

MalonamidaseE2 (MAE2) was found both in the plant cytosolic fraction and in the Bradyrhizobium japonicum bacteroids of soybean nodules, and was proposed to play an important role in the transfer of fixed nitrogen (NH₃) to plant cells. The enzyme in the cytosol was identical in its molecular size and was immunologically homologous with MAE2 from free-living B. japonicum. The enzyme was not found in soybean root that is not infected by symbiotic bacteria. B. japonicum MAE2 presence in the periplasmic space was confirmed by the determination of the enzyme activity using intact bacteria in the presence of Triton X-100 (0.02%). Under microaerobic conditions, B. japonicum secreted MAE2 into the medium. However, the presence of malonate did not affect the secretion, indicating that malonate is not directly involved in the secretion of MAE2. The evidence described above strongly suggests that B. japonicum MAE2 located in the periplasmic space may be secreted into the peribacteroidal space of nodules during symbiosis.