Isolation of an Asm−mutant ofRhizobium japonicumdefective in symbiotic N2-fixation

A mutant strain of Rhizobium japonicum (CJ9) unable to assimilate ammonium (Asm−) was isolated following mutagenesis with N-methyl N-nitro-nitrosoguanidine (NTG). Glutamate synthase activity was not detectable in cell-free extracts of the mutant strain in contrast to the wild type and revertant strains. Although mutant CJ9 induced nitrogenase activity in an ‘in vitro’ assay system under microaerobic conditions, it failed to fix nitrogen (acetylene reduction) in soybean root nodules. These properties of mutant CJ9 constitute a new Asm− mutant class in Rhizobium spp.