Rhizobiumjaponicum to Azotobacter vinelandii of Genes Required for Nodulation

Recently, much attention has focused on the role that plant lectins play in the establishment of the N2-fixing symbiosis between legumes and bacteria belonging to the genus Rhizobium (1-4, 10). Lectins from legumes interact with the 0 antigen-containing lipopolysaccharides of their rhizobial symbionts (10). The 0-antigen fractions of wild-type and non-nodulating mutant strains of Rhizobium japonicum have been shown to differ in antigenic and chemical properties (6). Thus the 0-antigen component of the lipopolysaccharides appears to play an important role in forming a successful N2-fixing symbiotic association between R. japonicum and soybean plants. Genes which seem to be involved in the initial stages of infection of white clover by R. trifolii can be transferred, by transformation (1), to mutant strains ofAzotobacter vinelandii unable to fix N2 (Nif-). These genes control the synthesis of an R. trifolii surface receptor which binds to a clover lectin (4). Therefore, we decided to determine whether genes required by R. japonicum for nodulation could be transferred to A. vinelandii. The intergeneric transformation crosses were conducted as previously described (7), except that 2% sucrose replaced glucose in the transformation medium, and the donor strain was lysed in 0.1% sodium dodecyl sulfate. Wild-type R. japonicum 61A76 (obtained from J. Burton, Nitragin Co., Milwaukee, Wis.) was the donor, and A. vinelandii Nif mutant strain UW10 (8) was the recipient. The transformation frequency was 9.7 x 10-7 Nif transformants per cell plated. The rever.Aon frequency of strain UW10 was less than 3.3 x 10iper cell plated. The recipient and transformant cells appeared