Symbiotic nitrogen fixation: molecular cloning of Rhizobium genes involved in exopolysaccharide synthesis and effective nodulation.

A transposon (Tn5)-induced mutant (strain ANU437) of Rhizobium trifolii was isolated in which no water-soluble exopolysaccharide (EPS) could be detected. This mutant was also incapable of forming nitrogen-fixing root nodules on clover plants. Molecular cloning has demonstrated that the Tn5 transposon was responsible for both of these mutant phenotypes and that there is a direct correlation between EPS synthesis in this bacterial strain and its ability to carry out symbiotic nitrogen fixation. In the mutant ANU437, Tn5 was located in a 9.4-kb EcoRI fragment that was cloned into the amplifiable plasmid pBR322. The recombinant plasmid was used as a hybridization probe to isolate the corresponding wild-type DNA sequence of R. trifolii from a lambda Charon 28 genomic clone bank. This DNA sequence was subcloned into the broad host range conjugative plasmid RP4 and introduced into the Escherichia coli strain RR1. It was then transferred to the mutant ANU437 by conjugation. The acquisition of the wild-type DNA sequence by the mutant ANU437 resulted in the restoration of its ability to synthesize normal levels of EPS and to form nitrogen-fixing nodules on white and subterranean clovers.