Two Transmissible Plasmids in Rhizobium leguminosarum strain 300

SUMMARY: The transposon Tn5, which specifies kanamycin resistance, was inserted into transmissible plasmids of Rhizobium leguminosarum strain 1062, a derivative of strain 300. In this way several kanamycin-resistant derivatives of the two smallest plasmids (pRL8JI and pIJ1001) were obtained, but there was no evidence that any of the other plasmids of strain 1062 was transmissible. Kanamycin-sensitive derivatives, each apparently cured of either pRL8JI or pIJ1001, still induced nitrogen-fixing nodules on peas and were not phenotypically distinct from the parental strain. Both plasmids were transmissible at low frequency to other R. leguminosarum strains, but they could be mobilized efficiently by pRLlJI, another transmissible R. leguminosarum plasmid. When Tn5-marked pIJ1001 was transferred to a strain of R. phaseoli, the majority of the transconjugants lost the ability to nodulate Phaseolus beans, the normal host for this species. This was due to the loss from R. phaseoli of a nodulation plasmid which was apparently incompatible with pIJ1001.