Site specific transposition of Tn7 into a Rhizobium meliloti megaplasmid

Summary Transposon Tn7 was shown to insert specifically into the megaplasmid of different Rhizobium meliloti strains. Tn7 transposition could not be detected in other Rhizobium strains such as R. trifolii, R. leguminosarum, R. phaseoli and R. japonicum. In R. meliloti strains, two unique sites in the megaplasmid were observed into which Tn7 can transpose at different frequencies. Only one copy of Tn7 could be detected in the megaplasmid and the insertion sites for Tn7 are outside the nif and nod region. Tn7 transposition in R. meliloti showed a marked preference for sites on plasmid RP4 compared to the megaplasmid sites. Attempts to cure Tn7 from the megaplasmid were unsuccessful. This site specific transposition of Tn7 in R. meliloti provides an additional genetic tool to further manipulate this important plasmid in symbiotic nitrogen fixation.