Transposition of Tn 1 to the Rhizobium meliloti genome.

A derivative of the IncP1 plasmid RP4, carrying the thermoinducible prophage Mucts62, was obtained in Escherichia coli K12 J53 (RP4). It was impossible to maintain the hybrid plasmid RP4::Mucts62 in Rhizobium meliloti GR4. Thus, it was used as a vehicle for introducing the ampicillin-resistant transposon Tn1 into the R. meliloti genome. Transposition of Tn1 did not generate auxotrophic strains, suggesting that the insertion of Tn1 into the R. meliloti genome was relatively specific. Two chromosomal hot spots for Tn1 insertion were identified by cotransductional analysis, after general transduction by phage DF2. Plasmid-curing experiments, carried out by heat treatment, revealed that symbiotic plasmid(s) also contain at least one site for Tn1 insertion.