A derivative of the IncP1 plasmid RP4, carrying the thermoinducible prophage Mucts62, was obtained in Escherichia coli K 12 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 ampicillinresistant transposon Tnl into the R. meliloti genome.
Transposition of Tnl did not generate auxotrophic strains, suggesting that the insertion of Tnl into the R. meliloti genome was relatively specific. Two chromosomal hot spots for Tnl 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 Tnl insertion.
for introducing transposable elements into a bacterial genome whenever a method for selecting against the maintenance of the plasmid is available (Kleckner et al. 1977). Thus, the instability of Mu: :plasmids in Rhizobia offers a simple procedure for introducing drug-resistant transposons into these species. In this way, Beringer et al. (1978) have constructed the plasmid pJB4JI, a suitable vehicle for transferring the kanamycin-resistant transposon Tn 5 to Rhizobium.
Tn 1 is an ampicillin-resistant transposon found in the Inc P1 plasmid RP4 (Datta et al. 1971). Transpositions of Tnl to other replicons are well known (Hedges and Jacob 1974;Heffron et al. 1977;Hernalsteens et al. 1977). We report here the transfer of Tn 1 to Rhizobium meliloti GR4 and the analysis of some basic features of the transposition process.