Isolation and characterization of R-plasmid variants with enhanced chromosomal mobilization ability in Escherichia coli K-12

Enhanced Chromosome Mobilizing (ECM) plasmids derived from the IncP-1 plasmid R68 were isolated in Escherichia coli K-12 by the same methods which have given similar plasmids such as R68.45 in Pseudomonas aeruginosa. The chromosome mobilizing properties of such plasmids in E. coli were similar to those of R68.45 but while retaining the ability to transfer to P. aeruginosa they did not mobilize the chromosome of that organism. Restriction enzyme analysis of two such plasmids, pMO163 and pMO168, showed that they both possessed an additional segment of DNA. With pMO163, an addition of 0.8 kb is located near the TnA region and is characterized by the cleavage site pattern SmaI-HpaI-PstI-BamHI. For pMO168, the additional DNA segment is located at a different site, about 4.0 kb anti-clockwise from the EcoRI site. It was also characterized by the sites SmaI-(HpaI-PstI)-BamHI. No sequence homology has been found between the additional segments of either pMO163 or pMO168 and IS21 of R68.45. However homology of these additional segments was found with the E. coli K-12 chromosome suggesting that pMO163 and pMO168 arise by the acquisition of a transposable element from the E. coli K-12 chromosome.