CHARACTERIZATION OF THE DRUG-RESISTANCE PLASMID R2418 - RESTRICTION MAP AND ROLE OF INSERTION AND DELETION IN ITS EVOLUTION

Escherichia coli 2418 strain is resistant to beta-lactam antibiotics ( ampicillin, carbenicillin, and cephalothin), streptomycin, tetracyclin e, kanamycin. and chloramphenicol. This strain contains at least two c onjugative plasmids (R2418 and R2418S) encoding resistance to beta-lac tam antibiotics and resistance to both beta-lactam antibiotics and str eptomycin, respectively. Restriction endonuclease mapping of plasmid D NAs indicates that the plasmid R2418S has evolved from R2418 DNA by th e insertion of 2.5-kb DNA between BamHI and PvuII sites, and deletion of 0.5-kb DNA within the EcoRI-EcoRV region. The 2.5-kb DNA insert is responsible for streptomycin resistance. This evolution is also associ ated with a reduction in the efficiency of conjugal transfer for the p lasmid R2418S. The conjugal transfer of streptomycin resistance occurs only through the coresidence of the conjugative plasmid R2418 or R241 8S in the donor cell. In accordance with the hypothesis that the Sm-r determinant is due to a putative transposon, plasmid-free transconjuga nts resistant to streptomycin only were isolated. Southern blot analys is of HindIII chromosomal digests extracted from these transconjugants shows that the Sm-r determinant is inserted into different sites in c hromosomal DNA.