Translocation of a plasmid DNA sequence which mediates ampicillin resistance: molecular nature and specificity of insertion

A series of recombinant plasmids was generated in Escherichia coli in which the TEM β lactamase translocon (TnA) was inserted into the small plasmid RSF1010. RSF1010 is a 5.5 x 10⁶ dalton nonconjugative plasmid which confers resistance to streptomycin and sulfonamide. The recombinant plasmids can be classified into three clearly defined phenotypic groups. Group I is ampicillin, streptomycin and sulfonamide resistant. Group II is ampicillin and sulfonamide resistant but has lost streptomycin resistance. Group III is ampicillin resistant but is sensitive to sulfonamide and shows a simultaneous 30 fold reduction in the minimal inhibitory concentration of streptomycin. It was possible to map the site of insertion of TnA within RSF1010 by electron microscope studies of DNA heteroduplexes formed between RSF1010 and recombinant plasmids. Insertions of TnA occur at, at least, 12 distinct sites in a region corresponding to one third of the RSF1010 DNA molecule. Those insertions giving rise to particular phenotypes are clustered. Insertions of TnA like insertion sequences (IS) appear to give rise to strongly polar mutations.