Electron microscope heteroduplex studies of sequence relations among bacterial plasmids: identification and mapping of the insertion sequences IS1 and IS2 in F and R plasmids.

Heteroduplex experiments between the plasmid R6 and one strand of the deoxyribonucleic acid (DNA) of a lambda phage carrying the insertion sequence IS1 show that IS1 occurs on R6 at the two previously mapped junctions of resistance transfer factor (RTF) DNA with R-determinant DNA. From previous heteroduplex experiments, it then follows that IS1 occurs at the same junctions in R6-5, R100-1, and R1 plasmids. Heteroduplex experiments with the DNA from a lambda phage carrying the insertion sequence IS2 show that one copy of IS2 occurs in R6, R6-5, and R100-1 (but not R1) at a point within the RTF with coordinates 67.5 TO 68.9 kilobase units (kb). In an accompanying paper, Ptashne and Cohen (1975) show that the insertion sequence IS3 occurs on R6 and R6-5. R100-25, a traC mutant, differs from its parent R100-1 only in that it contains an additional copy of IS1 inserted within the tra gene region of 82.1 kb. R100-31, atraX, TC-s mutant of R100-1, is deleted in R100-1 sequences starting at one of the IS3 termini (46.9 kb) and extending with RTF to 61.0 kb. Heteroduplex studies of F plasmids with the DNA of a lambda phage bearing insertion sequence IS2 show that the sequence of F with coordinates 16.3-17.6F is IS2. The occurrence of IS1 at the two junctions of R-determinant DNA and RTF DNA in R plasmids provides a structural basis to explain the mechanism of the previously observed formation of molecules containing one RTF unit and several tandem copies of the R-determinant unit, when R plasmids in Proteus mirabilis are grown in the presence of antibiotics, and the segregation of an R plasmid into an RTF unit and an R-determinant unit. In general, correlation of our results with previous studies shows that insertion sequences play a role in a variety of F- and R-related intra- and intermolecular recombination phenomena.