Involvement of multiple translocating DNA segments and recombinational hotspots in the structural evolution of bacterial plasmids

Abstract

Previous studies from our laboratory (Kopecko & Cohen, 1975) have shown that two different antibiotic resistance-coding segments of the pSC50 plasmid are capable of recA-independent translocation to the small non-conjugative plasmid pSC101. Both of the transposable units are structurally defined DNA segments containing 130 to 145-nucleotide long inverted repeat DNA sequences at each end. The present electron microscope heteroduplex investigations indicate that one of the transposable genetic elements (TnA) is contained within the length of the other (TnS) on both the parental and recombinant plasmids. The movement of TnA and TnS among plasmids as discrete structural units implies that specific sites at their termini are involved in the translocation process. Although multiple insertion sites for these elements exist on the pSC101 plasmid, the non-random distribution of recipient loci suggests that insertion is site-specific and that it may involve recognition of a frequently recurring nucleotide sequence. In addition to their role in the intergenomic transfer of DNA sequences, the termini of transposable genetic elements can serve as hotspots for other types of recombinational events: in the R1-19 plasmid, a Km-Nm resistance segment that separates the right terminus of TnS from an IS1 (insertion sequence) element can be deleted spontaneously, bringing the termini of TnS and IS1 in juxtaposition. Recombination of another DNA segment containing inverted repeats and a gene for Km-Nm resistance occurs at the left terminus of TnS. Our findings suggest that evolutionary divergence of several groups of related cointegrated antibiotic resistance plasmids has occurred segmentally by a series of site specific recombinational events involving the translocation, insertion, and/or deletion of structurally defined segments of DNA.