Transposase-dependent formation of circular IS256 derivatives in Staphylococcus epidermidis and Staphylococcus aureus

IS256 is a highly active insertion sequence (IS) element of multiresistant staphylococci and enterococci. Here we show that, in a Staphylococcus epidermidis clinical isolate, as well as in recombinant Staphylococcus aureus and Escherichia coli carrying a single IS256 insertion on a plasmid, IS256 excises as an extrachromosomal circular DNA molecule. First, circles were identified that contained a complete copy of IS256. In this case, the sequence connecting the left and right ends of IS256 was derived from flanking DNA sequences of the parental genetic locus. Second, circle junctions were detected in which one end of IS256 was truncated. Nucleotide sequencing of circle junctions revealed that (i) either end of IS256 can attack the opposite terminus and (ii) the circle junctions vary significantly in size. Upon deletion of the IS256 open reading frame at the 3' end and site-directed mutageneses of the putative DDE motif, circular IS256 molecules were no longer detectable, which implicates the IS256-encoded transposase protein with the circularization of the element.