Identification and characterization of the linear IS3 molecules generated by staggered breaks.

Insertion sequences IS3 encodes two, out-of-phase, overlapping open reading frames, orfA and orfB. The OrfAB transframe protein that is IS3 transposase is produced by -1 translational frameshifting between orfA and orfB. Efficient production of the IS3 transposase in the cells harboring the IS3-carrying plasmid has been shown to generate miniplasmids as well as characteristic minicircles, called IS3 circles, consisting of the entire IS3 sequence and one of the 3-base pair sequences flanking IS3 in the parental plasmid. Here, we show that the IS3 transposase also generates the linear molecules of IS3 with 3-nucleotide overhangs at the 5'-ends. The nucleotide sequences of the overhangs are the same as those flanking IS3 in the parental plasmid, suggesting that the linear IS3 molecules are generated from the parental plasmid DNA by staggered double strand breaks at the end regions of IS3. The linear IS3 molecules are likely to be the early intermediates in the transposition reaction, which proceeds in a non-replicative manner.