ISOLATION AND CHARACTERIZATION OF IS1 CIRCLES

Transposase encoded by insertion sequence IS1 is produced from two out -of-phase reading frames by translational frameshifting that occurs in a run of adenines. An IS1 mutant with a single adenine insertion in t he run of adenines efficiently produces transposase, resulting in gene ration of miniplasmids by deletion for a region adjacent to IS1 from a plasmid carrying the IS1 mutant. Here, we found that besides miniplas mids, cells harboring the plasmid contained minicircles without the re gion required for replication. Cloning and DNA sequencing of the minic ircles revealed that most of them were IS1 circles consisting of the e ntire IS1 sequence and a sequence, 5-9 bp in length, which intervenes between terminal inverted repeats, IRL and IRR, of IS1. Analysis of mo re IS1 circles isolated by polymerase chain reaction revealed that the intervening sequence was derived from the region flanking either IRL or IRR in the parental plasmid, suggesting that IS1 circles are genera ted by an excision event from the parental plasmid. The IS1 circles ma y be formed due to the cointegration reaction occurring within the par ental plasmid carrying IS1. (C) 1997 Elsevier Science B.V.