Structural requirement for IS50-mediated gene transposition.

Replicative transposition is signaled by the formation of cointegrates in which donor and target replicons are joined by direct repeats of a transposable element. Elements not generating such cointegrates may move by a conservative breaking and joining process. The IS50 elements forming the terminal repeats of Tn5 [which carries the determinant for kanamycin resistance (Kanr)] contain genes and sites necessary for transposition and mediate the movement of any DNA segment they bracket. To determine if IS50 generates cointegrates, the products of transposition from pBR322::Tn5 plasmids to an F factor in recA-Escherichia coli were examined. With monomeric pBR322::Tn5 plasmids, transposition of Kanr (from Tn5) was generally not accompanied by movement of the determinant for ampicillin resistance (Ampr) (from the pBR322 vector). With dimeric pBR322::Tn5 plasmids, by contrast, half of the transpositions of kanr were accompanied by transposition of ampr. Restriction endonuclease analyses indicated that these F-Kanr Ampr chimeras contained inserts of a single copy of the pBR322 vector sequence bracketed by one Tn5 element and one IS50 element or by a pair of Tn5 elements. None of 79 chimeras tested was a true cointegrate. Because IS50 seems to move only a segment of the donor replicon it is proposed that IS50 transposition is conservative.