A minimal system for Tn7 transposition: The transposon-encoded proteins TnsA and TnsB can execute DNA breakage and joining reactions that generate circularized Tn7 species

In the presence of ATP and Mg2+, the bacterial transposon Tn7 translocates via a cut and paste mechanism executed by the transposon-encoded proteins T nsA + TnsB + TnsC + TnsD. We report here that in the presence of Mn2+, TnsA + TnsB alone can execute the DNA breakage and joining reactions of Tn7 rec ombination. ATP is not essential in this minimal system, revealing that thi s cofactor is not directly involved in the chemical steps of recombination. In both the TnsAB and TnsABC + D systems, recombination initiates with dou ble-strand breaks at each transposon end that cut Tn7 away from flanking do nor DNA. In the minimal system, breakage occurs predominantly at a single t ransposon end and the subsequent end-joining reactions are intramolecular, with the exposed 3' termini of a broken transposon end joining near the oth er end of the Tn7 element in the same donor molecule to form circular trans poson species. In contrast, in TnsABC + D recombination, breaks occur at bo th ends of Tn7 and the two ends join to a target site on a different DNA mo lecule to form an intermolecular simple insertion. This demonstration of th e capacity of TnsAB to execute breakage and joining reactions supports the view that these proteins form the Tn7 transposase. (C) 2000 Academic Press.