Binding of the Tn3 transposase to the inverted repeats of Tn3.

The transposase protein and the inverted repeat sequences of Tn3 are both essential for Tn3 cointegrate formation and transposition. We have developed two assays to detect site-specific binding of transposase to the inverted repeats: (1) a nitrocellulose filter binding assay in which transposase preferentially retains DNA fragments containing inverted repeat sequences, and (2) a DNase 1 protection assay in which transposase prevents digestion of the inverted repeats by DNase 1. Both assays show that transposase binds directly to linear, duplex DNA containing the inverted repeats. The right inverted repeat of Tn3 binds slightly more strongly than the left one. Site-specific binding requires magnesium but does not require a high energy cofactor.