Specific DNA cleavage mediated by the integrase of conjugative transposon Tn916

The conjugative transposon Tn916 encodes a protein called INTTn916 which, based on DNA sequence comparisons, is a member of the integrase family of site specific recombinases, Integrase proteins such as INTlambda, FLP, and XERC/D that promote site-specific recombination use characteristic, conserved amino acid residues to catalyze the cleavage and ligation of DNA substrates during recombination. The reaction proceeds by a two-step transesterification reaction requiring the formation of a covalent protein DNA intermediate. Different requirements for homology between recombining DNA sites during integrase-mediated site-specific recombination and Tn916 transposition suggest that INTTn916 may use a reaction mechanism different from that used by other integrase recombinases. We show that purified INTTn916 mediates specific cleavage of duplex DNA substrates containing the Tn916 transposon ends and adjacent bacterial sequences, Staggered cleavages occur at both ends of the transposon, resulting in 5' hydroxyl protruding ends containing coupling sequences. These are sequences that are transferred with the transposon from donor to recipient during conjugative transposition. The nature of the cleavage products suggests that a covalent protein-DNA linkage occurs via a residue of INTTn916 and the 3'-phosphate group of the DNA. INTTn916 alone is capable of executing the strand cleavage step required for recombination during Tn916 transposition, and this reaction probably occurs by a mechanism similar to that of other integrase family site-specific recombinases.