Conjugative transposition: Tn916 integrase contains two independent DNA binding domains that recognize different DNA sequences.

Transposition of the conjugative transposon Tn916 requires the activity of a protein, called Int, which is related to members of the integrase family of site-specific recombinases. This family includes phage lambda integrase as well as the Cre, FLP and XerC/XerD recombinases. Different proteins, consisting of fragments of Tn916 Int protein fused to the C-terminal end of maltose binding protein (MBP) were purified from Escherichia coli. DNase I protection experiments showed that MBP-INT proteins containing the C-terminal end of Int bound to the ends of the transposon and adjacent plasmid DNA. MBP-INT proteins containing the N-terminal end of Int bound to sequences within the transposon close to each end. Competition binding experiments showed that the sites recognized by the C- and N-terminal regions of Int did not compete with each other for binding to MBP-INT. We suggest that Tn916 and related conjugative transposons are unique among members of the integrase family of site-specific recombination systems because the presence of two DNA binding domains in the Int protein might allow Int to bridge recombining sites, and this bridging seems to be the sole mechanism ensuring that only correctly aligned molecules undergo recombination.