The bipartite DNA-binding domain of Tc3 transposase, Tc3A, was crystallized in complex with its transposon recognition sequence. In the structure the two DNA-binding domains form structurally related helix-turn-helix (HTH) motifs. They both bind to the major groove on a single DNA oligomer, separated by a linker that interacts closely with the minor groove. The structure resembles that of the transcription factor Pax6 DNA-binding domain, but the relative orientation of the HTH-domain is different. The DNA conformation is distorted, characterized by local narrowing of the minor groove and bends at both ends. The protein-DNA recognition takes place through base and backbone contacts, as well as shape-recognition of the distortions in the DNA. Charged interactions are primarily found in the N-terminal domain and the linker indicating that these may form the initial contact area. Two independent dimer interfaces could be relevant for bringing together transposon ends and for binding to a direct repeat site in the transposon end. In contrast to the Tn5 synaptic complex, the two Tc3A DNA-binding domains bind to a single Tc3 transposon end.
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