The phage-encoded excisionase protein Tn916-Xis adopts a winged-helix structure that consists of a three-stranded anti-parallel beta-sheet that packs against a helix-turn-helix (HTH) motif and a third C-terminal alpha-helix. It is encoded for by Tn916, which also codes for the integrase Tn916-Int. The protein interacts with DNA by the insertion of helix alpha-2 into the major groove and the contact of the hairpin that connects strands beta-2 and beta-3 with the adjacent phosphodiester backbone and/or minor groove. Tn916-Xis stimulates phage excision and inhibits viral integration by stabilising distorted DNA structures [1].

1: The structure of the excisionase (Xis) protein from conjugative transposon Tn916 provides insights into the regulation of heterobivalent tyrosine recombinases. Abbani M, Iwahara M, Clubb RT; J Mol Biol. 2005;347:11-25. PMID:15733914