The phage-encoded excisionase protein (Xis, Swiss:P03699) is involved in excisive recombination by regulating the assembly of the excisive intasome and by inhibiting viral integration. It adopts an unusual 'winged'-helix structure in which two alpha helices are packed against two extended strands. Also present in the structure is a two-stranded anti-parallel beta-sheet, whose strands are connected by a four-residue 'wing'. During interaction with DNA, helix alpha2 is thought to insert into the major groove, while the wing contacts the adjacent minor groove or phosphodiester backbone. The C-terminal region of Xis is involved in interaction with phage-encoded integrase (Int), and a putative C-terminal alpha helix may fold upon interaction with Int and/or DNA [1].

1: Regulation of directionality in bacteriophage lambda site-specific recombination: structure of the Xis protein. Sam MD, Papagiannis CV, Connolly KM, Corselli L, Iwahara J, Lee J, Phillips M, Wojciak JM, Johnson RC, Clubb RT; J Mol Biol 2002;324:791-805. PMID:12460578