a, A schematic representation of the integrative complex. The P′ arm engages the N domains of subunits bound to C and B′ core sites. The P arm is positioned over the N domains of subunits on the C′ and B core sites by a compound bend induced by IHF. d, During excision, the P′ arm is oriented similarly as in integration, whereas the P arm could form a more compact loop upon binding to Xis and/or Fis (see text). b, c, e, f, Docking models of the hypothetical integrative (b, c) and excisive (e, f) complexes based on our crystal structures and that of an IHF–DNA complex36 are shown in front and back views.