Action at a distance in Mu DNA transposition: an enhancer‐like element is the site of action of supercoiling relief activity by integration host factor (IHF).

The first committed step in the in vitro strand transfer reaction of a mini‐Mu donor molecule is the formation of a Type 1 complex in which the Mu ends are held together in a non‐covalent protein‐DNA complex. Efficient formation of this complex at high levels of donor supercoiling (sigma approximately ‐0.06) requires the Mu A and Escherichia coli HU proteins. At in vivo levels of supercoiling, efficient reaction also requires E. coli integration host factor (IHF). We demonstrate that this supercoiling relief activity of IHF is mediated through an IHF binding site in the Mu early promoter region. This site is part of a larger enhancer‐like element which includes operator 1 (01) and part of operator 2 (02) with the IHF site in between. The enhancer‐like element stimulates the initial rate of the in vitro reaction 100‐fold and acts in a distance‐independent fashion. Inversion of the orientation of the element results in a total loss of enhancer activity in the absence of IHF. However, a 10‐fold stimulation in the initial rate of reaction is induced by the addition of IHF. Furthermore, correct helical phasing between 01 and 02 is required for maximal activity. The results indicate that a specific geometrical configuration of the enhancer‐like element, which includes a sharp bend between 01 and 02, is required for optimal induction of synapsis.