Three-site synapsis during Mu DNA transposition: a critical intermediate preceding engagement of the active site.

The chemical steps of bacteriophage Mu DNA transposition take place within a higher order nucleoprotein structure. We describe a novel intermediate that precedes the previously characterized transpososomes and directly demonstrates the interaction of a distant enhancer element with recombination regions. The transpositional enhancer interacts with the Mu left and right ends to form a three-site synaptic (LER) complex. Under normal reaction conditions, the LER complex is rapidly converted into the more stable Mu transpososomes. However, mutation of the Mu terminal nucleotides results in accumulation of the LER and a failure to form the type 0 transpososome. During the transition from LER to type 0, the Mu DNA termini and the active site of the transposase engage in a catalytically competent conformation.