Target immunity during Mu DNA transposition. Transpososome assembly and DNA looping enhance MuA-mediated disassembly of the MuB target complex.

The Mu transpososome can distinguish between proximal and distal DNA during the selection of a site for transposition. This phenomenon, termed target immunity, involves MuA-stimulated removal of MuB oligomers from sites near the Mu genome. Using a combination of ensemble and single-molecule fluorescence methods, we show that the MuA tetramer can stably associate with the DNA-bound MuB oligomer and is more efficient than monomeric MuA at stimulating the dissociation of MuB from DNA. In addition, we demonstrate that DNA looping is essential for efficient disassembly of the MuB oligomer. We propose a model in which the MuA tetramer forms a multivalent complex with the MuB oligomer and catalyzes the processive removal of MuB from DNA.