Abstract
Transposition of Mu involves transfer of the 3′ ends of Mu DNA to the 5′ ends of a staggered cut in the target DNA. We find that cleavage at the 3′ ends of Mu DNA precedes cutting of the target DNA. The resulting nicked species exists as a noncovalent nucleoprotein complex in which the two Mu ends are held together. This cleaved donor complex completes strand transfer when a target DNA, Mu B protein, and ATP are provided. Mu end DNA sequences that have been precisely cut at their 3′ ends by a restriction endonuclease, instead of by Mu A protein and HU, are efficiently transferred to a target DNA upon subsequent incubation with Mu A protein, Mu B protein, and ATP. Cleavage of the Mu ends therefore cannot be energetically coupled with joining these ends to a target DNA. We discuss the DNA strand transfer mechanism in view of these results, and propose a model involving direct transfer of the 5′ ends of the cut target DNA, from their original partners, to the 3′ ends of Mu.
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