Precise excision of bacteriophage Mu DNA.

The temperate bacteriophage Mu is a transposable element that can integrate randomly into bacterial DNA, thereby creating mutations. Mutants due to an integrated Mu prophage do not give rise to revertants, as if Mu, unlike other transposable elements, were unable to excise precisely. In the present work, starting with a lacZ::Muc62(Ts) strain unable to form Lac+ colonies, we cloned a lacZ+ gene in vivo on a mini-Mu plasmid, under conditions of prophage induction. In all lac+ plasmids recovered, the wild-type sequence was restored in the region where the Mu prophage had been integrated. The recovery of lacZ+ genes shows that precise excision of Mu does indeed take place; the absence of Lac+ colonies suggests that precise excision events are systematically associated with loss of colony-forming ability.