A NOVEL ILLEGITIMATE RECOMBINATION EVENT - PRECISE EXCISION AND REINTEGRATION WITH THE MU-GEM MUTANT PROPHAGE

The bacteriophage Mu is known to insert its DNA more or less randomly within the Escherichia coli chromosome, as do transposable elements, b ut unlike the latter, precise excision of the prophage, thereby restor ing the original sequence, is not observed with wild-type Mu, although it has been reported with certain defective mutants. We show here tha t the mutant prophage Mu gem2ts can excise precisely from at least thr ee separate loci - malT, lac and thyA (selected as Mal(+), Lac(+) and Thy(+), respectively). This excision occurs under permissive condition s for phage development, is observed in fully immune (c(+)) lysogens, and is independent of RecA and of Mu transposase. Mu gemts2 excision i s invariably accompanied by reintegration of a Mu gem2ts prophage else where in the chromosome. In the case of Mal(+) revertants, this propha ge is systematically located at 94 min on the E. coli chromosome. Mu g em2ts excision therefore sheds some light on the long-standing paradox of the lack of precise Mu excision.