In vitro mutagenesis of a circular DNA molecule by using synthetic restriction sites

A method for mutagenizing circular DNA molecules has been developed that uses synthetic oligodeoxynucleotide restriction sites as mutagens. A single synthetic restriction site is introduced at random by cleaving circular DNA with a nonspecific double-strand endonuclease. The restriction site is then ligated to the ends and the molecule is subsequently recircularized. These small additions to the genome are mapped by digestion with the appropriate restriction enzyme. Rearrangements such as duplications and deletions can be engineered at will by using the added restriction sites. This technique has been used to produce a fine-structure map of RSF1050, a ColE1 derivative, 60% of which is a transposable DNA sequence encoding the TEM beta-lactamase (Tn3). A subset of the mutations, mapping within a narrow region of Tn3, result in an increased frequency of Tn3 transposition; mutations in other regions abolish transposition entirely.