Lethality of palindromic DNA and its use in selection of recombinant plasmids

A plasmid derived from ColE1 is constructed so that the removal of one restriction endonuclease HindIII fragment allows the ends of the remaining single fragment (the replicator) to be joined, generating a palindromic sequence 2394 bp in length. The circular species thus produced gives rise to transformants of E. coli at very low frequency. Since the palindromic sequence is effectively lethal to a plasmid containing it, the replicator will give rise to more transformants when the restriction fragment originally removed from it is replaced by another. This principle can be exploited to allow the efficient molecular cloning of unselected restriction fragments.