Instability of Palindromic DNA in Escherichia coli

The lack of palindrome formation during recombinant DNA cloning experiments is a paradox that has been largely overlooked. In the present context, palindromic molecules are denned as DNA molecules that have twofold rotational symmetry in which the symmetry is perfect right up to the axis of symmetry, as distinguished from inverted repeats, in which a nonsymmetrical region separates the inverted symmetries. Collins and Hohn (1978) demonstrated the absence of palindromes among recombinants during the first use of cosmid cloning vehicles in which polymers of the cloned fragment were preferentially produced. Earlier attempts to clone large palindromic DNAs, such as the ribosomal RNA genes of ciliates (J. Collins and J. Engberg, unpubl; J. Gall, pers. comm.) or of slime molds (R. Firtel, pers. comm.), failed to give hybrids containing the axis of symmetry. Perricaudet et al. (1977) demonstrated that a large palindromic region in a λ clone was highly unstable and...