Large palindromes in the λ phage genome are preserved in a rec+ host by inhibiting λ DNA replication

A large palindrome carried by phage λ has been shown to prevent growth of the phage on a rec+ strain of Escherichia coli. The phage do form plaques on recBC sbcB strains, but the palindrome is not stable - deletions that either destroy the palindrome or diminish its size overgrow the original engineered palindrome-containing phage. The authors have prepared stocks of λ carrying a palindrome that is 2 x 4200 base pairs long. λ phage were density labeled by UV induction of lysogens grown in minimal medium containing [13C] glucose and 15NH4Cl. These phage stocks are produced by induction of a lysogen in which the two halves of the palindrome are stored at opposite ends of the prophage and are of sufficient titer (109 phage per ml) to enable one-step growth experiments with replication-blocked phage. They find that the large palindrome as well as a lesser palindrome of 2 x 265 base pairs are recovered intact among particles carrying unreplicated chromosomes following such an infection of a rec+ host. they propose that DNA replication drives the extrusion of palindromic sequences in vivo, forming secondary structures that are substrates for the recBC and sbcB gene products