Viability of λ phages carrying a perfect palindrome in the absence of recombination nucleases

Abstract

In Escherichia coli in vitro constructions of perfect palindromes larger than 30 base pairs (bp) long have in general been unstable^1–8. A perfect palindrome has the unique possibility of forming a cruciform structure, and it is this feature which probably results in its instability. Negative supercoiling favours the formation of the cruciform conformation, which in turn causes the molecule to relax^1,2. This relaxation may render replicons containing large perfect palindromes inviable⁴. An alternative hypothesis for inviability has been that the cruciform interferes with replication by favouring strand switching by polymerase I⁹. Here we show that the simultaneous absence of two recombination nucleases, the recBC product, exonuclease V, and the sbcB product, exonuclease I, confers viability on a derivative of phage λ carrying a perfect palindrome of inverted repeat length 1,600 bases. This observation suggests a third hypothesis—that nucleolytic cleavage of the cruciform is responsible for the inviability of the phage. Such an activity has been shown in vitro for T4 exonuclease VII¹⁰.