Palindromes as substrates for multiple pathways of recombination in Escherichia coli.

A 246-bp imperfect palindrome has the potential to form hairpin structures in single-stranded DNA during replication. Genetic evidence suggests that these structures are converted to double-strand breaks by the SbcCD nuclease and that the double-strand breaks are repaired by recombination. We investigated the role of a range of recombination mutations on the viability of cells containing this palindrome. The palindrome was introduced into the Escherichia coli chromosome by phage lambda lysogenization. This was done in both wt and sbcC backgrounds. Repair of the SbcCD-induced double-strand breaks requires a large number of proteins, including the components of both the RecB and RecF pathways. Repair does not involve PriA-dependent replication fork restart, which suggests that the double-strand break occurs after the replication fork has passed the palindrome. In the absence of SbcCD, recombination still occurs, probably using a gap substrate. This process is also PriA independent, suggesting that there is no collapse of the replication fork. In the absence of RecA, the RecQ helicase is required for palindrome viability in a sbcC mutant, suggesting that a helicase-dependent pathway exists to allow replicative bypass of secondary structures.