Chain-bias of Escherichia coli Rec-mediated lambda patch recombinants is independent of the orientation of lambda cos.

Chi is a hotspot for homologous recombination mediated by the RecBCD (Rec) pathway of Escherichia coli. For Rec-mediated recombination of phage lambda, the orientation of lambda cos in the lambda chromosome dictates the direction of travel of RecBCD enzyme through DNA and dictates which orientation of Chi or Chi-like sequences will be active in stimulating recombination. I previously found that Rec-mediated lambda patch heteroduplexes, stimulated by Chi or not, are chain-biased; at the lambda P locus, recombinant information resides on the lambda r chain. This bias exists in the presence or absence of Chi sites. Reported herein is the finding that r-chain-bias at the P locus is independent of the orientation of lambda cos and thus also independent of the orientation of active Chi's or Chi-like sequences and of the direction of travel of RecBCD enzyme. These results disprove previously elaborated models in which a chain-specific nick at Chi initiates recombination, and imply that some other chain-distinguishing process is involved with recombination. Replication and transcription are candidates for such a process.