Chain bias in Chi-stimulated heteroduplex patches in the lambda ren gene is determined by the orientation of lambda cos.

Heteroduplex patch recombinants have received information in one DNA chain but have not recombined flanking markers. Evidence regarding which chain is exchanged bears on the structure of recombination intermediates. The direction of travel along DNA of RecBCD recombinase, the central enzyme in the Escherichia coli RecBCD pathway of homologous recombination, is determined in phage lambda by the orientation of the packaging origin, cos. cos is a double-chain cut site which serves as a preferred entry site for RecBCD. Using partially denaturing gels to resolve heteroduplex molecules, we have examined patch recombinants at the lambda ren gene. We report that the transferred information in Chi-stimulated patches at ren can occur on either chain, but is biased to the chain ending 5' at the right of the lambda map (the lambda r chain) in phage carrying cos in its normal orientation. The chain bias switches in favor of the chain that ends 3' at the right (the lambda l chain) when RecBCD travel direction is reversed by inverting cos. We entertain models that accommodate these and other results pertaining to the structure of RecBCD-mediated recombinants.