Chi-stimulated patches are heteroduplex, with recombinant information on the phage lambda r chain.

Generalized recombination in Escherichia coli is elevated near Chi sites. In vitro, RecBCD enzyme can nick Chi a few nucleotides 3' of the terminal GG of the Chi sequence (5'-GCTGGTGG). The simplest model in which this nick at Chi participates in Chi function predicts that in phage lambda, Chi-stimulated recombinants not crossed-over for flanking markers (patches) should be heteroduplex, with recombinant information on the lambda I chain. I report here that patches are heteroduplex, but that recombinant information occurs primarily on the lambda r chain. This result rules out the simplest model in which the nick at Chi promotes initiation of recombination, forces reconsideration of Chi's role in recombination, and bears on molecular models for Rec-mediated recombination.