Coupling with packaging explains apparent nonreciprocality of Chi-stimulated recombination of bacteriophage lambda by RecA and RecBC functions.

Chi (chi, 5'-GCTGGTGG) is a recombinator in RecA- and RecBC-mediated recombination in Escherichia coli. In vegetative recombination between two bacteriophage lambda strains, one with and the other without Chi (a+ chi +b- X a- chi 0b+), the chi-containing recombinant (a- chi +b-) is less abundant than the non-chi-containing recombinant (a+ chi 0b+). Previously this was taken was evidence for nonreciprocality of chi-stimulated exchange. This inequality, however, is now seen to result from an event at cos (lambda's packaging origin) that both activates Chi and initiates DNA packaging. An event at rightward cos leads to activation of leftward chi on the same chromosome for an exchange to its left. From the resulting circulating dimer (--cos-a+- chi 0-b+-cos-a-- chi +-b- --), the cos that activated chi is more likely to be used for rightward packaging initiation than is the cos from the other parent. Consistent with this coupling model is "biased packaging" in lambda carrying two cos sites per monomer genome. When their maturation is dependent on dimerization by chi-stimulated exchange, the phage particles result more often from packaging from the cos that activates chi than from packaging from the other cos. Since Chi activation and packaging can be uncoupled, we infer that some early and reversible step in packaging activates chi. A strong candidate for this step is a double-strand break at cos that provides an oriented entry site for a recombinase.