Orientation of cohesive end site cos determines the active orientation of chi sequence in stimulating recA . recBC-mediated recombination in phage lambda lytic infections.

Sequence chi, 5'G-C-T-G-G-T-G-G, locally enhances homologous recombination by recA and recBC proteins of Escherichia coli. Previous work showed that, in phage lambda, chi is more active in one orientation (leftward) than in the other (rightward). Inverting cos, the sequence for the mature DNA ends of lambda, reverses this orientation dependence: the rightward chi becomes fully active, and the leftward chi becomes relatively inactive.

Generalized recombination: nucleotide sequence homology between Chi recombinational hotspots.

Chi sites stimulate generalized recombination catalyzed by the RecA-RecBC-dependent system of E. coli. This stimulation occurs over a region of several thousand base pairs surrounding the Chi site.

Directionality and nonreciprocality of Chi-stimulated recombination in phage lambda.

Chi's are genetic elements that stimulate generalized recombination in their locale in phage lambda. All Chi's wherever located on lambda's chromosome, act asymmetrically in crosses blocked in DNA replication: (1) They stimulate exchange primarily to their left on the conventional lambda map, and (2) the stimulated exchange is frequently nonreciprocal, the recombinant carrying the Chi element being produced less often than the complementary product.