Determinants of directionality in lambda site-specific recombination.

The DNA structural features governing directionality in lambda site-specific recombination are shown to reside in regions of the phage attachment site more than 70 bp to the left and more than 40 bp to the right of the cross-over region. Disposition of these sequences on the same attachment site in integration, and on different attachment sites in excision, determines the opposite effects of Xis protein upon the two reactions (stimulation of excision and inhibition of integration). The binding of Xis to two adjacent directly repeated sequences in the left phage arm is shown to occur in a highly cooperative manner, to alter the conformation of the DNA, and to produce a 32-fold stimulation of Int binding to an adjacent locus.