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. Author manuscript; available in PMC: 2017 Dec 1.
Published in final edited form as: Microbiol Spectr. 2015 Apr;3(2):MDNA3–0051-2014. doi: 10.1128/microbiolspec.MDNA3-0051-2014

FIGURE 13.

FIGURE 13

The basis for directionality in λ recombination. (A) An explanation for why Xis is required for excision. (B) Explanation for why the excision pathway is not run efficiently in reverse to perform integration. (C) Explanation for why Xis inhibits the normal integration reaction. The Xis, P2, and H1 sites cannot be occupied simultaneously (22). Schematics follow the same coloring scheme used in Fig. 11 and Fig. 12. Int subunits not bound to a core site are colored gray. doi:10.1128/microbiolspec.MDNA3-0051-2014.f13