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. Author manuscript; available in PMC: 2007 Jun 15.
Published in final edited form as: Curr Opin Struct Biol. 2005 Dec 20;16(1):42–50. doi: 10.1016/j.sbi.2005.12.003

Figure 1.

Figure 1

Integrative and excisive λ site-specific recombination. Integrative recombination between the phage attP and the bacterial attB sites (top) requires supercoiled attP DNA, Int and IHF. The Int pair bound to C and B core sites (red and pink arrows, respectively) nicks, exchanges and ligates the top strands (continuous lines) to form a HJ intermediate. The Int pair bound to C′ and B′ core sites then nicks, exchanges and ligates the bottom strands (dashed lines), thereby resolving the HJ to recombinant products attR and attL. attR and attL are shown here as the substrates of excisive recombination (bottom), a reaction that additionally requires Xis (and Fis, in vivo). Core-type Int-binding sites (C, C′, B and B′) and the seven base pair overlap region (sequence between cleavage sites, marked by black dots) comprise the ‘core region’. The arm regions contain the five arm-type Int-binding sites (P1, P2, P′1, P′2 and P′3, shown as green arrows) and the binding sites for the DNA-bending proteins IHF (H1, H2 and H′, blue triangles), Xis (X1 and X2, pink hexagons) and Fis (F, purple oval). The overlapping subsets of sites in the arm region that are used for either integrative or excisive recombination are shown as filled symbols.