Cover. Heterobivalent recognition of attL DNA by λ Integrase. The bacteriophage λ site-specific recombination system is responsible for phage integration during the establishment of lysogeny, and prophage excision during lysogenic induction. Catalyzed by λ Integrase protein (λ-Int), these reactions require binding to two dissimilar classes of DNA sequences in the recombination targets known as `arm-type' and `core-type' sites. DNA binding by λ-Int utilizes each of its three structural domains: the N-terminal domain (purple; PDB accession no. 1kjk) binds arm-type DNA sites, and the central core-binding domain {medium blue through green; PDB accession no. 1m97; also see the article by Swalla et al. in this issue [Nucleic Acids Res. (2003) 31, 805--818]} and C-terminal catalytic domain (green through red; modeled from PDB accession no. 4crx) both participate in binding to core-type DNA sites. In the model, a single λ-Int protein binds simultaneously to both the P′1 arm-type site (magenta backbone; green DNA bases) and the C′ core-type site (magenta backbone; yellow DNA bases). Such heterobivalent DNA recognition by λ-Int requires the 180° bend introduced by binding of integration host factor (light and dark gray; PDB accession no. 1ihf) to its H′ site. During recombination, bridging of the P′1 and C′ sites by λ-Int is believed to be important for assembly of higher-order nucleoprotein structures known as `intasomes', which are required for efficient catalysis of recombination. Illustration created with PyMOL (DeLano Scientific, San Carlos, CA).
[Table of Contents]