同源DNA双链之间的遗传重组在基因组的维持和增殖中扮演一个关键角色。这一过程的核心中间体是一个四通路的结合点（Holliday），连接着两个DNA双链。这两个链最终分开，形成两个单独的DNA双链。各种不同类型的结合点分解酶见于原核生物、真核生物和它们的病毒。这些核酸酶对于DNA分歧点（branchpoint）有高度选择性。现在，两个小组描述了两种不同的结合点分解酶的复合物的晶体结构，这两种分解酶分别是endonuclease 1 (phage T7) 和 endonuclease VII (phage T4)，每个都与DNA结合点结合在一起。这两种酶在结合时使结合点的结构显著扭曲。新的结构揭示了这两种酶何以对DNA结合点有选择性，也揭示了分解反应的原理。
原始出处:
Nature 449, 616-620 (4 October 2007) | doi:10.1038/nature06152; Received 1 June 2007; Accepted 7 August 2007; Published online 16 September 2007
Crystal structure of T4 endonuclease VII resolving a Holliday junction
Christian Biertümpfel1, Wei Yang1 & Dietrich Suck2
National Institute of Diabetes and Digestive and Kidney Diseases, Laboratory of Molecular Biology, 9000 Rockville Pike, Bethesda, Maryland 20892, USA
European Molecular Biology Laboratory, Structural and Computational Biology Programme, Meyerhofstr. 1, 69117 Heidelberg, Germany
Correspondence to: Wei Yang1Dietrich Suck2 Correspondence and requests for materials should be addressed to W.Y. (Email: wei.yang@nih.gov) or D.S. (Email: suck@embl.de).
Holliday proposed a four-way DNA junction as an intermediate in homologous recombination1, and such Holliday junctions have since been identified as a central component in DNA recombination and repair2. Phage T4 endonuclease VII (endo VII) was the first enzyme shown to resolve Holliday junctions into duplex DNAs by introducing symmetrical nicks in equivalent strands3. Several Holliday junction resolvases have since been characterized4, but an atomic structure of a resolvase complex with a Holliday junction remained elusive. Here we report the crystal structure of an inactive T4 endo VII(N62D) complexed with an immobile four-way junction with alternating arm lengths of 10 and 14 base pairs. The junction is a hybrid of the conventional square-planar and stacked-X conformation. Endo VII protrudes into the junction point from the minor groove side, opening it to a 14 Å 32 Å parallelogram. This interaction interrupts the coaxial stacking, yet every base pair surrounding the junction remains intact. Additional interactions involve the positively charged protein and DNA phosphate backbones. Each scissile phosphate that is two base pairs from the crossover interacts with a Mg2+ ion in the active site. The similar overall shape and surface charge potential of the Holliday junction resolvases endo VII, RuvC, Ydc2, Hjc and RecU, despite having different folds, active site composition and DNA sequence preference, suggest a conserved binding mode for Holliday junctions.
