Timsit Y, Westhof E, Fuchs R P, Moras D
Institut de Biologie Moléculaire et Cellulaire, Centre National de la Recherche Scientifique, Strasbourg, France.
Nature. 1989 Oct 5;341(6241):459-62. doi: 10.1038/341459a0.
The target sequence of the restriction enzyme NarI (GGCGCC) is a hot spot for the -2 frameshift mutagenesis (GGCGCC----GGCC) induced by the chemical carcinogens such as N-2-acetyl-aminofluorene. Of the guanine residues, all of which show equal reactivity towards the carcinogen, only binding to the 3'-most proximal guanine within the NarI site is able to trigger the frameshift event. We selected the non-palindromic dodecamer d(ACCGGCGCCACA), whose sequence corresponds to the most mutagenic NarI site in pBR322 DNA; for X-ray structure analysis. Its molecular structure determined at 2.8 A resolution reveals significant deviations from the structure of canonical B-form DNA, with partial opening of three G-C base pairs, high propeller twist values and sequence-dependent three-centred hydrogen bonds. This crystal structure shows a novel kind of packing in which helices are locked together by groove-backbone interactions. The partial opening of G-C base pairs is induced by interactions of phosphate anionic oxygen atoms with the amino group of cytosine bases. This provides a model for close approach of DNA molecules during biological processes, such as recombination.
限制性内切酶NarI的靶序列（GGCGCC）是由化学致癌物如N - 2 - 乙酰氨基芴诱导的 - 2移码突变（GGCGCC----GGCC）的热点。鸟嘌呤残基对致癌物均表现出相同的反应性，只有与NarI位点内最靠近3'端的鸟嘌呤结合才能引发移码事件。我们选择了非回文十二聚体d(ACCGGCGCCACA)，其序列对应于pBR322 DNA中最具诱变作用的NarI位点，用于X射线结构分析。其在2.8 Å分辨率下测定的分子结构显示出与典型B型DNA结构有显著偏差，三个G - C碱基对部分打开，螺旋桨扭转值高，且存在序列依赖性的三中心氢键。该晶体结构展示了一种新型堆积方式，其中螺旋通过沟 - 主链相互作用锁定在一起。G - C碱基对的部分打开是由磷酸阴离子氧原子与胞嘧啶碱基氨基的相互作用诱导的。这为生物过程如重组期间DNA分子的紧密靠近提供了一个模型。
