Suquet C, Smerdon M J
Department of Biochemistry and Biophysics, Washington State University, Pullman 99164-4660.
J Biol Chem. 1993 Nov 15;268(32):23755-7.
The major photoproduct formed in DNA, following absorption of ultraviolet (UV) light, is the cis-syn cyclobutane pyrimidine dimer (CPD). Formation of CPDs in DNA packaged into chromatin prior to UV irradiation results in a striking preference for these photoproducts to be oriented away from the histone surface in nucleosomes (Gale, J. M., Nissen, K. A., and Smerdon, M. J. (1987) Proc. Natl. Acad. Sci. U. S. A. 84, 6644-6648). In this report, we show that formation of nucleosomes onto UV-irradiated DNA results in a similar distribution of these photoproducts, indicating that preexisting CPDs in the DNA molecule can influence its rotational setting on the histone surface during nucleosome formation. This bias is less pronounced in the central three helical turns encompassing the dyad axis of nucleosomes, where the helix is overwound (10.7 bases/turn) relative to the "outer" portion of core DNA (10.0 bases/turn). Such a change in the rotational setting of DNA on the surface of newly formed nucleosomes could expose normally inaccessible DNA sequences to factors which control DNA processing.
DNA吸收紫外线（UV）后形成的主要光产物是顺式-顺环丁烷嘧啶二聚体（CPD）。在紫外线照射之前，包装在染色质中的DNA中CPD的形成导致这些光产物在核小体中明显倾向于远离组蛋白表面排列（Gale, J. M., Nissen, K. A., and Smerdon, M. J. (1987) Proc. Natl. Acad. Sci. U. S. A. 84, 6644 - 6648）。在本报告中，我们表明在紫外线照射过的DNA上形成核小体会导致这些光产物的类似分布，这表明DNA分子中预先存在的CPD在核小体形成过程中会影响其在组蛋白表面的旋转定位。这种偏向在围绕核小体二分轴的中央三个螺旋圈中不太明显，在那里螺旋相对于核心DNA的“外部”部分（10.0个碱基/圈）过度缠绕（10.7个碱基/圈）。新形成的核小体表面上DNA旋转定位的这种变化可能会使通常无法接近的DNA序列暴露于控制DNA加工的因子。
