Abstract：
Nucleosomes are the basic compaction unit of chromatin and nucleosome structure and their higher-order assemblies regulate genome accessibility. Many post-translational modifications alter nucleosome dynamics, nucleosome-nucleosome interactions, and ultimately chromatin structure and gene expression. Here, we investigate the role of two post-translational modifications associated with actively transcribed regions, H3K36me3 and H4K5/8/12/16ac, in the contexts of tri-nucleosome arrays that provide a tractable model system for quantitative single-molecule analysis, while enabling us to probe nucleosome-nucleosome interactions. Direct visualization by AFM imaging reveals that H3K36me3 and H4K5/8/12/16ac nucleosomes adopt significantly more open and loose conformations than unmodified nucleosomes. Similarly, magnetic tweezers force spectroscopy shows a reduction in DNA outer turn wrapping and nucleosome-nucleosome interactions for the modified nucleosomes. The results suggest that for H3K36me3 the increased breathing and outer DNA turn unwrapping seen in mononucleosomes propagates to more open conformations in nucleosome arrays. In contrast, the even more open structures of H4K5/8/12/16ac nucleosome arrays do not appear to derive from the dynamics of the constituent mononucleosomes, but are driven by reduced nucleosome-nucleosome interactions, suggesting that stacking interactions can overrule DNA breathing of individual nucleosomes. We anticipate that our methodology will be broadly applicable to reveal the influence of other post-translational modifications and to observe the activity of nucleosome remodelers.
摘要：
核小体是染色质和核小体结构的基本压实单元，它们的高阶组装调节基因组的可接近性。许多翻译后修饰改变核小体动力学，核小体-核小体相互作用，以及最终的染色质结构和基因表达。这里,我们研究了与活跃转录区域相关的两个翻译后修饰的作用，H3K36me3和H4K5/8/12/16ac，在为定量单分子分析提供易于处理的模型系统的三核小体阵列的背景下，同时使我们能够探测核小体-核小体相互作用。通过AFM成像的直接可视化显示，H3K36me3和H4K5/8/12/16ac核小体采用比未修饰的核小体明显更开放和更松散的构象。同样，磁镊子力谱显示，修饰的核小体的DNA外转包裹和核小体-核小体相互作用减少。结果表明，对于H3K36me3，在单核小体中看到的呼吸增加和外部DNA转向展开会在核小体阵列中传播到更开放的构象。相比之下,H4K5/8/12/16ac核小体阵列的更开放的结构似乎并不来自组成单核小体的动力学，但由减少的核小体-核小体相互作用驱动，这表明堆叠相互作用可以推翻单个核小体的DNA呼吸。我们预计我们的方法将广泛适用于揭示其他翻译后修饰的影响并观察核小体重塑物的活性。
