Nucleosome DNA unwrapping and its disassembly into hexasomes and tetrasomes is necessary for genomic access and plays an important role in transcription regulation. Previous single-molecule mechanical nucleosome unwrapping revealed a low- and a high-force transitions, and force-FRET pulling experiments showed that DNA unwrapping is asymmetric, occurring always first from one side before the other. However, the assignment of DNA segments involved in these transitions remains controversial. Here, using high-resolution optical tweezers with simultaneous single-molecule FRET detection, we show that the low-force transition corresponds to the undoing of the outer wrap of one side of the nucleosome (∼27 bp), a process that can occur either cooperatively or noncooperatively, whereas the high-force transition corresponds to the simultaneous unwrapping of ∼76 bp from both sides. This process may give rise stochastically to the disassembly of nucleosomes into hexasomes and tetrasomes whose unwrapping/rewrapping trajectories we establish. In contrast, nucleosome rewrapping does not exhibit asymmetry. To rationalize all previous nucleosome unwrapping experiments, it is necessary to invoke that mechanical unwrapping involves two nucleosome reorientations: one that contributes to the change in extension at the low-force transition and another that coincides but does not contribute to the high-force transition.

中文翻译：

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核小体及其分解产物机械展开过程中结构转变的分配

核小体 DNA 展开及其分解为六体和四体是基因组访问所必需的，并且在转录调控中起着重要作用。先前的单分子机械核小体展开揭示了低力和高力的转变，力-FRET 拉动实验表明 DNA 展开是不对称的，总是首先从一侧发生在另一侧之前。然而，参与这些转换的 DNA 片段的分配仍然存在争议。在这里，使用同时进行单分子 FRET 检测的高分辨率光学镊子，我们表明低力转换对应于核小体一侧 (~27 bp) 外包装的解开，这个过程可以发生合作或不合作，而高力转换对应于从两侧同时展开~76 bp。这个过程可能随机地导致核小体分解成六体和四体，我们建立了它们的展开/重新包裹轨迹。相比之下，核小体重新包装不会表现出不对称性。为了使所有以前的核小体展开实验合理化，有必要调用机械展开涉及两个核小体重新定向：一个有助于在低力转换时改变延伸，另一个重合但不有助于高力转换。核小体重新包装不会表现出不对称性。为了使所有以前的核小体展开实验合理化，有必要调用机械展开涉及两个核小体重新定向：一个有助于在低力转换时改变延伸，另一个重合但不有助于高力转换。核小体重新包装不会表现出不对称性。为了使所有以前的核小体展开实验合理化，有必要调用机械展开涉及两个核小体重新定向：一个有助于在低力转换时改变延伸，另一个重合但不有助于高力转换。