Histone divergence in Trypanosoma brucei results in unique alterations in nucleosome structure

Eukaryotes have an array of diverse mechanisms for organising and using their genomes, but the histones that make up chromatin are highly conserved. Unusually, histones from Kinetoplastids are highly divergent. The structural and functional consequences of this variation are unknown. Here, we have biochemically characterised nucleosome core particles (NCPs) from the Kinetoplastid parasite Trypanosoma brucei. A structure of the T. brucei NCP reveals that global histone architecture is conserved, but specific sequence alterations lead to distinct DNA and protein interaction interfaces. The T. brucei NCP is unstable and has weakened DNA binding overall. However, dramatic changes at the H2A-H2B interface introduce local reinforcement of DNA contacts. The T. brucei acidic patch has altered topology and is refractory to known binders, indicating that the nature of chromatin interactions in T. brucei may be unique. Overall, our results provide a detailed molecular basis for understanding evolutionary divergence in chromatin structure. Sequencing fragments of Widom 601 145 bp DNA produced from limited miccrococcal nuclease digestion of Homo sapiens and Trypanosoma brucei nucleosome core particles

真核生物拥有多样化的基因组组织与利用策略，但构成染色质的组蛋白却高度保守。不同寻常的是，动质体类（Kinetoplastida）生物的组蛋白存在高度显著的分化，这种变异所引发的结构与功能效应迄今仍未明确。本研究通过生化手段，对动质体寄生虫布氏锥虫（Trypanosoma brucei）的核小体核心颗粒（nucleosome core particles, NCPs）进行了表征。布氏锥虫NCP的结构显示，其组蛋白的整体架构仍保持保守，但特定的序列变异使其形成了独特的DNA与蛋白质相互作用界面。布氏锥虫的NCP稳定性较差，整体与DNA的结合能力均有所减弱；不过，其组蛋白H2A-H2B界面发生的显著变化，却实现了局部DNA结合的强化。布氏锥虫组蛋白的酸性补丁（acidic patch）拓扑结构发生改变，且无法与已知的结合蛋白结合，这表明布氏锥虫染色质相互作用的模式可能具有独特性。综上，本研究结果为理解染色质结构的进化分化提供了详实的分子基础。本研究通过有限微球菌核酸酶（micrococcal nuclease）消化人类（Homo sapiens）与布氏锥虫的核小体核心颗粒，获取了Widom 601 145 bp DNA的测序片段。