BACKGROUND DNA packaging into chromatin regulates all DNA-related processes and at chromosomal ends could affect both essential functions of telomeres: protection against DNA damage response and telomere replication. Despite this primordial role of chromatin, little is known about chromatin organization, and in particular about nucleosome positioning on unmodified subtelomere-telomere junctions in Saccharomyces cerevisiae. RESULTS By ChEC experiments and indirect end-labeling, we characterized nucleosome positioning as well as specialized protein-DNA associations on most subtelomere-telomere junctions present in budding yeast. The results show that there is a relatively large nucleosome-free region at chromosome ends. Despite the absence of sequence homologies between the two major classes of subtelomere-telomere junctions (i.e.: Y'-telomeres and X-telomeres), all analyzed subtelomere-telomere junctions show a terminal nucleosome-free region just distally from the known Rap1-covered telomeric repeats. Moreover, previous evidence suggested a telomeric chromatin fold-back structure onto subtelomeric areas that supposedly was implicated in chromosome end protection. The in vivo ChEC method used herein in conjunction with several proteins in a natural context revealed no evidence for such structures in bulk chromatin. CONCLUSIONS Our study allows a structural definition of the chromatin found at chromosome ends in budding yeast. This definition, derived with direct in vivo approaches, includes a terminal area that is free of nucleosomes, certain positioned nucleosomes and conserved DNA-bound protein complexes. This organization of subtelomeric and telomeric areas however does not include a telomeric cis-loopback conformation. We propose that the observations on such fold-back structures may report rare and/or transient associations and not stable or constitutive structures.

中文翻译：

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天然酵母端粒区域上的体内染色质组织独立于顺式端粒环回构象。

背景技术装入染色质的DNA调节着所有与DNA有关的过程，并且在染色体末端可能影响端粒的两个基本功能：防止DNA损伤反应和端粒复制。尽管染色质具有这种原始的作用，但对染色质的组织，尤其是酿酒酵母中未修饰的亚端粒-端粒连接处的核小体定位了解甚少。结果通过ChEC实验和间接末端标记，我们对出芽酵母中存在的大多数亚端粒-端粒连接处的核小体定位以及专门的蛋白质-DNA缔合进行了表征。结果表明，在染色体末端存在较大的无核小体区域。尽管在两个主要类别的亚端粒-端粒连接之间没有序列同源性（即：Y' -端粒和X-端粒），所有分析的端粒-端粒连接处都显示一个末端无核小体区域，位于已知Rap1覆盖的端粒重复序列的远端。此外，先前的证据表明端粒亚基区域上的端粒染色质折返结构可能与染色体末端保护有关。在自然情况下，本文与几种蛋白质结合使用的体内ChEC方法没有发现大量染色质具有这种结构的证据。结论我们的研究允许在发芽酵母的染色体末端发现染色质的结构定义。该定义是通过直接体内方法得出的，其末端区域不含核小体，某些定位的核小体和保守的DNA结合蛋白复合物。然而，亚端粒和端粒区域的这种组织不包括端粒顺式环回构象。我们建议对这种折返结构的观察可能报告罕见和/或短暂的关联，而不是稳定或本构结构。