Genome-Wide Mapping of Yeast Histone Chaperone Anti-Silencing Function 1 Reveals Its Role in Condensin Binding with Chromatin

Genome-wide participation and importance of the histone chaperone Asf1 (Anti-Silencing Function 1) in diverse DNA transactions like replication, repair, heterochromatic silencing and transcription are well documented. Yet its genome-wide targets have not been reported. Using ChIP-seq method, we found that yeast Asf1 associates with 590 unique targets including centromeres, telomeres and condensin-binding sites. It is found selectively on highly transcribed regions, which include replication fork pause sites. Asf1 preferentially associates with the genes transcribed by RNA polymerase (pol) III where its presence affects RNA production and replication-independent histone exchange. On pol II-transcribed genes, a negative correlation is found between Asf1 and nucleosome occupancy. It is not enriched on most of the reported sites of histone exchange or on the genes, which are misregulated in the asf1Δ cells. Interestingly, chromosome-wide distributions of Asf1 and one of the condensin subunits, Brn1 show a nearly identical pattern. Moreover, Brn1 shows reduced occupancy at various condensin-binding sites in asf1Δ cells. These results along with high association of Asf1 with heterochromatic centromeres and telomeres ascribe novel roles to Asf1 in condensin loading and chromatin dynamics.

组蛋白伴侣Asf1（抗沉默因子1，Anti-Silencing Function 1）在复制、修复、异染色质沉默与转录等多种DNA代谢过程中的全基因组参与性及重要性已被广泛证实，但其全基因组结合靶点尚未见报道。本研究采用染色质免疫共沉淀测序（ChIP-seq，Chromatin Immunoprecipitation Sequencing）技术，发现酵母Asf1可与590个独特靶点结合，涵盖着丝粒、端粒及凝缩蛋白（condensin）结合位点。该蛋白选择性富集于高转录区域，其中包含复制叉停滞位点。Asf1优先结合RNA聚合酶（RNA polymerase, pol）III转录的基因，其结合可影响RNA生成及不依赖于复制的组蛋白替换过程。在RNA聚合酶II转录的基因区域中，Asf1的富集与核小体占据率呈负相关，但并未在多数已报道的组蛋白替换位点，或asf1基因缺失（asf1Δ）细胞中存在表达失调的基因区域富集。有趣的是，Asf1与凝缩蛋白亚基Brn1的全染色体分布模式几乎完全一致。此外，在asf1基因缺失细胞中，Brn1在各类凝缩蛋白结合位点的占据率显著降低。结合Asf1与异染色质区域的着丝粒、端粒存在高度结合的现象，上述结果表明Asf1在凝缩蛋白加载及染色质动态调控中具有全新的生物学功能。