期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：52
页码：15048-15053
DOI：10.1073/pnas.1615546113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceTwo forms of heterochromatin, constitutive and facultative, cause gene silencing in eukaryotes. In Neurospora crassa, H3K27me2/3-marked facultative heterochromatin reversibly represses scores of specialized genes, whereas H3K9me3-marked constitutive heterochromatin permanently silences repetitive DNA. Interactions between heterochromatin provide a structural framework for the genome, and this is thought to be functionally important. Histone marks underlying constitutive and facultative heterochromatin are nonessential in N. crassa, permitting tests of their roles in genome organization and gene expression. Although linkages between regions of constitutive heterochromatin are the most prominent feature of the 3D structure of the genome, loss of the facultative mark has a much greater effect on genome architecture than does loss of key features of constitutive heterochromatin, i.e., H3K9me3 and Heterochromatin Protein 1. High-throughput chromosome conformation capture (Hi-C) analyses revealed that the 3D structure of the Neurospora crassa genome is dominated by intra- and interchromosomal links between regions of heterochromatin, especially constitutive heterochromatin. Elimination of trimethylation of lysine 9 on histone H3 (H3K9me3) or its binding partner Heterochromatin Protein 1 (HP1)--both prominent features of constitutive heterochromatin--have little effect on the Hi-C pattern. It remained possible that di- or trimethylation of lysine 27 on histone H3 (H3K27me2/3), which becomes localized in regions of constitutive heterochromatin when H3K9me3 or HP1 are lost, plays a critical role in the 3D structure of the genome. We found that H3K27me2/3, catalyzed by the Polycomb Repressive Complex 2 (PRC2) member SET-7 (SET domain protein-7), does indeed play a prominent role in the Hi-C pattern of WT, but that its presence in regions normally occupied by H3K9me3 is not responsible for maintenance of the genome architecture when H3K9me3 is lost. The Hi-C pattern of a mutant defective in the PRC2 member N. crassa p55 (NPF), which is predominantly required for subtelomeric H3K27me2/3, was equivalent to that of the set-7 deletion strain, suggesting that subtelomeric facultative heterochromatin is paramount for normal chromosome conformation. Both PRC2 mutants showed decreased heterochromatin-heterochromatin contacts and increased euchromatin-heterochromatin contacts. Cytological observations suggested elimination of H3K27me2/3 leads to partial displacement of telomere clusters from the nuclear periphery. Transcriptional profiling of {Delta}dim-5, {Delta}set-7, {Delta}set-7; {Delta}dim-5, and {Delta}npf strains detailed anticipated changes in gene expression but did not support the idea that global changes in genome architecture, per se, led to altered transcription.
