Journal Article

Variable chromatin structure revealed by in situ spatially correlated DNA cleavage mapping

Straight, Aaron F., Denny, Sarah K., Greenleaf, William J., Risca, Viviana I.

2017

Overview

The first genome-wide map of human chromatin conformation at the 1–3 nucleosome (50–500 base pair) scale, obtained using ionizing radiation-induced spatially correlated cleavage of DNA with sequencing (RICC-seq), which identifies spatially proximal DNA–DNA contacts. Nucleosome-scale chromatin structure Genome-scale mapping methods are gradually revealing the three-dimensional organization of chromatin, but we lack an understanding at a spatial precision below 10 nm, which encompasses local nucleosome–nucleosome interactions. Here, William Greenleaf and colleagues develop a technique that they term RICC-seq, which identifies spatially proximal DNA–DNA contacts. They report the first genome-wide map of human chromatin fibre conformation at the one-to-three nucleosome scale in intact nuclei. The results indicate that compaction of two-start helical fibres organizes condensed chromatin structure, and the stacking of alternating nucleosomes varies between open and repressed chromatin states. The technique can potentially be extended to study condensed nucleic acid conformations in other biological systems. Chromatin structure at the length scale encompassing local nucleosome–nucleosome interactions is thought to play a crucial role in regulating transcription and access to DNA 1 , 2 , 3 . However, this secondary structure of chromatin remains poorly understood compared with the primary structure of single nucleosomes or the tertiary structure of long-range looping interactions 4 . Here we report the first genome-wide map of chromatin conformation in human cells at the 1–3 nucleosome (50–500 bp) scale, obtained using ionizing radiation-induced spatially correlated cleavage of DNA with sequencing (RICC-seq) to identify DNA–DNA contacts that are spatially proximal. Unbiased analysis of RICC-seq signal reveals regional enrichment of DNA fragments characteristic of alternating rather than adjacent nucleosome interactions in tri-nucleosome units, particularly in H3K9me3-marked heterochromatin. We infer differences in the likelihood of nucleosome–nucleosome contacts among open chromatin, H3K27me3-marked, and H3K9me3-marked repressed chromatin regions. After calibrating RICC-seq signal to three-dimensional distances, we show that compact two-start helical fibre structures with stacked alternating nucleosomes are consistent with RICC-seq fragmentation patterns from H3K9me3-marked chromatin, while non-compact structures and solenoid structures are consistent with open chromatin. Our data support a model of chromatin architecture in intact interphase nuclei consistent with variable longitudinal compaction of two-start helical fibres.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/208/177 / 631/337/100/101 / 631/57/2268 / Binding sites / Calibration / Chromatin / Chromatin - chemistry / Chromatin - genetics / Chromatin - metabolism / Chromatin Assembly and Disassembly / Deoxyribonucleic acid / DNA / DNA - chemistry / DNA - metabolism / DNA Cleavage / DNA damage / DNA sequencing / Epigenesis, Genetic / Fibroblasts / Genome, Human / Genomes / Histones - chemistry / Histones - metabolism / Humanities and Social Sciences / Humans / Ionizing radiation / letter / Methods / Molecular Conformation / multidisciplinary / Nucleic Acid Conformation / Nucleosomes - chemistry / Nucleosomes - genetics / Nucleosomes - metabolism / Observations / Physical Chromosome Mapping - methods / Physiological aspects / Proteins / Science