Predicting scale-dependent chromatin polymer properties from systematic coarse-graining

Here the authors provide a quantitative description of chromatin as bead-spring polymers. The study predicts the 3D size of chromatin beads given the genomic length and computes how stretchable and bendable chromatin is and how soft chromatin beads are. Simulating chromatin is crucial for predicting genome organization and dynamics. Although coarse-grained bead-spring polymer models are commonly used to describe chromatin, the relevant bead dimensions, elastic properties, and the nature of inter-bead potentials are unknown. Using nucleosome-resolution contact probability (Micro-C) data, we systematically coarse-grain chromatin and predict quantities essential for polymer representation of chromatin. We compute size distributions of chromatin beads for different coarse-graining scales, quantify fluctuations and distributions of bond lengths between neighboring regions, and derive effective spring constant values. Unlike the prevalent notion, our findings argue that coarse-grained chromatin beads must be considered as soft particles that can overlap, and we derive an effective inter-bead soft potential and quantify an overlap parameter. We also compute angle distributions giving insights into intrinsic folding and local bendability of chromatin. While the nucleosome-linker DNA bond angle naturally emerges from our work, we show two populations of local structural states. The bead sizes, bond lengths, and bond angles show different mean behavior at Topologically Associating Domain (TAD) boundaries and TAD interiors. We integrate our findings into a coarse-grained polymer model and provide quantitative estimates of all model parameters, which can serve as a foundational basis for all future coarse-grained chromatin simulations.

Automated summary

The authors provide a quantitative description of chromatin as bead-spring polymers, predicting the 3D size of chromatin beads and quantifying their stretchability, bendability, and softness. They systematically coarse-grain chromatin using Micro-C data and derive essential quantities for polymer representation. They also challenge the prevailing notion by showing that chromatin beads must be considered soft particles that can overlap, and in doing so, they derive an effective inter-bead soft potential and quantify an overlap parameter. Additionally, they analyze bond lengths and bond angles to gain insights into chromatin folding and local bendability.