Nucleosome Positioning and Dynamics

Nucleosomes are not distributed at random along the genome: their precise positions, and the regions left free of them, shape which regulatory sequences are accessible. Nucleosome positioning and its dynamic changes are a major determinant of gene regulation, and genome-wide maps of where nucleosomes sit have become a standard way to read chromatin accessibility.

Definition

Nucleosome positioning refers to the location of nucleosomes relative to the DNA sequence and to one another; together with their dynamic assembly, repositioning, and destabilization, it governs the local accessibility of regulatory DNA.

Scope

This topic covers what determines where nucleosomes are placed along DNA, the characteristic nucleosome-depleted regions at active promoters, the dynamics by which nucleosomes are assembled, moved, and destabilized, and the genomic methods used to map positioning and accessibility. It is a reference entry on chromatin organization and is not clinical guidance.

Core questions

• What determines where nucleosomes are positioned along the genome?
• Why are active promoters typically flanked by ordered nucleosomes and a nucleosome-depleted region?
• How is nucleosome positioning measured genome-wide, and what does it reveal about accessibility?

Key concepts

• Nucleosome-depleted region (NDR)
• Promoter nucleosome organization (+1 nucleosome)
• Statistical vs. sequence-directed positioning
• Nucleosome phasing and arrays
• Nucleosome turnover and destabilization
• MNase-seq and ATAC-seq mapping

Key theories

Statistical positioning around fixed barriers

Rather than each nucleosome being independently sequence-encoded, ordered arrays often arise statistically: a fixed barrier such as a bound factor or a nucleosome-depleted promoter region phases neighbouring nucleosomes into regular positions, a model supported by genome-scale mapping of nucleosome positions.

Mechanisms

Nucleosome positioning is shaped by several overlapping factors: the intrinsic bendability of the DNA sequence, the binding of transcription factors and the general transcription machinery, the action of ATP-dependent remodelers that slide and space nucleosomes, and the boundaries set by such fixed elements. Active promoters typically display a nucleosome-depleted region upstream of the transcription start site, flanked by well-positioned nucleosomes, with the so-called +1 nucleosome marking the start of an ordered downstream array. These positions are dynamic: nucleosomes are continually assembled during replication, destabilized or evicted during transcription, and repositioned by remodelers, and rapid turnover at regulatory regions keeps them accessible. Genome-wide maps generated by digesting chromatin with micrococcal nuclease, or by profiling open chromatin with transposase-based ATAC-seq, reveal these positions and the accessibility landscape.

Clinical relevance

Nucleosome positioning and accessibility profiling underpin maps of regulatory elements across human cell types and are widely used to study how gene regulation differs between healthy and diseased tissues. This entry describes the organization and measurement of nucleosomes for reference and is not a basis for diagnosis or treatment.

History

Although individual well-positioned nucleosomes were known earlier, genome-scale mapping transformed the field in the mid-2000s, when high-resolution surveys in yeast located nucleosomes across an entire genome and revealed the ordered organization around promoters. Reviews integrated these findings into models of statistical and factor-directed positioning, and the later development of transposase-based ATAC-seq made profiling of open chromatin and nucleosome positions fast and broadly applicable.

Key figures

• B. Franklin Pugh
• Oliver Rando
• Steven Henikoff
• Jason Buenrostro

Seminal works

• yuan-2005
• jiang-2009

Frequently asked questions

What is a nucleosome-depleted region?

It is a stretch of DNA, typically just upstream of an active gene's start site, that is largely free of nucleosomes and therefore accessible to transcription factors and the transcription machinery.

How is nucleosome positioning measured across the genome?

Common approaches digest chromatin with micrococcal nuclease to map protected nucleosomal DNA, or use transposase-based ATAC-seq to profile accessible, nucleosome-free regions, both read out by high-throughput sequencing.