ATAC-Seq is a method for identifying open chromatin regions, areas of the genome where DNA is not tightly wrapped around nucleosomes and is accessible to transcription factors and other regulatory proteins.

Principle

A hyperactive derivative of Tn5 transposase (Tn5-transposome complex) simultaneously fragments and tags accessible DNA with sequencing adapters in a process called tagmentation. Regions of open chromatin are preferentially cut and tagged by Tn5 because nucleosome-bound DNA is physically inaccessible. The tagged fragments are then PCR-amplified and sequenced.

Protocol

1. Harvest 50,000 viable cells (the recommended minimum). For low-input experiments, as few as 500 cells can be used.
2. Lyse cells in cold lysis buffer (10 mM Tris-HCl pH 7.4, 10 mM NaCl, 3 mM MgCl₂, 0.1% IGEPAL CA-630) for 10 minutes on ice to isolate intact nuclei.
3. Pellet nuclei and resuspend in transposition reaction mix containing Tn5 transposome loaded with Illumina-compatible adapters.
4. Incubate at 37 °C for 30 minutes with gentle mixing.
5. Purify the tagmented DNA using a column- or bead-based cleanup.
6. PCR-amplify for 10–15 cycles, incorporating barcoding indices.
7. Sequence on an Illumina platform (paired-end 50 bp is standard).

Data Analysis

Raw reads are:

1. Trimmed of adapter sequences and aligned to the reference genome.
2. Filtered to remove mitochondrial reads (which can comprise 30–50% of reads).
3. Corrected for Tn5 insertion bias (Tn5 has a weak sequence preference for certain dinucleotides).
4. Called for peaks of open chromatin using MACS2 or Genrich.
5. Annotated to identify peaks in promoters, enhancers, CTCF-binding sites, and other regulatory elements.

The resulting peaks define the accessible chromatin landscape. Differential accessibility between conditions identifies regulatory elements that change activity during differentiation, disease progression, or drug treatment.

Quality Control

• Fragment size distribution: Tn5 produces fragments of two sizes. Nucleosome-free fragments are <100 bp, mononucleosomal fragments are 180–247 bp, and dinucleosomal fragments are 315–400 bp. A good ATAC-Seq library shows a clear nucleosome ladder pattern.
• Fraction of reads in peaks (FRiP): a FRiP score >0.3 indicates high signal-to-noise.
• Transcriptional start site (TSS) enrichment: reads should be enriched at TSSs. A TSS enrichment score >5 indicates good quality.

Applications

ATAC-Seq is used to identify regulatory elements (promoters, enhancers, silencers), study transcription factor footprinting (where Tn5 cannot cut because a transcription factor is bound), and profile chromatin dynamics across cell states and cell types. Single-cell ATAC-Seq (scATAC-Seq) extends the method to individual cells, revealing cell-type-specific regulatory landscapes in heterogeneous tissues.

Comparison with DNase-Seq and MNase-Seq

ATAC-Seq requires 10–100× fewer cells than DNase-Seq (50,000 vs. 10–50 million), takes 1–2 days instead of 3–4 days, and has a simpler workflow (one enzyme instead of DNase I titration). MNase-Seq maps nucleosome positions (not open chromatin) and requires 10–50 million cells. ATAC-Seq has largely replaced both methods for routine chromatin accessibility profiling.