KAS-ATAC Sequencing: High-Resolution Mapping of Simultaneously Accessible and Single-Stranded DNA in the Genome

Study Background and Research Question

The precise regulation of gene expression in eukaryotes depends on the dynamic interplay between chromatin accessibility and the formation of single-stranded DNA (ssDNA) during active transcription. Regulatory DNA elements, such as enhancers and promoters, often become nucleosome-depleted and accessible to proteins, while the transcription process itself generates transient ssDNA bubbles. Traditional approaches, like ATAC-seq for open chromatin and KAS-seq for ssDNA detection, have been invaluable for mapping these features separately. However, an integrated method to capture DNA fragments that are both physically accessible and ssDNA-rich in a single experimental workflow has been lacking. The reference study by Marinov and Greenleaf (2025) addresses this gap with the development of KAS-ATAC sequencing.

Key Innovation from the Reference Study

KAS-ATAC sequencing is the first protocol to enable genome-wide profiling of DNA fragments that are simultaneously accessible and contain ssDNA regions within native chromatin. The innovation lies in combining the specificity of N3-kethoxal—a membrane-permeable azide-functionalized nucleic acid probe that selectively reacts with unpaired guanines in ssDNA—with the sensitivity and resolution of Tn5 transposase-mediated transposition from ATAC-seq. This synergy allows researchers to directly interrogate the intersection of chromatin structure and transcriptional activity, providing a nuanced view of cis-regulatory element (cRE) activation and RNA polymerase engagement in situ.

Methods and Experimental Design Insights

The protocol begins with the in vivo labeling of genomic DNA using N3-kethoxal (3-(2-azidoethoxy)-1,1-dihydroxybutan-2-one). This probe penetrates cellular membranes and covalently modifies unpaired guanines in ssDNA, introducing an azide moiety that is subsequently leveraged in a bioorthogonal click chemistry reaction to attach biotin. The workflow then employs Tn5 transposase to transpose sequencing adapters into accessible chromatin regions. After fragmentation and biotin pulldown, the enriched DNA fragments are used to construct sequencing libraries. The resulting dataset reflects the landscape of DNA that is both accessible and contains ssDNA, characteristic of active regulatory elements and sites of ongoing transcription (see protocol details).

Protocol Parameters

• N3-kethoxal labeling: Incubate live cells with 5 mM N3-kethoxal for 5–10 minutes at 37°C to selectively tag unpaired guanines in ssDNA regions.
• Click chemistry biotinylation: Treat the labeled DNA with DBCO-biotin under copper-free conditions to achieve bioorthogonal conjugation via the azide moiety.
• Tn5 transposition: Incubate nuclei with Tn5 transposase and sequencing adapters at 37°C for 30 minutes to target accessible chromatin.
• Streptavidin pulldown: Capture biotinylated, accessible DNA fragments using streptavidin-coated magnetic beads.
• Library preparation: Amplify and purify DNA fragments for high-throughput sequencing using standard PCR and size-selection steps.

These literature-backed parameters are optimized for mammalian cell lines, but may require adjustment for primary tissues or non-model organisms.

Core Findings and Why They Matter

The study demonstrates that KAS-ATAC sequencing can robustly identify DNA regions where chromatin is open and ssDNA is present—primarily marking active cREs and sites of RNA polymerase activity. This dual profiling reveals regulatory elements not only by their accessibility but also by their functional engagement in transcription. Notably, the approach distinguishes between open chromatin that is transcriptionally active (containing ssDNA bubbles from polymerase activity) and regions that are merely accessible but not engaged in active transcription.

Such resolution is crucial for dissecting the regulatory logic of gene expression, as it enables the mapping of enhancer activity, promoter engagement, and transcriptional pausing in a unified experiment. This integrated view supports comprehensive charting of regulatory networks and can refine the annotation of functional genomic elements, as shown by Marinov and Greenleaf (2025).

Comparison with Existing Internal Articles

Previous internal resources highlight the transformative impact of N3-kethoxal in bioorthogonal labeling and the detection of unpaired guanines in both RNA and DNA. For example, the article "N3-kethoxal: Revolutionizing RNA and DNA Structural Probing" outlines the probe's utility in mapping secondary structures and CRISPR off-targets, while "N3-kethoxal: Next-Gen RNA Structure Probing & DNA Mapping" emphasizes its live-cell compatibility and efficiency in click chemistry labeling.

The key advance in the KAS-ATAC protocol is the integration of N3-kethoxal-based ssDNA labeling with ATAC-seq's chromatin accessibility mapping in a single workflow, which goes beyond the applications described in earlier internal articles. This protocol enables the direct identification of DNA fragments that are both open and transcriptionally engaged, rather than inferring activity from separate datasets. Compared to studies focused on RNA secondary structure probing or RNA-protein interaction identification (see internal review), KAS-ATAC provides a chromatin-centric perspective that leverages the same chemical principles for a novel genomic application.

Limitations and Transferability

While KAS-ATAC sequencing offers a powerful approach for profiling accessible, ssDNA-containing regions, it is subject to certain limitations. The specificity of N3-kethoxal labeling relies on the presence of unpaired guanines, which may vary depending on sequence context and local chromatin structure. Additionally, the protocol requires careful optimization of reaction times and reagent concentrations to balance efficient labeling with minimal genomic perturbation. Transferability to primary tissues or rare cell populations may necessitate further troubleshooting, especially for nuclei isolation and DNA recovery steps. Nonetheless, the core methodology is adaptable to a wide range of cell types, and the integration of bioorthogonal click chemistry labeling is compatible with downstream multi-omics workflows.

Research Support Resources

Researchers planning to implement KAS-ATAC sequencing can benefit from the detailed protocol provided by Marinov and Greenleaf (2025). For practical workflows requiring selective ssDNA labeling, N3-kethoxal (SKU A8793) is commercially available and validated for both in vitro and in vivo nucleic acid probing. The product's high solubility and membrane permeability support efficient labeling in a variety of biological systems. For additional technical insights, see the in-depth discussions of N3-kethoxal applications in internal reviews and protocol optimization guides.