KAS-ATAC Sequencing: A High-Resolution Approach for Mapping Accessible and ssDNA-Containing Genome Regions

Study Background and Research Question

Understanding the regulatory landscape of the genome requires precise methods for identifying regions of open chromatin and single-stranded DNA (ssDNA) bubbles, both of which are central to gene regulation. Cis-regulatory elements (cREs) such as promoters, enhancers, and insulators play pivotal roles in modulating transcription, and their activation status is closely linked to chromatin accessibility. Conventional approaches, such as ATAC-seq, have enabled genome-wide profiling of open chromatin, while complementary methods like KAS-seq have provided insight into regions of active transcription by labeling ssDNA. However, a gap remains in capturing both chromatin accessibility and ssDNA content simultaneously. The reference protocol by Marinov and Greenleaf (2025) introduces KAS-ATAC sequencing to address this need, offering a unified assay to map genomic regions that are both accessible and enriched for ssDNA.

Key Innovation from the Reference Study

The principal innovation of the KAS-ATAC protocol is its ability to concurrently profile regions of chromatin that are physically accessible (nucleosome-depleted) and those containing ssDNA on the same DNA fragments. This dual mapping is achieved by integrating the chemical specificity of N3-kethoxal, a membrane-permeable nucleic acid probe, with the transposase-based labeling strategy of ATAC-seq. N3-kethoxal (also known as 3-(2-azidoethoxy)-1,1-dihydroxybutan-2-one) selectively forms covalent adducts with unpaired guanines in ssDNA, introducing an azide functional group that enables subsequent click chemistry-based biotinylation. By combining this chemical labeling with Tn5 transposase-mediated adapter insertion, the method allows for specific enrichment, sequencing, and computational analysis of DNA fragments that are both accessible and contain ssDNA structures.

Methods and Experimental Design Insights

The KAS-ATAC workflow can be summarized in several critical steps:

• Labeling with N3-kethoxal: Intact cells or nuclei are incubated with N3-kethoxal to covalently tag unpaired guanines in ssDNA regions. This step exploits the probe's membrane permeability and high selectivity for accessible nucleic acids.
• ATAC-seq Transposition: Following chemical labeling, the chromatin is treated with a hyperactive Tn5 transposase, which inserts sequencing adapters at accessible DNA regions, marking sites of nucleosome depletion.
• Click Chemistry Biotinylation: The azide-labeled DNA is subjected to bioorthogonal click chemistry, typically a copper-catalyzed azide-alkyne cycloaddition, to attach a biotin moiety for affinity enrichment.
• Pulldown and Library Preparation: Biotinylated DNA fragments are isolated via streptavidin pulldown, and standard library preparation protocols are used to generate sequencing-ready material.
• Data Processing: Sequencing reads are aligned and analyzed to map the genomic distribution of accessible, ssDNA-containing regions, providing insights into the regulatory genome.

This integrated protocol enables the simultaneous detection of open chromatin and ssDNA bubbles, which are signatures of active regulatory elements and sites of engaged RNA polymerase, respectively.

Protocol Parameters

• N3-kethoxal labeling: Incubate cells or nuclei with N3-kethoxal under conditions optimized for efficient penetration and guanine modification (e.g., 37°C, 5-10 min, as per the protocol).
• ATAC-seq transposition: Use Tn5 transposase at concentrations compatible with the sample type; incubation typically at 37°C for 30 min.
• Click chemistry reaction: Perform copper-catalyzed azide-alkyne cycloaddition using a biotin-alkyne reagent; reaction times and copper concentrations should be optimized to minimize background labeling.
• Streptavidin pulldown: Use magnetic beads for efficient recovery of biotinylated DNA; wash stringently to reduce non-specific binding.
• Library amplification: Follow standard PCR protocols, adjusting cycle number to avoid overamplification and maintain representation of enriched fragments.

Core Findings and Why They Matter

The KAS-ATAC method provides comprehensive maps of simultaneously accessible and ssDNA-rich genomic regions. According to the reference study, this approach reveals the fine-scale organization of active cREs, including promoters and enhancers, and captures the presence of RNA polymerase-associated ssDNA bubbles. The resulting data enable researchers to dissect the interplay between chromatin accessibility and transcriptional activity at single-fragment resolution, advancing our understanding of gene regulation mechanisms. Furthermore, the protocol supports multi-omic strategies, as the covalent nature of N3-kethoxal labeling allows for the integration of additional molecular readouts on the same DNA molecules.

Comparison with Existing Internal Articles

Several internal resources discuss the utility of N3-kethoxal in genomic and transcriptomic research. For example, one article highlights the probe's role in high-resolution RNA secondary structure probing and accessible DNA mapping in living cells, emphasizing its membrane-permeable and azide-functionalized nature. Another internal guide addresses practical considerations in assay reproducibility and workflow reliability when using N3-kethoxal (SKU A8793), offering best practices and troubleshooting insights. While these sources focus on standalone applications of N3-kethoxal for RNA and DNA labeling, the KAS-ATAC protocol distinguishes itself by integrating this chemistry with ATAC-seq to provide simultaneous mapping of chromatin accessibility and ssDNA content. This combination extends the probe’s utility from targeted nucleic acid modification to comprehensive regulatory genomics.

Limitations and Transferability

Despite its advantages, the KAS-ATAC method presents certain limitations. The specificity of N3-kethoxal for accessible, unpaired guanines means that regions lacking these features may be underrepresented. Additionally, the efficiency of click chemistry and pulldown steps can impact recovery and data quality. Sample preparation parameters, such as cell permeability and chromatin state, require careful optimization to ensure consistent labeling and transposition. While the protocol is broadly applicable to various eukaryotic systems, adaptation to specific cell types or low-input samples may necessitate further refinement. The transferability of findings to in vivo contexts is promising but may be influenced by chromatin complexity and tissue heterogeneity, as discussed in the internal literature.

Research Support Resources

To facilitate adoption of the KAS-ATAC workflow, researchers can utilize N3-kethoxal (SKU A8793), a well-characterized, azide-functionalized nucleic acid probe suitable for both in vitro and in vivo labeling of unpaired guanines. This reagent underpins the core labeling chemistry described in the reference protocol and is available with detailed handling and storage guidelines. For further reading on best practices, data reproducibility, and advanced workflow integration, see the internal Q&A article and related resources.