Flap Endonuclease 1 (FEN1) is a critical enzyme extensively utilized in next-generation sequencing (NGS) library preparation due to its precise structure-specific endonuclease activity involved in DNA replication and repair processes. FEN1 belongs to the RAD2 superfamily of nucleases and plays a pivotal role in removing RNA primers and processing flap structures generated during DNA strand displacement synthesis, which is essential for Okazaki fragment maturation and long-patch base excision repair. In the context of NGS, these activities permit the efficient and accurate processing of DNA fragments necessary for high-quality library construction.

During the preparation of NGS libraries, DNA fragments that have undergone shearing or enzymatic fragmentation often feature overhanging or flap-like single-stranded DNA structures. FEN1 specifically recognizes the 5′ flap structures in these intermediates. The enzyme cleaves the flap at the junction between single-stranded and double-stranded DNA by binding to the flap base and threading the flap through its active site to make a precise cut. This action removes unwanted single-stranded tails, producing clean, ligatable blunt or nearly blunt ends suitable for adapter ligation, a crucial step in library preparation.

FEN1 functions through two main mechanisms: a "tracking mechanism," where the enzyme loads onto the 5′ end of the flap and cleaves at the flap–duplex junction, and a "flap threading mechanism," where the flap strand is threaded through the enzyme to reach the catalytic site for cleavage. This specificity prevents non-target cleavage, maintaining the fidelity of the DNA fragments for sequencing.

Structurally, FEN1 interacts with proliferating cell nuclear antigen (PCNA), which stimulates its endonuclease activity, helping coordinate its function during DNA replication and repair. This interaction enhances the efficiency of flap cleavage, ensuring genome stability—crucial for the integrity of library templates in NGS workflows.

In summary, FEN1 is indispensable in NGS library preparation owing to its ability to process flap structures precisely, clean DNA fragment ends, and thereby facilitate the generation of high-quality sequencing libraries. Its role in Okazaki fragment maturation and DNA repair mechanisms underlines its evolutionary adaptation as an essential enzyme for maintaining genome fidelity, a property that directly translates into improved accuracy and reliability in NGS applications.

Key Roles of FEN1 in NGS Library Preparation

• Removal of 5′ flap structures on fragmented DNA to create ligatable ends.
• Precise endonucleolytic cleavage at flap–duplex junctions to avoid sequence duplication or errors.
• Interaction with PCNA to enhance flap cleavage efficiency.
• Maintenance of DNA integrity critical for library quality and sequencing accuracy.

Molecular Mechanisms

• Flap recognition and cleavage via tracking or threading mechanisms.
• Coordination with DNA polymerase and ligase in DNA maturation pathways.
• Role in base excision repair and replication fork stability contributing to genome stability.