Terminal deoxynucleotidyl transferase (TdT) is an essential enzyme used in next-generation sequencing (NGS) applications, particularly in the preparation of sequencing libraries. TdT is a unique DNA polymerase that catalyzes the template-independent addition of deoxynucleotides to the 3′ hydroxyl terminus of single-stranded DNA molecules, enabling the addition of homopolymeric tails or customized nucleotide sequences without the need for a DNA template.

In NGS library preparation, TdT plays a crucial role in adding homopolymeric tails—most commonly polyadenine (poly-A) tails—to fragmented DNA or cDNA molecules. This tailing facilitates subsequent amplification and adapter ligation steps necessary for sequencing. For instance, short or degraded DNA fragments, which lack the suitable ends for ligation or amplification, can be efficiently tail-labeled by TdT to generate uniform and compatible 3′ ends for primers to anneal during PCR amplification. This template-independent tailing avoids the need for specific DNA end configurations, making TdT-based approaches highly versatile for diverse sample types, including challenging samples like formalin-fixed paraffin-embedded (FFPE) tissues or ancient DNA.

One notable NGS method utilizing TdT is Terminal Transfer Amplification and Sequencing (TTAS), which relies on two rounds of tailing and amplification to amplify minute amounts of short DNA fragments without traditional ligation steps. TdT adds a poly-A tail at the 3′ end of DNA, enabling oligo-T primers to anneal and extend DNA linearly. This method is beneficial for input DNA that is highly fragmented or chemically modified, which otherwise poses challenges for conventional library preparation methods. TdT tailing also facilitates constructing variable-sized DNA libraries and screening polynucleotide sequences based on binding affinity, by increasing the length complexity and enabling PCR amplification with universal primers.

Beyond tailing, TdT is also instrumental for labeling DNA ends with fluorescent or chemically modified nucleotides for functionalization prior to sequencing or analysis. This labeling aids in tracking, detection, or increasing nuclease resistance of DNA fragments within NGS workflows.

Key Roles of Terminal Transferase in NGS

• Template-independent addition of homopolymeric tails (e.g., poly-A) to DNA 3′ ends.
• Facilitation of amplification and adapter ligation for fragmented or degraded DNA.
• Enabling ligation-free library preparation methods like TTAS.
• Functionalization of DNA ends with fluorescent or modified nucleotides for detection.
• Versatility across various sample types, including FFPE and ancient DNA.

Molecular Mechanisms and Applications

• TdT adds deoxynucleotides without a DNA template, extending 3′ ends to create priming sites.
• Enhances uniform amplification by generating compatible tail sequences for oligo primers.
• Used in assays for variable-sized DNA library creation for binding affinity studies.
• Applied in high-efficiency, low-bias library construction for NGS of challenging samples.