7 Types of DNA Ligases: Functions and Features
DNA ligases are indispensable in DNA replication, recombination, and repair. They catalyze phosphodiester bond formation between adjacent nucleotides to seal nicks or gaps in DNA strands. In molecular cloning, site-directed mutagenesis, and library construction, choosing the right ligase depends on end type, reaction temperature, and required cofactor.
【Function】
Efficiently catalyzes phosphodiester bond formation between adjacent 5′-phosphate and 3′-hydroxyl groups on duplex DNA; ligates both cohesive (sticky) and blunt ends; can repair nicks in duplex DNA and some DNA/RNA hybrids. It is the most widely used ligase in routine cloning.
【Features & Applications】
Operates over ~4–37 °C; cloning commonly at 16 °C overnight or 25 °C for shorter reactions to balance activity and end-pairing efficiency. Uses ATP as cofactor (provided in dedicated ligation buffer). Because it works well on both sticky and blunt ends, T4 is the first choice for most vector–insert ligations and nick sealing.
【T4 DNA Ligase: Ligation Requirements and Outcomes】
End type of exogenous DNA | Ligation requirements | Ligation outcomes |
Asymmetric sticky ends | After digestion with two restriction enzymes, the vector should be purified to improve ligation efficiency. | The restriction sites at the junction between vector and insert are often retained; background from non-recombinant clones is low; the insert can be directionally inserted into the vector. |
Symmetric sticky ends | Linearized vector DNA usually requires dephosphorylation with phosphatase. | The restriction sites at the junction between vector and insert are often retained; recombinant plasmids may contain tandem copies of the insert; the insert can ligate into the vector in both orientations. |
Blunt ends | Requires high concentrations of DNA and ligase. | Restriction sites at the junction between vector and insert are lost; recombinant plasmids may contain tandem copies of the insert; background from non-recombinant clones is relatively high. |
【Recommended Usage】
In routine molecular cloning, a 1:3–1:5 molar ratio of vector to insert in a 20 μL ligation reaction is commonly used. For blunt-end ligation or samples with low DNA concentration, the insert proportion can be moderately increased or the reaction time extended. To ensure high transformation efficiency, the volume of ligation mixture added to competent cells should generally not exceed 10% of the total cell volume. If the ligation system contains a relatively high amount of enzyme or elevated levels of salts, ATP, etc.—especially for electroporation—it is recommended to purify the ligation products before transformation to minimize inhibitory effects on cells.
II. T3 DNA Ligase
【Function】
Catalyzes phosphodiester bond formation between adjacent 5′-phosphate and 3′-hydroxyl groups on duplex DNA; often shows higher efficiency on A/T overhangs than on C/G overhangs, fitting specific end structures.
【Features & Applications】
Compared with T4, T3 tolerates higher-salt buffers and can maintain good efficiency under such conditions. Also requires ATP (in the supplied buffer). Preferable when ligation must proceed in elevated-salt conditions or with A/T overhangs.
【Reaction Substrates and End-Compatibility】
T3 DNA ligase can be used for ligation of sticky ends, blunt ends, and nicks in double-stranded DNA, making it suitable for plasmid construction, fragment assembly, and nick repair. Aside from the fact that ligation efficiency is typically higher for A/T overhangs than for C/G overhangs, good ligation performance can also be achieved for blunt-ended DNA fragments in an appropriate buffer system (with additives such as PEG).
【Recommended Usage】
During ligation, ensure that the final ATP concentration in the reaction is approximately 1 mM; if preparing buffer in-house, remember to supplement ATP. For blunt-end ligation or cases with low DNA concentration, increasing the amount of enzyme or extending the reaction time can improve the proportion of positive clones. When performing ligation in high-salt conditions, T3 DNA ligase is recommended as the preferred choice, and the reaction time and DNA input should be optimized according to the NaCl concentration and whether PEG 6000 is included in the system.
III. T7 DNA Ligase
【Function】
Primarily ligates cohesive-ended duplex DNA (5′-phosphate/3′-OH); shows good activity on sticky ends but low efficiency on blunt ends; addition of PEG6000 (e.g., ≥20%) can partially enhance blunt-end ligation.
【Features & Applications】
Substrate preference for sticky ends; not suitable for T/A cloning or blunt-end ligation. Requires ATP. Owing to its stronger specificity for cohesive ends, it is useful when enforcing ligation directionality or minimizing blunt-end self-ligation.
【Substrate Preference and Ligation Characteristics】
T7 DNA ligase exhibits a pronounced substrate preference for double-stranded DNA with complementary cohesive (sticky) ends generated by restriction endonucleases, and can efficiently catalyze phosphodiester bond formation between adjacent 5′-phosphate and 3′-hydroxyl termini under conditions of relatively low nonspecific background. In contrast, its ligation activity toward blunt-ended double-stranded DNA and single base overhangs used in T/A cloning is extremely low, and only limited blunt-end ligation can be observed when relatively high mass fractions of PEG6000 are included in the reaction system. Compared with T4 DNA ligase, which possesses a broader substrate spectrum and robust activity toward multiple end configurations, T7 DNA ligase displays a narrower substrate range and a stronger selectivity for cohesive ends. Consequently, in reaction systems where cohesive and blunt ends coexist, T7 DNA ligase can help suppress blunt end–mediated nonspecific ligation and vector self-ligation, thereby improving the specificity and purity of the desired recombinant products.
【Recommended Usage】
If both blunt ends and complementary sticky ends are present in the reaction, and the experimental design aims to ligate only the sticky ends while avoiding blunt-end self-ligation or nonspecific joining, T7 DNA ligase is a preferred option. For routine sticky-end cloning, it is recommended to:
1）Set the molar ratio of vector to insert at 1:3–1:5;
2）Use the supplied ligation buffer (containing PEG6000 and ATP) and avoid making major, self-directed changes to buffer components;
3）For systems with low DNA input or weaker end compatibility, moderately increase the enzyme amount and/or extend the reaction time. If downstream requirements make it necessary to work in a PEG-free system, a ligation buffer without PEG can be used instead; however, note that T7 DNA ligase activity will drop markedly under these conditions. In such cases, it functions best as a selective ligation tool that “primarily ligates sticky ends and essentially does not ligate blunt ends.”
IV. Thermostable DNA Ligase (e.g., Ampligase)
【Function】
Catalyzes ligation of adjacent 3′-OH and 5′-phosphate at elevated temperatures; typically uses NAD as cofactor. Retains activity and structural stability at high temperature, enabling ligation under stringent hybridization conditions (e.g., high-temperature oligonucleotide probe ligation).
【Features & Applications】
Half-life can be tens of hours at 65 °C and retains activity for some time even at 95 °C; tolerates hundreds of PCR-like thermal cycles; offers high hybridization stringency and sequence specificity. Virtually inactive on blunt ends; shows low but detectable activity on 2- or 4-nt overhangs. Common in probe-based high-temperature ligation assays such as genotyping, SNP detection, and ligation-dependent amplification.
【Enzymatic Characteristics and Reaction Environment】
Thermostable DNA ligases typically have an optimal working temperature of about 55–70 °C, require NAD⁺ as a cofactor, and also depend on Mg²⁺ for catalysis. Unlike ATP-dependent ligases such as T4/T3/T7 DNA ligase, NAD⁺ is more stable at elevated temperatures. Together with the intrinsic heat-stable structure of thermostable ligases, this allows them to maintain high activity and good batch-to-batch reproducibility during repeated temperature cycling. Typical ligation buffers are mildly alkaline Tris-based systems containing Mg²⁺, NAD⁺, and reducing agents, balancing oligonucleotide hybridization with enzymatic activity.
【Recommended Usage】
The ligation temperature for thermostable DNA ligase is generally set about 3–10 °C below the Tm of the probe or oligonucleotide (for example, when Tm≈70 °C, 60–65 °C is suitable). Reactions should be performed in a buffer containing NAD⁺, Mg²⁺, and an appropriate amount of reducing agent. Depending on experimental needs, either isothermal ligation (for high-temperature ligation readout only) or a cycling program combined with PCR can be used. In practice, use high-purity PCR products or genomic DNA as templates whenever possible, and avoid excessive EDTA, protein, or surfactant carryover to ensure ligation efficiency and reliable site discrimination.
【Function】
Uses NAD⁺ as cofactor to ligate adjacent 3′-OH and 5′-phosphate in duplex DNA; functions in E. coli DNA replication and repair to seal nicks.
【Features & Applications】
Compared with T4, has poor acceptance of RNA and virtually no activity on blunt-ended duplex DNA, but shows higher specificity in cohesive-end ligation and nick repair. Uses NAD (provided in its buffer), not ATP. Chosen when modeling/leveraging prokaryotic repair mechanisms or in certain cDNA synthesis/modification workflows.
【Substrate Characteristics】
In vivo, E. coli DNA ligase is mainly responsible for joining Okazaki fragments behind the replication fork and sealing single-strand nicks during DNA damage repair. Therefore, its substrate preference closely matches the prokaryotic replication environment: it favors correctly paired double-stranded DNA nick sites, and its activity drops markedly at ligation regions containing mismatched bases or structural distortions. In vitro, this ligase has almost no ability to join typical blunt-ended double-stranded DNA, but under appropriate conditions it can catalyze highly specific ligation of complementary sticky ends generated by restriction endonucleases or of engineered nicked substrates. As a result, it provides a model that is closer to physiological conditions for studying prokaryotic DNA repair fidelity, nick processing, and mismatch correction.
【Recommended Usage】
When using E. coli DNA ligase, ensure that sufficient NAD⁺ is supplied in the reaction system (typically provided by the matched 10× buffer), and avoid reusing the ATP-type buffers commonly employed for T4/T3 ligation. This enzyme is better suited for ligating sticky ends or pre-formed nick sites at near-physiological temperatures (e.g., 25–37 ℃). The substrates should preferably be double-stranded DNA with accurately paired terminal bases. It is not recommended for systems containing large amounts of RNA, incompletely annealed substrates, or a high proportion of blunt ends. If the primary goal is molecular cloning, T4 DNA ligase or other universal ligases are still recommended.
VI. Taq DNA Ligase
【Function】
At elevated temperatures, ligates the 5′-phosphate and 3′-OH of two adjacent oligonucleotides only when both are perfectly base-paired to the same template with no gap or mismatch; mismatches or gaps strongly inhibit ligation, enabling highly specific base discrimination.
【Features & Applications】
Uses NAD as cofactor; active ~37–75 °C, typically with thermocycling. Generally does not ligate standard 4-nt overhangs from restriction enzymes but can efficiently ligate longer (e.g., ~12-bp) overhangs or probe junctions under appropriate conditions. Widely used in LCR, SNP genotyping, and allele-specific ligation requiring very high sequence specificity.
【Substrate and Recognition Features】
Taq DNA ligase is derived from the thermophilic bacterium Thermus aquaticus. It is an NAD⁺-dependent thermostable ligase that is extremely sensitive to substrate geometry and base-pairing status. Only when two oligonucleotides are fully complementary to the template DNA at the target site, and when there is absolutely no gap or mismatch at the ligation junction, can it efficiently catalyze formation of the phosphodiester bond between a 5′ phosphate and a 3′ hydroxyl at high temperature. This nearly “all-or-none” recognition mode makes Taq DNA ligase highly suitable as a single-base-resolution sequence discrimination tool, but not as a universal ligase for routine fragment cloning.
【Recommended Usage】
In practice, the reaction temperature for Taq DNA ligase is typically set slightly below the Tm of the probes or oligonucleotides to maintain stable, highly specific hybridization at elevated temperature while maximally suppressing mismatched pairing. The reaction system must supply sufficient NAD⁺ and use a buffer compatible with high-temperature ligation. Templates should be high-quality PCR products or well-prepared genomic DNA, and excessive salts, EDTA, or protein contaminants should be avoided to prevent interference with enzyme activity and hybridization. For allele-specific detection, the variant site should be positioned at the ligation junction or at the 3′ end of the downstream probe to fully exploit the enzyme’s extreme mismatch sensitivity.
VII. SplintR-type DNA Ligase (RNA-splint–dependent DNA ligase)
【Function】
Efficiently ligates two adjacent single-stranded DNA (ssDNA) oligos when they are co-hybridized by a complementary RNA “splint.” The RNA splint pairs with both ssDNA ends to create a local duplex-like structure that the ligase recognizes, sealing the 5′-phosphate and 3′-OH.
【Features & Applications】
Specialized for RNA-splint–guided ssDNA ligation; ideal for padlock-probe–based microRNA assays, RASL-seq, SNP genotyping, and alternative splicing analyses. Its high affinity and catalytic efficiency on RNA–DNA splints enable highly sensitive (sub-nanomolar) RNA quantification.
【Substrate Characteristics and Recognition Advantages】
SplintR-type DNA ligases show relatively limited activity toward typical DNA–DNA or DNA–RNA hybrid substrates, but exhibit exceptionally high affinity and catalytic efficiency for the specific ternary complex of “an RNA splint + two adjacent ssDNA probes.” When an RNA strand serves as a “splint” and hybridizes complementarily to the terminal regions of two ssDNA fragments, it locally forms a highly ordered, almost fully duplex hybrid structure. This enables the enzyme to precisely recognize the base-pairing status near the ligation junction and efficiently catalyze bond formation between the 5′ phosphate and 3′ hydroxyl. As a result, even in complex sample backgrounds, the ligase can selectively “amplify” probe-ligation events that match a particular RNA sequence, thereby markedly improving detection sensitivity and sequence discrimination for low-abundance RNA molecules.
【Recommended Usage】
When using an RNA-splint–dependent DNA ligase, the target RNA is usually designed to constitute part or all of the “template/splint” so that two DNA probes anneal adjacently on it. To ensure ligation specificity and efficiency, it is recommended to precisely design the hybridization regions of the DNA probes and the RNA splint according to the length and GC content of the target RNA, keeping the bases around the ligation junction strictly complementary and minimizing interference from secondary structures. Appropriate amounts of Mg²⁺, salt, and reducing agent should be included in the buffer, and the reaction temperature should be set slightly below the Tm of this local hybridization region so that the RNA–DNA splint structure remains stable while nonspecific annealing is suppressed. When working with complex samples such as serum or tissue extracts, the purity and degradation state of the template RNA have a major impact on ligation efficiency; if necessary, pre-enrichment or pretreatment steps prior to reverse transcription can be introduced to improve detection sensitivity and reliability.
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