DNA ligations can be frustrating. Sometimes they don’t work for no obvious reason. Our top 5 DNA ligation tips should improve the efficiency of your ligations, and will hopefully increase your cloning success rate!
1. Aliquot the ligase buffer
The ATP in the ligase buffer is essential for the DNA ligation reaction, but is broken down by repeated freeze-thaw cycles. To avoid this, aliquot the ligase buffer from each new stock of DNA ligase. Make the aliquots small enough for single-use (e.g. 5 µL), and make sure to completely defrost and mix your buffer well before you aliquot.
2. Heat the DNA just before ligation
When setting up a cohesive-ended ligation, mix the vector and insert fragments first and heat to 65°C for 5 minutes before adding the remaining reaction components. This heating step disrupts any vector/vector or insert/insert cohesive-end interactions that may otherwise interfere with the desired vector/insert interaction, reducing ligation efficiency.
3. Check the pH
The optimum pH range for DNA ligation is between 7.6 and 8.0. Depending on how the DNA fragments were prepared, the pH of your ligation mixture may lie outside of this range. You can check the pH of your ligation mixture by pipetting approximately 0.2 µL of the mix onto narrow range pH paper (e.g. pH 6-8). If required, adjust the pH using 0.2 µL drops of 2M Tris base or 1M HCl.
4. Include polyethylene glycol (PEG)
As with any chemical reaction, the concentration of the reaction components can greatly influence the speed of the ligation reaction. PEG is a hydrophobic molecule that takes up space in the reaction, effectively increasing the concentration of the aqueous reaction components e.g. DNA, ATP and ligase. Adding PEG (e.g. PEG 8000) to a final concentration of 5-15% may increase ligation efficiency. Bear in mind however that PEG concentrations above 5% can reduce transformation efficiency. In addition, heat inactivation or extended incubation of ligation reactions containing PEG can also decrease transformation efficiency.
5. Add a restriction enzyme just before transformation
This neat trick can be used to circumvent high background resulting from undigested vector. If the vector fragment removed during the preparative digest contains a unique restriction site, adding the respective restriction enzyme to the ligation reaction will selectively digest any intact vector, preventing it from being transformed. Adding 1 µL of the enzyme 5-10 minutes prior to transformation should be sufficient.
Originally published on August 24, 2007. Revised and updated on June 1, 2016.