1. Non-homologous End Joining (NHEJ)
DNA Ligase IV is Specialized to Join Damaged Double Strand Breaks
Kaitlyn E. Walsh, Mike Conlin, Dale A. Ramsden 
Non-homologous End Joining (NHEJ)
L1-4 chimera
Types of DSBs
Making the L1-4 Chimera
Sticky ends – normal base pairing
Ligation Fidelity of Lig1-4 Chimera
Mispair – G:T mispairing which distorts the double helix
Interaction
NHEJ Efficiency
Oxidized Guanine – most common DNA damage produced by radiation and reactive oxygen species
This mutant was made by combining the catalytic domains of Ligase 1 with the domains of Ligase 4 that allow it to interact with the rest of the NHEJ complex.
Y298V mutation
Conclusions
Comparing Ligase Structures
Ligation Fidelity of Lig4 Y298V
Ligase 4 has a unique tolerance for ligating damaged DSBs, which is contributed to by multiple residues.
Ligase 1 is poor at ligating damaged ends in NHEJ
The 298 tyrosine and the 345 lysine are important for tolerating both mispairs and oxidized bases
Double strand breaks (DSBs), are a lethal form of DNA damage resulting from a variety of sources including ionizing radiation and immune system development. These breaks can have associated end damage that can influence repair. Both damaged and undamaged ends are repaired through NHEJ and ligated in the last step of the pathway by Ligase IV (Lig4).
NHEJ Efficiency
Blundel lab, University of Cambridge
Hypothesis
K345F mutation
Future Directions
Ligation Fidelity of Lig4 K345F
Ligase Interactions at a DNA Break
We propose that DNA Ligase IV (Lig4) is specialized to ligate damaged DSBs.
Testing the hypothesis:
Exchanging the Lig4 active region with another ligase (Lig1)
Mutating specific residues unique to Lig4 to look like other ligases
What is the impact of the damage tolerated by Lig4?
Is Lig4 damage tolerance important for cell survival following treatment with ionizing radiation and chemotherapeutics?
NHEJ Efficiency