Scientists from the Division of Structural Biology at The Institute of Cancer Research, London, alongside colleagues from the University of Bristol carried out a series of experiments to study proteins involved at a crucial stage in a process called homologous recombination, which is used by cells to repair breaks in DNA. The project was funded by Cancer Research UK.
DNA breaks, which disrupt genes and cause them to move to other chromosomes, are a cause of cancer and consequently cells need to have effective ways of repairing them.
The researchers studied DNA recombination in bacteria, which are relatively easier to study than humans but share a very similar system for the repair of DNA breaks. The research focussed on the process leading up to the recruitment to DNA of a protein called RecA, which is essential for high fidelity DNA repair.
In bacteria, broken DNA ends are processed by a multi-protein complex called AddAB/RecBCD, which trims off the DNA at the broken ends and prepares it for loading of the RecA protein.
The study used a technique called X-ray crystallography to visualise the molecular structure of the proteins. The researchers initially revealed how the protein complex processes the broken DNA ends to unwind DNA, prior to trimming it off in preparation for loading of RecA.
The study went on to show that the recruitment of RecA is initiated after a rearrangement in the structure of AddAB/RecBCD, caused by the complex encountering a distinctive DNA sequence called Chi. The rearrangement, which the researchers were able to visualise directly, pauses the complex as it shuttles along a DNA strand. The complex then recruits RecA to help complete the repair process.
Although RecA is not involved in DNA repair in human cells, a closely related protein called Rad51 does the same job after the DNA break has been processed in a similar manner.
Dr Mark Dillingham, Senior Lecturer in the School of Biochemistry at Bristol University, said: “The role of the specific sequence Chi in promoting structural re-arrangements within DNA molecules was first appreciated in the early 1970s. Subsequent work then led to an appreciation that Chi acted as a ‘recombination hotspot’ by controlling the activity of an enzyme that helps to re-join broken DNA strands.
“This exciting study has, for the first time, unveiled the interaction between the repair enzyme and the hotspot DNA sequence at the molecular level. It shows us in exquisite detail the parts of the enzyme that are responsible for Chi recognition, and provides us with novel insights into how this interaction may promote DNA repair.”