The enzyme RecQ helicase senses whether DNA in a D-loop has been paired correctly for repair by pausing for different lengths of time depending on whether the paired strands are a good or bad match. Image based on Protein Data Bank codes 1OYY and 1WUD. Image credit: Seol et al (CC BY 4.0)
Molecules of DNA carry instructions for all of the biological processes that happen in cells. Therefore, it is very important that cells maintain their DNA and quickly repair any damage. DNA molecules are made of two strands that twist together to form a double helix. The most reliable way to repair damage affecting both DNA strands involves a process known as homologous recombination. In this process, one of the strands of the broken DNA joins up with a strand of an identical or similar DNA molecule to make a triple-stranded structure known as a D-loop. This allows the cell to rebuild the damaged DNA using the intact DNA as a template.
To ensure that the DNA is repaired correctly, enzymes known as RecQ helicases bind to and unwind D-loops if the strand pairs are poorly matched, whilst not disrupting pairs that are correctly matched. It remains unclear, however, how these enzymes are able to distinguish whether DNA strands in D-loops are a good or bad pair.
To address this question, Seol, Harami et al. measured how individual RecQ helicases from a bacterium known as Escherichia coli unwind DNA. The experiments showed that the enzymes were better able to unwind sections of double-stranded DNA that were less stable than other sections of DNA (indicating the two strands may be a bad match). This causes the helicase to pause at stable sections of the DNA as it unwinds the double helix of the D-loop. Further experiments showed that a region of the helicase known as the HRDC domain increased the duration of these pauses, leading to a dramatic decrease in the unwinding speed.
Seol, Harami et al. propose that this difference in unwinding speed prevents RecQ from unwinding legitimate matching D-loops while permitting rapid disruption of illegitimate D-loops that could lead to damaged DNA being repaired incorrectly. Mutations in the human versions of RecQ helicases lead to Bloom’s syndrome and Werner’s syndrome in which individuals are predisposed to developing cancer. Understanding how cells repair DNA may ultimately help to treat individuals with these and other similar conditions.
