Repair of Single-Stranded Breaks in Hyperthermophilic DNA

This review focuses on a fundamental problem faced by hyperthermophiles that grow at temperatures above 95 °C. At such high temperatures, both linear and open circular DNA denature, leading to strand separation and loss of the double-helical structure. One strategy to prevent denaturation is to maintain DNA in a closed circular form, where the topological constraint on DNA conformations increases the melting temperature by 30-40 °C. However, this conformational restriction is lost when a single-stranded break occurs - a common type of DNA lesion. In hyperthermophiles, circular DNA containing a single-stranded nick begins to unwind and partially denature. Although DNA-bound proteins can slow this process, they protect only a fraction of the double helix. As a result, repairing such damage requires not only restoration of the strand integrity but also restoration of the original linking number between complementary strands. Reverse gyrase, a thermophile-specific enzyme that catalyzes positive supercoiling in closed circular DNA, fulfills this essential role in the DNA repair pathway.