A single-strand annealing protein clamps DNA to detect and secure homology

Repair of DNA breaks by single-strand annealing (SSA) is a major mechanism for the maintenance of genomic integrity. SSA is promoted by proteins (SSAPs) like eukaryotic RAD52 and lambda phage Redß that use a short single-stranded region to find sequence identity and initiate homologous recombination. However, it is unclear how SSAPs detect homology and catalyze annealing. Using single-molecule experiments, we provide evidence that homology is recognized by Redß monomers that weakly hold single DNA strands together. Once annealing begins, dimerization of Redß clamps the double-stranded region and nucleates nucleoprotein filament growth. In this manner, DNA clamping ensures and secures a successful search for DNA sequence homology. The clamp is characterized by a structural change of Redß and a remarkable stability against force up to 200 pN. Our findings not only present a detailed explanation for SSAP action but also identify the DNA clamp as a very stable, non-covalent, DNA-protein interaction.