Homologous recombination is a universal genetic process of biological significance, interconnecting DNA replication and repair. Recombination is essential for generating genetic diversity, enabling the adaptation and evolution of organisms. The single strand annealing pathway of homologous recombination is mediated by a two-component recombinase composed of an exonuclease and an annealase. The exonuclease binds to double-stranded DNA breaks degrading one strand, while the annealase binds to the exposed single-stranded DNA promoting recombination between homologous regions. Despite half a century of research, the molecular mechanisms underlying this process are poorly understood. Our understanding has been hampered in-part by the limited availability of structural data, particularly for annealases.
We used cryo-EM to generate a structural model of the essential recombination function (ERF) annealase from bacteriophage P22. Initial imaging indicated severe preferred particle orientation, which was overcome by the addition of the zwitterionic detergent CHAPSO. The final 3-D model was refined to 4.3 Å, which allowed manual de novo atomic modelling to obtain a partial atomic model of ERF. Based on our structure, we predict a RAD52-like β-β-β-α fold, responsible for DNA binding. This fold had been previously predicted to be present in several annealases, including ERF [1].
In summary, after the discovery of ERF in 1970 [2], we have finally determined its first structure, a significant addition to the limited cache of structural information on annealases.