RadA protein from Archaeoglobus fulgidus forms rings, nucleoprotein filaments and catalyses homologous recombination

Proteins that catalyse homologous recombination have been identified in all living organisms and are essential for the repair of damaged DNA as well a s for the generation of genetic diversity. In bacteria homologous recombina tion is performed by the RecA protein, whereas in the eukarya a related pro tein called Rad51 is required to catalyse recombination and repair. More re cently, archaeal homologues of RecA/Rad51 (RadA) have been identified and i solated. In this work we have cloned! and purified the RadA protein from th e hyperthermophilic, sulphate-reducing archaeon Archaeoglobus fulgidus and characterised its in vitro activities. We show that (i) RadA protein forms ring structures in solution and binds single- but not double-stranded DNA t o form nucleoprotein filaments, (ii) RadA is a single-stranded DNA-dependen t ATPase at elevated temperatures, and (iii) RadA catalyses efficient D-loo p formation and strand exchange at temperatures of 60-70 degreesC. Finally, we have used electron microscopy to visualise RadA-mediated joint molecule s, the intermediates of homologous recombination. Intriguingly, RadA shares properties of both the bacterial RecA and eukaryotic Rad51 recombinases.