Life without replication origins: the role of Hel308

Hel308 is a monomeric 3' to 5' DNA helicase conserved throughout archaea and metazoans, but absent from bacteria and fungi. Hel308 acts analogously to RecQ-family helicases, which serve as guardians of genome stability. Hel308 has been implicated in a variety of overlapping DNA repair pathways, including homologous recombination. However, the exact role of Hel308 remains elusive. We are using Haloferax volcanii as a model archaeon to study cellular role of Hel308. H. volcanii is highly unusual in that it is capable of accelerated growth in the absence of DNA replication origins; instead, it uses recombination-dependent replication. By contrast, replication origins are essential in the closely-related species Haloferax mediterranei. Our preliminary bioinformatic analysis suggests that H. mediterranei encodes a recombination-suppression factor that prevents recombination-dependent replication in the absence of origins. Using a genetic screen, we have identified Hel308 as such a recombination-suppression factor in H. mediterranei. Notably, H. mediterranei Hel308 differs from its H. volcanii homologue by the addition of a 16-amino acid proline-rich highly-disordered region. The project will use a combination of genetics, biochemistry and structural biology to investigate differences between the Hel308 helicases from H. volcanii and H. mediterranei, and how they promote recombination-dependent and origin-dependent DNA replication, respectively.