Genetic analysis of the Hel308 helicase in the archaeon Haloferax volcanii

Hel308 is a RecQ family DNA helicase that is conserved in metazoans and archaea but is absent from bacteria and fungi. Hel308 family helicases are implicated in DNA repair, homologous recombination and genome stability, but the exact role of Hel308 is largely unknown. Strains deleted for hel308 are sensitive to DNA inter-strand crosslinks, which are potent blocks to DNA replication. In this study, the archaeon Haloferax volcanii was used as a model organism to study the role of Hel308. In archaea, homologous recombination is catalysed by polymerisation of the RadA recombinase onto ssDNA; the mediator RadB assists this process. Strains deleted for radB exhibit decreased levels of recombination and an increased sensitivity to DNA damaging agents. In this study, strains deleted for hel308 in combination with radB exhibited in an improvement in both these phenotypes, suggesting that Hel308 acts as an anti-recombinase to antagonise RadA filament formation. Genetic analysis of point mutants in Hel308 revealed that that the helicase activity of Hel308 is separate to its role in the regulation of recombination, which appears to rely heavily on the correct structural conformation of Hel308. Analysis of these point mutations suggests that Hel308 may act in regulating the pathway choice for the resolution of homologous recombination intermediates. This study showed that H. volcanii contains a second Hel308 helicase named Hel308b, which lacks the ‘auto-inhibitory’ domain 5 found in canonical Hel308 helicases. Deletion of hel308b does not lead to sensitivity to DNA inter-strand crosslinks but does result in defects in homologous recombination