Hjm/Hel308A DNA Helicase fromSulfolobus tokodaiiPromotes Replication Fork Regression and Interacts with Hjc Endonuclease In Vitro

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Abstract
Hjm and Hel308a are novel, RecQ-like DNA helicases recently identified in the euryarchaeotesPyrococcus furiosusandMethanothermobacter thermautotrophicus, respectively. In this study, an Hjm/Hel308 homologue (designated StoHjm) fromSulfolobus tokodaii, a hyperthermophilic archaeon belonging to theCrenarchaeotasubdomain of archaea, was cloned, purified, and characterized. Unlike Hjm and Hel308a, which unwind DNA in a 3′-to-5′ direction, StoHjm unwound DNA in both 3′-to-5′ and 5′-to-3′ directions. Remarkably, StoHjm exhibited structure-specific single-stranded-DNA-annealing and fork regression activities in vitro. In addition, gel filtration, affinity pulldown, and yeast two-hybrid analyses revealed that StoHjm physically interacted with StoHjc, the Holliday junction-specific endonuclease fromS. tokodaii. This interaction may have functional significance, because the unwinding activity of StoHjm was inhibited by StoHjc in vitro. These results may suggest that the Hjm/Hel308 family helicases, in association with Hjc endonucleases, are involved in processing of stalled replication forks.