Hjm/Hel308A DNA helicase from Sulfolobus tokodaii promotes replication fork regression and interacts with Hjc endonuclease in vitro

Hjm and Hel308a are novel, RecQ-like DNA helicases recently identified in the euryarchaeotes Pyrococcus furiosus and Methanothermobacter thermautotrophicus, respectively. In this study, an Hjm/Hel308 homologue (designated StoHjm) from Sulfolobus tokodaii, a hyperthermophilic archaeon belonging to the Crenarchaeota subdomain 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 from S. 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.