The human coronavirus 229E replicase gene encodes a
protein, p66HEL, that contains a putative
zinc finger structure linked to a putative superfamily
(SF) 1 helicase. A histidine-tagged form of this protein,
HEL, was expressed using baculovirus vectors in insect
cells. The purified recombinant protein had in vitro ATPase
activity that was strongly stimulated by poly(U), poly(dT),
poly(C), and poly(dA), but not by poly(G). The recombinant
protein also had both RNA and DNA duplex-unwinding activities
with 5′-to-3′ polarity. The DNA helicase activity
of the enzyme preferentially unwound 5′-oligopyrimidine-tailed,
partial-duplex substrates and required a tail length of
at least 10 nucleotides for effective unwinding. The combined
data suggest that the coronaviral SF1 helicase functionally
differs from the previously characterized RNA virus SF2
helicases.