Severe acute respiratory syndrome coronavirus nonstructural protein 13 (SCV nsP13), a superfamily 1 helicase, plays a central role in viral RNA replication through the unwinding of duplex RNA and DNA with a 5' single-stranded tail in a 5' to 3' direction. Activity of helicase is dependent on magnesium. Despite its putative role in viral RNA replication, nsP13 readily unwinds duplex DNA by cooperative translocation. The accumulation of duplex RNA/nsP13 complexes increased as the concentration of nsP13 increased. An increased ATP concentration in the unwinding of duplex RNA relieved the decrease in duplex RNA unwinding.