Dda Helicase Tightly Couples Translocation on Single-Stranded DNA to Unwinding of Duplex DNA: Dda Is an Optimally Active Helicase

Helicases utilize the energy of ATP hydrolysis to unwind double-stranded DNA while translocating on the DNA. Mechanisms for melting the duplex have been characterized as active or passive, depending on whether the enzyme actively separates the base pairs or simply sequesters single-stranded DNA that forms due to thermal fraying. Here we show that Dda translocates unidirectionally on single-stranded DNA at the same rate at which it unwinds double-stranded DNA in both ensemble and single molecule experiments. Further, the unwinding rate is largely insensitive to the duplex stability and to applied force. Thus, Dda transduces all of its translocase activity into DNA unwinding activity so that the rate of unwinding is limited by the rate of translocation and the enzyme actively separates the duplex. Active and passive helicases have been characterized by dividing the velocity of DNA unwinding in base pairs per second (Vun) by the velocity of translocation on single-stranded DNA in nucleotides per second (Vtrans). If the resulting fraction is 0.25, then a helicase is considered to be at the lower end of the “active” range. In the case of Dda, the average DNA unwinding velocity was 257 ± 42 bp/s and the average translocation velocity was 267 ± 15 nucleotides/s. The Vun/Vtrans value of 0.96 places Dda in a unique category of being an essentially “perfectly” active helicase.