Cooperative translocation enhances the unwinding of duplex DNA by SARS coronavirus helicase nsP13

Lee, Na-Ra; Kwon, Hyun-Mi; Park, Kkothanahreum; Oh, Sangtaek; Jeong, Yong-Joo; Kim, Dong-Eun

The results of this study clearly show that increasing the ssDNA tail length enhances the unwinding amplitude. This result was also previously observed for SF1 and SF2 helicase (26, 41) ; longer ssDNA tails provide more chances for helicase molecules to bind to ssDNA and multiple binding leads to an enhanced unwinding efficiency. In the present experimental setup, where the enzyme concentration was greater than the substrate concentration, all ssDNA tails were assumed to be initially bound by nsP13. The stacked oligomers and forward movement of nsP13 caused by the binding of multiple nsP13s on the ssDNA tail are believed to be the main reasons for the enhanced unwinding on substrates with a longer ssDNA loading strand. Taken together, when nsP13 concentration was in excess, the amplitude of ssDNA formation increased as the length of either the ssDNA overhang or gap increased. These results suggest that multiple nsP13 molecules may bind to the ssDNA loading strand and unwind longer duplex DNA with a higher processivity.