Cooperative translocation enhances the unwinding of duplex DNA by SARS coronavirus helicase nsP13.
SARS coronavirus encodes non-structural protein 13 (nsP13), a nucleic acid helicase/NTPase belonging to superfamily 1 helicase, which efficiently unwinds both partial-duplex RNA and DNA. In this study, unwinding of DNA substrates that had different duplex lengths and 5'-overhangs was examined under single-turnover reaction conditions in the presence of excess enzyme. The amount of DNA unwound decreased significantly as the length of the duplex increased, indicating a poor in vitro processivity. However, the quantity of duplex DNA unwound increased as the length of the single-stranded 5'-tail increased for the 50-bp duplex. This enhanced processivity was also observed for duplex DNA that had a longer single-stranded gap in between. These results demonstrate that nsP13 requires the presence of a long 5'-overhang to unwind longer DNA duplexes. In addition, enhanced DNA unwinding was observed for gapped DNA substrates that had a 5'-overhang, indicating that the translocated nsP13 molecules pile up and the preceding helicase facilitate DNA unwinding. Together with the propensity of oligomer formation of nsP13 molecules, we propose that the cooperative translocation by the functionally interacting oligomers of the helicase molecules loaded onto the 5'-overhang account for the observed enhanced processivity of DNA unwinding.
Lee
Na-Ra
NR
Department of Bio and Nanochemistry, Kookmin University, Seoul 136-702, Republic of Korea.
Kwon
Hyun-Mi
HM
Park
Kkothanahreum
K
Oh
Sangtaek
S
Jeong
Yong-Joo
YJ
Kim
Dong-Eun
DE
D004247:DNA / Q000737:chemistry / Q000378:metabolism*; D004265:DNA Helicases / Q000378:metabolism*; D007700:Kinetics; D045473:Severe acute respiratory syndrome-related coronavirus / Q000201:enzymology*; D017361:Viral Nonstructural Proteins / Q000378:metabolism*
D017361:Viral Nonstructural Proteins; D004247:DNA; D004265:DNA Helicases