Mechanism of Nucleic Acid Unwinding by SARS-CoV Helicase

Adedeji, Adeyemi O.; Marchand, Bruno; te Velthuis, Aartjan J. W.; Snijder, Eric J.; Weiss, Susan; Eoff, Robert L.; Singh, Kamalendra; Sarafianos, Stefan G.

Interestingly, nsp12 translocates on the template strand in a 39R59 direction during RNA synthesis. On the other hand, unlike pestivirus or flavivirus helicases, SARS-CoV nsp13 binds a 59 overhang and moves 59R39 to unwind double stranded RNA [34, 35] . Despite the fact that the enzymatic functions of nsp13 and nsp12 have opposite polarities (59-39 vs 39-59 of the template strand)nsp12 and nsp13 may work together to carry out the following RC functions: 1) Synthesis of subgenomic transcripts containing the leader sequence derived from the 59end of the genome [34, 35] . To accomplish this, nsp12 would have to copy RNA in a 39R59 template direction, followed by pausing at transcription regulatory element sites. Such pauses may trigger motion in the opposite direction allowing nsp13 (aided by nsp12 and possibly other viral and host proteins) to unwind the 39 end of the nascent RNA primer and facilitate transfer to the complementary region of the 59-leader genomic sequence [34, 35] . 2) Data in Figure 3B suggest that nsp13 is capable of disrupting secondary structures of nucleic acid, only when they can be accessed by an upstream 59-single stranded region. Hence, stem-loop ''roadblocks'' are likely to be cleared by RCs that approach the stem loop from the 59 ssRNA direction. Ongoing biochemical experiments focus on characterizing RC functions that involve nsp13.