Dual functionality for the SARS-CoV nsp(7+8) RNA polymerase at the genomic 3ʹ end

Despite identification of two virus-encoded RNA-dependent RNA polymerases (RdRps) genome, many critical steps in coronavirus RNA synthesis remain to be elucidated at the molecular level. Here, using the SARS-coronavirus model, we describe intriguing biochemical observations suggesting that the initiation of minus-strand RNA synthesis and the polyadenylation of both genome-length and subgenomic mRNAs may depend on a unique platform that involves an interplay of the nsp(7+8) RNA polymerase with a conserved 3ʹ-terminal signature of the viral genome. Specifically, our in vitro studies implicate the 3ʹ-terminal GACOH, which can be polyadenylated or used as template for de novo initiation of minus strand synthesis. We hypothesise that this specificity is partly achieved through the template-independent formation of a 5ʹ-pppGpUOH product that is complementary to this sequence. Lastly we explore here the intrinsic regulation between the two SARS-CoV encoded RdRps and the implications thereof for regulation of their activities. nsp(7+8) activity on the SARS-CoV 3' end 181