The SARS-coronavirus nsp7+nsp8 complex is a unique multimeric RNA polymerase capable of both de novo initiation and primer extension

te Velthuis, Aartjan J.W.; van den Worm, Sjoerd H. E.; Snijder, Eric J.

In summary, our results provide important novel insights into the functionality of the SARS-CoV hexadecameric nsp(7+8) complex and demonstrate its activity as an RNA polymerase. In addition, our experiments and controls revealed and address a number of disparities between previous claims and hypotheses (12) , and our own observations. The 'primase hypothesis' previously formulated by Imbert and co-workers (12) remains an intriguing model to explain the initiation of SARS-CoV RNA synthesis and is a topic that will be addressed in detail elsewhere. Nevertheless, based on the primer extension activity of nsp(7+8) on non-structured RNA templates, we can no longer exclude the possibility that nsp(7+8) may synthesise substantially longer products than mere oligonucleotide primers in vivo, possibly stimulated by the presence of additional viral protein factors that could e.g. provide RNA-unwinding activity. Consequently, it is now a distinct possibility that CoV RNA synthesis involves structurally different and functionally separable RNA synthesising complexes [e.g. containing nsp12 or nsp(7+8)], each possessing their own dedicated RdRp characteristics and function in viral plus or minus strand RNA synthesis. It will therefore be crucial to study whether these different polymerase activities are part of the same enzyme complex and, if so, whether they can influence each other's activity or are subject to additional control mechanisms.