Fig. 1.

Initiation mechanisms of RNA synthesis by plus strand RNA viruses. Two basic types of mechanisms for RNA synthesis by plus strand RNA virus RNA polymerases are shown (162). (A) Primer-independent (de novo) initiation. De novo initiation involves starting an RNA chain, usually with a purine nucleoside triphosphate, templated by a pyrimidine at the 3’-end of the template strand or at an internal site. (B) Primer-dependent initiation. The primer is either an oligonucleotide or a protein. A 3’-hydroxyl group of a nucleotide or the hydroxyl group of a tyrosine or serine residue of a peptide/protein provides the hydroxyl group for the formation of a phosphodiester bond with the first nucleotide.