Back-priming mode of phi6 RNA-dependent RNA polymerase.

The RNA-dependent RNA polymerase of the double-stranded RNA bacteriophage ϕ6 is capable of primer-independent initiation, as are many RNA polymerases. The structure of this polymerase revealed an initiation platform, composed of a loop in the C-terminal domain (QYKW, aa 629–632), that was essential for de novo initiation. A similar element has been identified in hepatitis C virus RNA-dependent RNA polymerase. Biochemical studies have addressed the role of this platform, revealing that a mutant version can utilize a back-priming initiation mechanism, where the 3′ terminus of the template adopts a hairpin-like conformation. Here, the mechanism of back-primed initiation is studied further by biochemical and structural methods.