Direct evidence that the poly(A) tail of influenza A virus mRNA is synthesized by reiterative copying of a U track in the virion RNA template

The poly(A) tail of influenza virus mRNA is thought to be synthesized by re iterative copying of the U track near the 5' end of the virion RNA template . This has been widely accepted as a plausible hypothesis, but until now th ere has been no direct experimental evidence for it. Here, we report such d irect evidence based on the fact that (i) replacing the U track with an A t rack directs synthesis of products with poly(U) tails, both in vitro and in vivo, and (ii) interrupting the U track abolishes polyadenylation in vitro .