Once the (-) strand influenza viral RNAs enter the nucleus, they serve as templates for the synthesis of mRNAs. These molecules are then transported back to the cytoplasm, where they direct the synthesis of viral proteins. However, the mRNAs are not complete copies of the viral (-) strand RNAs – they are missing sequences from both the 5′- and 3′-ends. Therefore, to produce more viral (-) strand RNAs that are needed to assemble new virions, a full length (+) strand is produced, which in turn is copied to a full-length (-) strand RNA. The (-) strand RNAs are then used to assemble new virions. This process is best understood by referring to the figure, which distinguishes the processes of mRNA synthesis and replication. The (-) strand RNAs are those which are present in virions. The same processes occur for all eight segments of influenza viral RNA.
The enzyme that reproduces influenza RNA is known as an RNA-dependent RNA polymerase. This enzyme, which consists of the viral proteins PA, PB1, and PB2, is present in every virus particle. If this enzyme were absent from virions, they would never initiate infection, because the (-) strand viral RNAs cannot be translated into protein, and the cell has no enzymes which can copy such long RNA molecules. Of course, additional molecules of the viral RNA polymerase are made in infected cells, but the enzyme that is brought in with the virion is crucial for initiating the infectious cycle.
The influenza viral RNA polymerase is a primer-dependent enzyme. The enzyme cannot copy the (-) strand RNA template without a small piece of RNA that aligns on the template RNA and provides a starting point for RNA synthesis. The primers for influenza viral mRNA synthesis are produced from the cell’s own collection of mRNA molecules. The influenza viral RNA polymerase actually cleaves cell mRNAs near their 5′-ends, generating the primers it requires for RNA synthesis. This process has been called cap snatching, because the primers are made from the 5′-ends of cell mRNAs, which have an unusual chemical structure called a ‘cap’ (box labeled ‘c’ in illustration). Cap-snatching is also a feature of mRNA synthesis of other viruses.
Because RNA-dependent RNA polymerases are not found in mammalian cells, they are an excellent target for inhibition by antiviral compounds. At least one new antiviral drug that targets this enzyme, T-705, is currently in development.