Synthesis of influenza virus: new impetus from an old enzyme, RNA polymerase I

Reverse genetics systems, i.e., systems for the generation of virus entirely from cloned cDNA, have been established for most nonsegmented negative-sense RNA viruses. In contrast, the generation of influenza A viruses (whose genome is composed of eight segments of negative-sense RNA) was not possible until 1999, likely due to the inherent technical difficulties of providing all eight viral RNAs as well as the four viral proteins required for replication and transcription. In 1999, we (Neumann et al., 1999, Proc. Natl. Acad. Sci. USA 96, 9345–9350) and others (Fodor et al., 1999, J. Virol. 73, 9679–9682) demonstrated the generation of influenza A virus from plasmids, relying on the cellular enzyme RNA polymerase I for the synthesis o influenza viral RNAs. In this review, we provide background on RNA polymerase I transcription and discuss its use for the generation of influenza virus from cloned cDNAs.