The RNA-dependent RNA polymerase of influenza A virus is responsible for both transcription and replication of negative-sense viral RNA. It is thought that a "switching" mechanism regulates the transition between these activities. We demonstrate that, in the presence of preexisting viral RNA polymerase and nucleoprotein (NP), influenza A virus synthesizes both mRNA (transcription) and cRNA (replication) early in infection. We suggest that there may be no switch regulating the initiation of RNA synthesis and present a model suggesting that nascent cRNA is degraded by host cell nucleases unless it is stabilized by newly synthesized viral RNA polymerase and NP.

The influenza A virus RNA-dependent RNA polymerase consists of three subunits PB1, PB2 and PA. The 59 and 39 terminal sequences of the viral RNA (vRNA) form the viral promoter and are bound by the PB1 subunit. The putative promoter-binding sites of the PB1 subunit have been mapped in previous studies but with contradictory results. The aim of the current study was to investigate the function of two evolutionary conserved regions in PB1 -from aa 233 to 249 and 269 to 281, which lie immediately N-and C-terminal, respectively, of a previously proposed binding site for the 39 end of the vRNA promoter. The previously proposed binding site extended from aa 249 to 256 and centred on two phenylalanine residues (F251 and F254). However, the fact that F251 is required for polymerase activity was not confirmed here. Instead, it was proposed that the 233-249 region contains a new 59 vRNA promoter-binding site, and arginine residues crucial for this activity were characterized. However, residues 269-281 were unlikely to be directly involved in promoter binding. These results are discussed in relation to the previous studies and a new model for vRNA promoter binding to the influenza RNA polymerase is presented.