Molecular model of a eucaryotic transcription complex: functions and movements of influenza P proteins during capped RNA-primed transcription.

We present a model for the functions and movements of the influenza virus P proteins (PB1, PB2, and PA) as they transcribe the virion RNAs (vRNAs) into messenger RNAs (mRNAs). Using ultraviolet-light-induced crosslinking, we show that the P proteins as a complex move from the 3' ends of the vRNA templates down the elongating mRNAs. PB2 binds the cap 1 structure of heterologous RNAs, which are cleaved to generate capped primer fragments. PB1, initially found at the first residue added onto the primer, moves to the 3' ends of the growing mRNA chains, indicating that it most likely catalyzes each nucleotide addition. PA and PB2 move down the growing chains in concert with PB1. PB2 is also associated with the cap during the first 11-15 nucleotides of chain growth, but then dissociates from the cap as the P protein complex moves further down the mRNA chains.