among influenza A viruses (IAV) and have a promoter function. Hoffmann's 8-plasmid
reverse genetics vector system introduced mutations at position 4, C nucleotide (C4) to U
nucleotide (U4), of the 3′ ends of neuraminidase (NA) and matrix (M) vRNAs of wild-type
A/PR/8/34 (PR8). This resulted in a constellation of C4 and U4 vRNAs coding for low
(polymerases) and relatively high (all others) copy number proteins, respectively. U4 has …