Transcription
Eight expression plasmids (pHW2000) corresponding to the viral genomic segments were transfected to express all influenza genes. The first eight genetic constructs above are for the H1N1 fraction and the later 8 are for the H3N2 fractions of the vaccine. The plasmid contains bidirectional sets of expression cassettes: Cytomegalovirus promoter - Bovine growth hormone terminator (sense) and Human polymerase I promoter - Mus musculus polymerase I terminator (antisense). The combination of transcription in both directions and interaction with host cellular machinery results in the production of infectious virions.

For protein expression of the influenza genes, transcription by RNA polymerase II is directed from the Cytomegalovirus promoter and terminates at the bovine hormone terminator. For segments VII and VII (matrix protein 2 and nuclear export protein, respectively), the transcripts will be spliced to join the initial coding sequences to the later sequences and form a mature mRNA. The transcripts receive 5' caps and 3' poly-adenylated tails. Thus, the transcripts are translated into proteins. To produce viral RNA to function as genomic material, transcription must produce negative sense (antisense) cRNA. Thus, transcription is directed from the human polymerase I promoter and terminates at the Mus musculus polymerase I terminator. These transcripts have a 5' triphosphate group and no polyadenylated tail. Thus, they are not translated into proteins.

Sequence source
H1N1 modified virion (Lot no. 241-192) - first eight genetic constructs above
Influenza strain A/swine/Minnesota/37866/1999 (H1N1): HA and NA proteins

Influenza strain A/Swine/Texas/4199-2/98 (H3N2): PB1, PB2, PA, NP, M1, M2, NS1 and NEP

H3N2 modified virion (Lot no. 241-196) - later eight genetic constructs above
Influenza strain A/Swine/Texas/4199-2/98 (H3N2): PB1, PB2, PA, HA, NP, NA, M1, M2, NS1 and NEP

Note:
For both modified virions, the nonstructural protein 1 (NS1) has been carboxyl-truncated and codes for amino acids 1 to 126 of the native protein. The truncation is related to the attenuation (reduction in virulence) of the seed virus.