Figure 1.
Schematic representation of the pol I–pol II transcription system for synthesis of vRNA and mRNA. The cDNA of each of the eight influenza virus segments is inserted between the pol I promoter (pIh) and the pol I terminator (tI). This pol I transcription unit is flanked by the pol II promoter (pIICMV) of the human cytomegalovirus and the polyadenylation signal (aIIBGH) of the gene encoding bovine growth hormone. After transfection of the eight expression plasmids, two types of molecules are synthesized. From the human pol I promoter, negative-sense vRNA is synthesized by cellular pol I. The synthesized vRNA contains the noncoding regions (NCR) at the 5′ and 3′ ends. Transcription by pol II yields mRNAs with 5′ cap structures and 3′ poly(A) tails; these mRNAs are translated into viral proteins. The ATG of the viral cDNA is the first ATG downstream of the pol II transcription start site.