Influenza A and B viruses share common sequences and potentially similar panhandle structures in the terminal noncoding regions of virion RNA (vRNA). Interesting differences exist, however, in the number of conserved nucleotides at the 5h and 3h ends of the vRNAs, in base pairs constituting the panhandle duplex, and the length of uridine stretch (U stretch) juxtaposed to the RNA duplex. To analyse the contribution of these signals to the specificity between the two viruses, a transient ribonucleoprotein transfection method was used for the expression of the chloramphenicol acetyltransferase (CAT) reporter gene flanked by the noncoding nucleotides derived from influenza B vRNA. While the base pairing in the RNA duplex was primarily important for template activity, mismatch mutations G11iG12h and C12iA13h in the terminal RNA

The roles of the 3-and 5-terminal nucleotides and the panhandle structure of influenza B virus virion RNA were analyzed in vitro by transcription of model RNA templates with influenza B virus RNA polymerase. The results suggest that the stability of the panhandle and breathing of the extreme ends of the panhandle are important factors for efficient transcription. Influenza B virus polymerase appears to be more tolerant of mutations in the panhandle structure than influenza A virus polymerase. This is consistent with the greater degree of heterogeneity observed naturally in the 3-terminal nucleotides of the virion RNA of influenza B virus than in influenza A virus.