Figure 1.

Mutations at the M segment-specific non-coding nucleotides of the influenza A vRNA and cRNA promoters. Mutations at the X_14′ and Y₁₃ nucleotides of the vRNA promoter were introduced by site-directed mutagenesis, named vX_14′Y₁₃ or vXY (left, black). Position 1′ means the starting point of the 5′ UTR, while position 1 does the terminating point of the 3′ UTR. Their complementary RNA (cRNA) cassettes were designed and named cY′_13′X′₁₄ or cYX (right, red). According to the hybridization ability of the target nucleotides within the vRNA and cRNA promoters, they are classified into the three groups. wt, wild-type. Paired or unpaired nucleotides at the target site are highlighted in red or blue boxes