Mutagenesis of the serial deletion of base pairs in the N-SL2 stem. (A) RNA secondary structure prediction of the mutants. (i) Schematic representation of the predicted secondary structure of the mutants, D1-D4. The dashed boxes indicate the increasing base pair deletions from the bottom of N-SL2. (ii) RNA secondary structure prediction of the mutant D5. The remaining nucleotides of N-SL2 are highlighted in gray. N-SL1' and N-SL2' represent the regenerated stem-loop structures after the deletions. (iii) Predicted secondary structure of mutant D6. The remaining nucleotides of N-SL2 are indicated by gray shading, which disappeared and became linear nucleotides. (B) RT-PCR analysis of the mutants D1-D6 as described in Figure 2.