Splicing order of introns 1 and 2.

(A) Splice intermediates expected for RT-PCR reactions A and B if intron 1 is spliced before intron 2, or vice versa. (B) RT-PCR products obtained for primer pairs A and B, separated on a 1% agarose gel. For each reaction, a PCR amplification was performed without prior reverse transcription (RT−) to exclude DNA contamination. The bands correspond to the unspliced exon 1–intron 1–exon 2 product for reaction A and the spliced intron 1–exon 2–exon 3 product for reaction B, demonstrating splicing of intron 2 before intron 1 (see Table 2).