Comparison of intron secondary structures. The intron sequences are in upper case font, while the 5' and 3' exons are in lower case. Each of the introns includes a P1, P7 and P10 (formed by the 3' end of the intron and part of the 3' exon, paired with part of P1), as well as a pairing region between the middle portion of the intron and the 3' exon (labeled PM). The 5' and 3' splice sites are indicated by large arrowheads. The site that holds the guanosine that initiates the first trans-esterification reaction is indicated (G site with arrow).