Differences between group I intron classes (IA to IE). Shown are secondary structure representatives for the group I intron classes [26–33]. The IA to ID classes are commonly found in bacteria. The IE class is also depicted for comparative purposes. For all group I intron RNA structures the catalytic core is highlighted in yellow. Beside each secondary structure model is a sequence logo alignment of the P7:P7′ pairing for the intron subclasses. The P7:P7′ pairing is important because it is a highly conserved region and is diagnostic for discriminating between various group I intron subclasses. With regards to the sequence logos the information content at each position (in bits, from 0 to 2) is represented by the height of the nucleotide. A score of 2 bits corresponds to high conservation, while a score of 0 corresponds to low conservation. The number of sequences used to generate each sequence logo is indicated below the intron subtype. Asterisks indicate the possible locations of peripheral insertions within the intron. The catalytic domain is highlighted in yellow.