• P4 P6 RNA Ribozyme Domain
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Figure 1. The secondary structure diagram of all three domains that make up the group I intron (4).

Figure 2. (a) A basic pathway for group I intron self-splicing. Step (1) is the first transesterification that takes place, resulting in a cleavage at the 5’-splice site. The intron goes through a conformation stage (Conf.) where the 3’-guanosine replaces the exogenous guanosine. The splicing comes to completion with the second transesterification step (2) with a cleavage at the 3’-spliesite and ligation of the two exons (5). (b) A basic pathway for group II intron self-splicing. In step (1) an internal adenosine acts as a nucleophile and attacks the 5’-splic site while forming a lariat-type structure. In step (2), the 3’ end of the 5’-exon cleaves the 3’-splice site, ligating the two exons and excising a lariat intron. 

Table 1. Important hydrogen bonds between the adenosine residues in the A-rich bulge and the nucleotide residues in the P4 segment (6).

Table 2. Important hydrogen bonds between the adenosine residues in the tetraloop and the nucleotides in the tetraloop receptor region (6).

Table 3. Location of metal ligands in the P4-P6 domain (9).