Secondary and tertiary structure of group II introns. The diagram shows the secondary structure of the Oceanobacillus iheyensis group II intron. The structure is characterized by six domains (D1 to D6) and D1 is formed by smaller subdomains (D1(i), D1(ii), D1A, D1B, D1C, D1D1, and D1D2). Each subdomain of D1 and D2, D3, D4, D5 and D6 are depicted in different colors. The inset shows the tertiary structure of the Oceanobacillus iheyensis group II intron (same color code). The four active site metal ions are represented as yellow (Mg2+) and violet (K+) spheres, respectively. The figure was adapted from .