(b) The X-ray structure of the 414-nt group I intron from Tetrahymena thermophila drawn in paddle form with its P4-P6 domain blue, its P3-P9 domain green, the catalytically active 3' ωG residue yellow, the base triples of the P7 domain red, the A-rich bulge brown, the tetraloop pink, and the tetraloop receptor cyan. The inferred positions of Mg²⁺ ions are represented by orange spheres. Click on the "Color Ramp the RNA" button to color the polynucleotide chain in rainbow order from its 5' end (blue) to its 3' end (red). (c) As in Part b but rotated 140° about the vertical axis to better show the A-rich bulge and the interaction between the tetraloop and the tetraloop receptor. [Based on an X-ray structure by Thomas Cech, University of Colorado. PDBid 1X8W.]