Model of the <i>Tetrahymena</i> Ribozyme Transition State during the First Step of Splicing

<p>The three identified catalytic metal ions (M_A, M_B, and M_c) and their transition-state interactions with the U(-1) 3′-oxygen, G nucleophile 3′-oxygen, and G 2′-OH, respectively, along with the M_A and M_C interactions with the scissile phosphate <i>pro</i>-<i>S</i>_P oxygen, are shown by red dots [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030277#pbio-0030277-b35" target="_blank">35</a>,<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030277#pbio-0030277-b36" target="_blank">36</a>]. The 2′-OH of U(-1) participates in a hydrogen-bonding network with the 2′-OH of A207 and the exocyclic amine of the G·U wobble pair [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030277#pbio-0030277-b58" target="_blank">58</a>,<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030277#pbio-0030277-b72" target="_blank">72</a>] and donates a hydrogen bond to the adjacent 3′-oxygen in the transition state [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030277#pbio-0030277-b71" target="_blank">71</a>]; the hydrogen bonds are shown as hashed lines.