Sandbox S24

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spinqualitylabelsall models Caption for this structure Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image   Print3D tRNA CCA-adding polymerase matures the essential 3'-CCA terminus of transfer RNA without any nucleic-acid template. However, it remains unclear how the correct nucleotide triphosphate is selected in each reaction step and how the polymerization is driven by the protein and RNA dynamics. Here we present complete sequential snapshots of six complex structures of CCA-adding enzyme and four distinct RNA substrates with and without CTP (cytosine triphosphate) or ATP (adenosine triphosphate). The CCA-lacking RNA stem extends by one base pair to force the discriminator nucleoside into the active-site pocket, and then tracks back after incorporation of the first cytosine monophosphate (CMP). Accommodation of the second CTP clamps the catalytic cleft, inducing a reorientation of the turn, which flips C74 to allow CMP to be accepted. In contrast, after the second CMP is added, the polymerase and RNA primer are locked in the closed state, which directs the subsequent A addition. Between the CTP- and ATP-binding stages, the side-chain conformation of Arg 224 changes markedly; this is controlled by the global motion of the enzyme and position of the primer terminus, and is likely to achieve the CTP/ATP discrimination, depending on the polymerization stage. Throughout the CCA-adding reaction, the enzyme tail domain firmly anchors the TPsiC-loop of the tRNA, which ensures accurate polymerization and termination.You may include any references to papers as in: the use of JSmol in Proteopedia or to the article describing Jmol to the rescue. Structural highlights tRNA highlighted sulfur O1 highlighted This is a sample scene created with SAT to color by Group, and another to make a transparent representation of the protein. You can make your own scenes on SAT starting from scratch or loading and editing one of these sample scenes. black background