Fig. 4. CTP and ATP molecules bound to the active site of A.fulgidus CCA-adding enzyme. Interactions between (A) CTP or (B) ATP and conserved amino acid residues of A.fulgidus CCA-adding enzyme are shown. The secondary structure elements are color coded as in Figure 1A. Hydrogen bonds are shown as dashed black lines. The superimposed simulated annealing omit map (gray) is contoured at 3.0σ. (C) Superimposition of CTP (green) and ATP (blue) bound to the active site of A.fulgidus CCA-adding enzyme. Three conserved carboxylates in the CTP-bound and ATP-bound forms are displayed with green and blue backbones, respectively. (D) The activities of CTP (left) and ATP (right) incorporation into tRNA-DC and tRNA-DCC, respectively, by A.fulgidus CCA-adding enzyme mutants. The bars on the graphs indicate the SDs of five independent experiments.