Fig. 4.

Structural organization of class II CCA-adding enzymes. Upper part The catalytically important motifs are indicated as red boxes with consensus sequences in the rainbow bar. In the three-dimensional structure (CCA-adding enzyme from Thermotoga marimita [46]), these motifs are indicated in dark red in the head and neck domain. Lower part The nucleotide-binding site (green) recognizes the incoming CTP and ATP (red) by the formation of Watson/Crick-like hydrogen bonds between the amino acid template EDxxR and the corresponding edge of the bases. The nucleotide-binding site structure is derived from the pdb entry of the B. stearothermophilus CCA-adding enzyme [45]