Figure 1.

Catalytically inactive open form and active closed form of CCA-adding enzyme during the CCA-adding reaction. (A) Transition from the open to closed form of the enzyme–RNA complex for CMP incorporation. For CMP incorporation, the catalytic cleft in the head domain relocates towards the neck domain and the 3′-end nucleotide flips. The complexes with mini-D₇₃C₇₄ and with mini-D₇₃C₇₄ and CTP are coloured magenta and marine blue, respectively. CTP is coloured blue. (B) Closed conformation for AMP incorporation. For AMP incorporation, the enzyme is fixed in the closed form and the 3′-end nucleotide is flipped. The complexes with mini-D₇₃C₇₄C₇₅ and with mini-D₇₃C₇₄C₇₅ and ATP are coloured magenta and green, respectively. ATP is coloured red.