National Center for
3WFS: Trna Processing Enzyme Complex 3
Translocation and rotation of tRNA during template-independent RNA polymerization by tRNA nucleotidyltransferase
Structure (2014) 22 p.315-325
The 3'-terminal CCA (CCA-3' at positions 74-76) of tRNA is synthesized by CCA-adding enzyme using CTP and ATP as substrates, without a nucleic acid template. In Aquifex aeolicus, CC-adding and A-adding enzymes collaboratively synthesize the CCA-3'. The mechanism of CCA-3' synthesis by these two enzymes remained obscure. We now present crystal structures representing CC addition onto tRNA by A. aeolicus CC-adding enzyme. After C(7)(4) addition in an enclosed active pocket and pyrophosphate release, the tRNA translocates and rotates relative to the enzyme, and C(7)(5) addition occurs in the same active pocket as C(7)(4) addition. At both the C(7)(4)-adding and C(7)(5)-adding stages, CTP is selected by Watson-Crick-like hydrogen bonds between the cytosine of CTP and conserved Asp and Arg residues in the pocket. After C(7)(4)C(7)(5) addition and pyrophosphate release, the tRNA translocates further and drops off the enzyme, and the CC-adding enzyme terminates RNA polymerization.