CCA-adding enzyme

UniProtKB accession:  O28126

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UniProtKB description:  Catalyzes the addition and repair of the essential 3'-terminal CCA sequence in tRNAs without using a nucleic acid template (PubMed:14592988, PubMed:25640237). Adds these three nucleotides in the order of C, C, and A to the tRNA nucleotide-73, using CTP and ATP as substrates and producing inorganic pyrophosphate (PubMed:14592988, PubMed:25640237). tRNA 3'-terminal CCA addition is required both for tRNA processing and repair (PubMed:25640237). Also involved in tRNA surveillance by mediating tandem CCA addition to generate a CCACCA at the 3' terminus of unstable tRNAs (PubMed:25640237). While stable tRNAs receive only 3'-terminal CCA, unstable tRNAs are marked with CCACCA and rapidly degraded (PubMed:25640237). The structural flexibility of RNA controls the choice between CCA versus CCACCA addition: following the first CCA addition cycle, nucleotide-binding to the active site triggers a clockwise screw motion, producing torque on the RNA (PubMed:25640237). This ejects stable RNAs, whereas unstable RNAs are refolded while bound to the enzyme and subjected to a second CCA catalytic cycle (PubMed:25640237).