CCA-adding enzyme builds the 3′-end CCA of tRNA without a nucleic acid template. The mechanism for the maintenance of fidelity during the CCA-adding reaction remains elusive. Here, we present almost a dozen complex structures of the class I CCA-adding enzyme and tRNA mini-helices (mini-D 73N74, mini-D73N74C75 and mini-D73C74N75; D73 is a discriminator nucleotide and N is either A, G, or U). The mini-D 73N74 complexes adopt catalytically inactive open forms, and CTP shifts the enzymes to the active closed forms and allows N74 to flip for CMP incorporation. In contrast, unlike the catalytically active closed form of the mini-D73C74C75 complex, the mini-D73N74C75 and mini-D73C 74N75 complexes adopt inactive open forms. Only the mini-D73C74U75 accepts AMP to a similar extent as mini-D73C74C75, and ATP shifts the enzyme to a closed, active form and allows U75 to flip for AMP incorporation. These findings suggest that the 3′-region of RNA is proofread, after two nucleotide additions, in the closed, active form of the complex at the AMP incorporation stage. This proofreading is a prerequisite for the maintenance of fidelity for complete CCA synthesis.
- Template-independent polymerase
ASJC Scopus subject areas
- Molecular Biology
- Biochemistry, Genetics and Molecular Biology(all)
- Immunology and Microbiology(all)