Enzyme structure and kinetics produce tRNA and nucleotide specificity of Schizosaccharomyces pombe CC- and A-adding enzymes.

Transfer RNAs (tRNAs) are transcribed and then processed through a series of post-transcriptional steps to produce mature forms that are charged with amino acids for translation. A CCA sequence is added at their 3' ends as the site of aminoacylation. Although a single enzyme typically adds the CCA tail without a nucleic acid template, Schizosaccharomyces pombe encodes two enzymes: CCA1 that adds the two cytosines and CCA2 that adds the adenosine. Here we explore how these two S. pombe enzymes evolved specificity to generate the 3' CCA tails. Enzymology (activity and kinetic assays) indicates distinct nucleotide addition specificity. CCA1 adds the sequential cytosines with a slower first addition followed by a quicker second addition. CCA2 then rapidly adds the terminal adenosine. Moreover, structural biology (crystal structures and molecular dynamics simulations) explains how the active site configuration and distances between active site and tRNA elbow binding residues of CCA1 and CCA2 restrict their tRNA substrate specificity. Together the data presented here describe how S. pombe CCA1 and CCA2 interrogate complete reaction complexes of tRNA and nucleotide substrates that arrange the particular components for active site catalysis and therefore specificity.