How the CCA-Adding Enzyme Selects Adenine over Cytosine at Position 76 of tRNA

Abstract

Adding CCA Translation of a gene sequence into protein is mediated by transfer RNA (tRNA), which has a specific cytosine-adenine (CCA) tail to which amino acids attach and is recognized by enzymes. The tail, however, does not have a DNA template, and instead CCA-adding enzymes bolt on the additional nucleosides. Crystal structures have shown how these enzymes achieve specificity for cytosine, but we did not know how they select the final adenine until Pan et al. (p. 937 ) described CCA-adding enzyme structures captured at several stages of the reaction. Crystallized enzymes were complexed with a tRNA mimic and the respective cytosine or adenine triphosphate. The final adenine was discovered to be incorporated by the mediation of a single Mg 2+ ion in the enzyme, but no more cytosine could be attached because its triphosphate could not then get into the right position for the reaction.