The CCA-adding enzyme is the smartest enzyme among a large number of template-independent RNA polymerases. It adds and/or synthesizes the defined sequence CCA onto the 3' end of tRNA without aid of nucleic acid template. The ternary structure of the enzyme complexed with tRNA primer lacking terminal A and incoming ATP analog has been determined. The structure shows the base moiety of ATP stacks with base moiety of the 3' end of tRNA primer and is recognized by complement amino acid residues through " Watson-Crick " like base pairing. These results suggest that the complex of RNA and protein together compose the template for the incoming ATP and the collaboration of RNA and protein determines the nucleotide specificity of the enzyme.