Characterization of a temperature-sensitive mutation that impairs the function of yeast tRNA nucleotidyltransferase

ATP(CTP) : tRNA nucleotidyltransferase catalyses the posttranscriptional addition of cytidine, cytidine and adenosine to the 3' ends of tRNAs. Previously, a temperature-sensitive phenotype in Saccharomyces cerevisiae resulting from a mutation in the CCA1 gene coding for this enzyme was identified. Here, we show that a single guanine-to-adenine transition in cca1-1 generates the temperature-sensitive phenotype. Alignment of the amino acid sequence of S. cerevisiae tRNA nucleotidyltransferase with other tRNA nucleotidyltransferases for which crystal structures have been solved suggests that the resulting Glu-to-Lys substitution is in a ss-turn connecting the structurally and functionally important head and neck domains of the protein. Proteins containing Gln, His or Phe at this position were constructed to further characterize the importance of this residue in enzyme structure and function. As with the Lys variant, the Phe and His variants generate a temperature-sensitive phenotype in isogenic yeast strains, further supporting the role of this position in maintaining the structure and function of this enzyme. Comparative biophysical and biochemical characterization of both the wild-type and variant proteins indicates that amino acid substitutions at this position can result in a structural change in the protein that reduces enzyme activity (both at the permissive and non-permissive temperatures), decreases the melting temperature of the protein and alters its stability at the non-permissive temperature (37 degrees C).