To identify trans-acting factors involved in mRNA decay in the yeast Saccharomyces cerevisiae, we have begun to characterize conditional lethal mutants that affect mRNA steady-state levels. A screen of a collection of temperature-sensitive mutants identified ts352, a mutant that accumulated moderately stable and unstable mRNAs after a shift from 23 to 37 degrees C (M. Aebi, G. Kirchner, J.-Y. Chen, U. Vijayraghavan, A. Jacobson, N.C. Martin, and J. Abelson, J. Biol. Chem. 265:16216-16220, 1990). ts352 has a defect in the CCA1 gene, which codes for tRNA nucleotidyltransferase, the enzyme that adds 3' CCA termini to tRNAs (Aebi et al., J. Biol. Chem., 1990). In a shift to the nonpermissive temperature, ts352 (cca1-1) cells rapidly cease protein synthesis, reduce the rates of degradation of the CDC4, TCM1, and PAB1 mRNAs three- to fivefold, and increase the relative number of ribosomes associated with mRNAs and the overall size of polysomes. These results were analogous to those observed for cycloheximide-treated cells and are generally consistent with models that invoke a role for translational elongation in the process of mRNA turnover.