Yeast Ribosomal Protein Deletion Mutants Possess Altered Peptidyltransferase Activity and Different Sensitivity to Cycloheximide

The major function of the ribosome is its ability to catalyze formation of peptide bonds, and it is carried out by the ribosomal peptidyltransferase. Recent evidence suggests that the catalyst of peptide bond formation is the 23S rRNA of the large ribosomal subunit. We have developed an in vitro system for the determination of peptidyltransferase activity in yeast ribosomes. Using this system, a kinetic analysis of a model reaction for peptidyltransferase is described with Ac-Phe-tRNA as the peptidyl donor and puromycin as the acceptor. The Ac-Phe-tRNA−poly(U)−80S ribosome complex (complex C) was isolated and then reacted with excess puromycin to give Ac-Phe-puromycin. This reaction (puromycin reaction) followed first-order kinetics. At saturating concentrations of puromycin, the first-order rate constant (k 3) is identical to the catalytic rate constant (k cat) of peptidyltransferase. This k cat from wild-type yeast strains was equal to 2.18 min-1 at 30 °C. We now present for the first time kinetic evidence that yeast ribosomes lacking a particular protein of the 60S subunit may possess significantly altered peptide bond-forming ability. The k cat of peptidyltransferase from mutants lacking ribosomal protein L24 was decreased 3-fold to 0.69 min-1, whereas the k cat from mutants lacking L39 was slightly increased to 3.05 min-1 and that from mutants lacking both proteins was 1.07 min-1. These results suggest that the presence of ribosomal proteins L24 and, to a lesser extent, L39 is required for exhibition of the normal catalytic activity of the ribosome. Finally, the L24 or L39 mutants did not affect the rate or the extent of the translocation phase of protein synthesis. However, the absence of L24 caused increased resistance to cycloheximide, a translocation inhibitor. Translocation of Ac-Phe-tRNA from the A- to P-site was inhibited by 50% at 1.4 μM cycloheximide for the L24 mutant compared to 0.7 μM for the wild type.