The Ribosome-Bound Chaperones RAC and Ssb1/2p Are Required for Accurate Translation in Saccharomyces cerevisiae

Abstract

The chaperone homologs RAC (ribosome-associated complex) and Ssb1/2p are anchored to ribosomes; Ssb1/2p directly interacts with nascent polypeptides. The absence of RAC or Ssb1/2p results in a similar set of phenotypes, including hypersensitivity against the aminoglycoside paromomycin, which binds to the small ribosomal subunit and compromises the fidelity of translation. In order to understand this phenomenon we measured the frequency of translation termination and misincorporation in vivo and in vitro with a novel reporter system. Translational fidelity was impaired in the absence of functional RAC or Ssb1/2p, and the effect was further enhanced by paromomycin. The mutant strains suffered primarily from a defect in translation termination, while misincorporation was compromised to a lesser extent. Consistently, a low level of soluble translation termination factor Sup35p enhanced growth defects in the mutant strains. Based on the combined data we conclude that RAC and Ssb1/2p are crucial in maintaining translational fidelity beyond their postulated role as chaperones for nascent polypeptides.

This work was supported by the Kultusministerium des Landes Sachsen-Anhalt (to S.R.), the Fonds der Chemischen Industrie (to S.R.), and by SFB 388 and 610 (to S.R.).

We thank Suzanne Ross, Andrej Mun, and Tina Wölfle for excellent technical assistance. We thank Magda Boguta for yeast strain GT197. We also thank Agnieszka Chacinska, Yves Dubaquié, Matthias Gautschi, Klaus Pfanner, and members of the laboratory for discussion and critically reading the manuscript.