All classes of tRNA-mediated nonsense suppression are much more efficient in yeast cell-free lysates prepared from a [psi +] strain than in those prepared from an isogenic [psi −] strain. Mixed [psi +]/[psi −] lysates do not support efficient suppression. Fractionation of the [psi −] lysate demonstrated the presence of an inhibitor of in vitro suppression that is loosely associated with the 80 S ribosome. The data indicate that the inhibitor is a factor involved in the termination of translation in this simple eukaryote.
