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The shift of recessive suppressor mutant of yeast Saccharomyces cerevisiae from permissive to restrictive conditions is accompanied by polysome decay and accumulation of 80 S ribosomes (Smirnov et al., 1976). In this paper some properties of 80 S ribosomes are studied. It is demonstrated that polysome decay under non-permissive conditions is not the consequence of the impairment of RNA synthesis. More than 70% of 80 S ribosomes accumulated under non-permissive conditions contain bound peptidyl-tRNAs localized in P-ribosomal site. tRNA moiety of bound peptidyl-tRNA is able to accept all 20 natural amino acids after chemical deacylation. Therefore it is not a specific isoacceptor species but rather total tRNA that is bound to ribosomes. The polypeptide residues of these peptidyl-tRNAs are heterogeneous in size. Their molecular weights are comparable with the molecular weights of the completed polypeptides. Some of the 80 S ribosomes accumulated under non-permissive conditions contain poly-A+ RNA. In conclusion, possible mechanism of the impairment of translation under non-permissive conditions in recessive suppressor strain is discussed.

The influence of peptidyl-tRNA on the dissociation of yeast 80 S ribosomes into subunits was studied. For this purpose temperature-sensitive (ts) suppressor strain of yeast Saccharomyces cervisiae carrying a defect in peptide chain termination was used. It was found that peptidyl-tRNA did not influence the dissociation of ribosomes either at high salt concentration or in the presence of dissociation factor (DF) from yeast. After dissociation of yeast ribosomes in 0.5 M KCl, peptidyl-tRNA remains bound to the 60 S subunit. Some characteristics of the termination process and release of nascent polypeptides from yeast ribosomes are discussed.

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