Exposure of Escherichia coli MRE 600 cells suspended in standard buffer (Tris-acetate (0.01 m, pH 7.8)-MgAc2, (0.01 M)-NH4Cl (0.06 M)-β-mercaptoethanol (0.002 m)) to 53° for 15 min causes a two- to threefold increase in the poly(phenylalanine) synthesis activity of their ribosomes. Binding of [3H]poly(U) and [3H]Phe-tRNA by ribosomes obtained from bacteria heated at 53° was similar to that of control ribosomes obtained from bacteria heated at 37°. Peptidyl-transferase activity, on the other hand, increased significantly. The degradation of poly(U) in the poly(phenylalanine) synthesis reaction mixture was lower with ribosomes obtained from heated cells as compared to control ribosomes. Therefore, the in vivo thermal activation of ribosomes is related both to the increase in peptidyltransferase and the decrease in nucleolytic activities. The activated ribosomes showed a normal sedimentation profile. Similar thermal treatment of the bacteria suspended in other media (for example, potassium phosphate, 0.25 m, pH 7.2) results in complete degradation of the 30S ribosomal subunit. The 50S particles which remain intact retain their peptidyltransferase activity.
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