1. The ability of Escherichia coli ribosomes to function in poly(U)-directed protein synthesis was measured at elevated temperatures by using thermostable supernatant factors from Bacillus stearothermophilus. The amount of polyphenylalanine synthesized at 55°C was about the same as at 37°C, but the rate of synthesis was increased approximately fivefold. At 60°C the activity of the ribosomes was halved. 2. E. coli ribosomes can sustain the loss of approx. 10% of the double-helical secondary structure of RNA without losing activity. 3. Within the active ribosome the double-helical secondary structure of the rRNA moiety is stabilized compared with isolated rRNA, as judged by enzymic hydrolysis and by measurements of E260. 4. The main products, over the range 0–55°C, of ribonuclease T1 digestion of the smaller subribosomal particle of E. coli were two fragments (s020,w 15S and 25.3S) of approximately one-quarter and three-quarters of the size of the intact molecule, revealing the presence of a ‘weak spot’ where intramolecular bonds appear insufficient to hold the fragments together. 5. Subribosomal particles of B. stearothermophilus were more stable to heating, by approx. 10°C, than those of E. coli, and the stabilization of double-helical secondary structure of the RNA moiety was more striking. 6. Rabbit reticulocyte ribosomes were active in poly(U)-directed protein synthesis at 45°C, and half the activity was lost after heating to 53°C. Active subribosomal particles of rabbit reticulocytes and of oocytes of Xenopus laevis, like the bacterial subribosomal particles, underwent a conformational change to a slower-sedimenting form on heating. The temperature range of the transition depended on the species. 7. Slower-sedimenting particles, whether produced by EDTA treatment or by heating, had different ‘melting’ profiles compared with active subribosomal particles, providing another indication of conformational differences. 8. Comparison of the properties of the various subribosomal particles revealed greater variation in the secondary structure of the protein moieties (judged by measurement of circular dichroism) than in the secondary structure of the RNA moieties, which appeared to have features in common.

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