Effects of cycloheximide on polyribosome function in reticulocytes

4 × 10 −6 m-Cycloheximide partially inhibited protein synthesis in reticulocytes, and caused a slight increase in the polyribosome content of the cells and the amount of nascent polypeptide material bound to polyribosomes. When reticulocytes were incubated with NaF, with chloramphenicol, or without added iron salts, polyribosomes were converted to single ribosomes. This conversion could be prevented by the addition of 4 × 10 −6 m-cycloheximide. When cycloheximide was added after polyribosome breakdown had taken place, the polyribosomes were re-formed. Re-formation of polyribosomes was accompanied by globin synthesis, and the polyribosomes formed carried nascent polypeptide chains. A higher concentration of cycloheximide (10 −4 m) caused breakdown of the polyribosomes during the first 20 minutes of incubation. However, polyribosomes breakdown did not take place when reticulocytes were incubated for 50 minutes with 1.5 × 10 −2 m-cycloheximide. 1.5 × 10 −2 m-cycloheximide prevented the breakdown of polyribosomes in cells incubated with NaF but did not cause polyribosome formation when added after breakdown had taken place. In reticulocytes incubated with 1.5 × 10 −2 m-cycloheximide, there was no detectable incorporation of amino acids into soluble protein or nascent polypeptide chains during 60 minutes of incubation. The results are consistent with the hypothesis that protein synthesis is accompanied by an exchange of ribosomes between the polyribosomal aggregates and the pool of single ribosomes, and that the detachment of ribosomes from the aggregates is slowed by 4 × 10 −6 m-cycloheximide and strongly inhibited by 1.5 × 10 −2 m-cycloheximide.