Actinomycin D inhibits hemoglobin synthesis in rabbit reticulocyte lysates and binds to a 15 S cytoplasmic particle involved in the initiation step of translation.

Actinomycin D inhibits hemoglobin synthesis in rabbit reticulocyte lysates by affecting initiation of globin polypeptide chains. To obtain information on the distribution of actinomycin-sensitive structures among the components of the translation machinery, a reticulocyte lysate is incubated with [3H]actinomycin. The tritium label is found associated with ribosomes actively engaged in protein synthesis and with a 15 S cytoplasmic particle. During cell-free incubation or when initiation is inhibited by aurintricarboxylic acid, the labeling of the 15 S particles increases, suggesting that they are relased from ribosomes into the cytoplasm and play a role in the initiation process. The 15 S cytoplasmic particles contain a DNAase-sensitive component, probably complexed with ribonucleic acids and proteins. A possible role of these DNA-containing particles in the initiation step of translation is discussed.