Mechanism of puromycin action: fate of ribosomes after release of nascent protein chains from polysomes.

The exchange of ribosomal subunits during the release of growing polypeptide chains by puromycin has been investigated in a bacterial cell-free system engaged in protein synthesis. The addition of spermidine, used as a stabilizing agent of 70S monomers, caused a strong inhibition of the subunit exchange. This result led us to conclude that upon premature release of unfinished protein chains by the antibiotic, the ribosomes fall off mRNA as 70S particles. This behavior is different from that occurring during physiological termination of translation, where the ribosomes detach in a dissociated form. Some implications of the postulated mechanism are also discussed.