The release of [14C]amino acid-containing polypeptides from rabbit reticulocyte ribosomes by puromycin has been studied and compared to the release occurring in a hemoglobin synthesizing cell-free system. Unlike the more naturally occurring release of a completed peptide chain, the puromycin-produced release of polypeptide from the ribosome does not require an ATP-generating system or the supernatant fraction, and occurs maximally in 1 or 2 min. Thus, completion of a chain is apparently a requisite for the naturally occurring release, but not for puromycin release. The polypeptide released by puromycin is not hemoglobin but resembles the completed molecule in having N-terminal valine. [14C]Puromycin, synthesized by methylating the tyrosyl analog with [14C]diazomethane is bound by the soluble polypeptide released from the ribosome. Saturation of the binding site on the puromycin-released polypeptide occurs at a puromycin concentration equivalent to that producing maximal inhibition of [14C]amino acid incorporation in a cell-free system and maximal release of polypeptide from the ribosomes. One residue of puromycin is bound to the released polypeptide for every N-terminal valine or one residue per polypeptide chain. It is hypothesized that puromycin may displace the soluble RNA binding the polypeptide chain to the ribosome in its action in releasing the chain from the ribosome.
Bibliographical noteFunding Information:
Support was provided by grants from the Atomic Energy Commission (Contract No. AT(3o-I)-2643) and from the National Cancer Institute, National Institutes of Health (Contract No. CRTY 5o18).