Puromycin causes release of growing polypeptide chains from ribosomes. Polyribosomes are not degraded during a short incubation with puromycin, but are degraded after longer incubations. The inhibitors of protein synthesis cycloheximide or sparsomycin, when added after puromycin, prevent polyribosome degradation. This indicates that ribosomes initiate new chains after premature termination by puromycin. Peptide chains initiated from internal positions have been isolated and studied. To label preferentially these chains the inhibitor of initiation NaF has been used. These incomplete chains aggregate and are markedly unstable, whereas incomplete chains initiated at the N-terminus and released prematurely by puromycin are stable. This differential stability may be explained by conformational differences between the two types of chains.
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