Alan R. Williamson

ROLE OF THE GENETIC MESSAGE IN POLYRIBOSOME FUNCTION.

Studies of the properties of polyribosomes from puromycin-treated rabbit reticulocytes have supplied confirming evidence for the dynamic model of polyribosome function. A net, orderly increase of polyribosomes has been shown to occur in the cell-free system by attachment of monomeric ribosomes to the genetic message at the codon corresponding to the N-terminal amino acid of globin. Ribosomes which lose their growing peptide chain at any point along the messenger RNA can initiate a new chain at that point, and these incomplete chains can be released when the ribosome reaches the end of the genetic message. Thus we conclude that: (1) a polypeptide chain is normally initiated only at the N-terminal amino acid because an 80 s ribosome can attach only at the corresponding point on the messenger; (2) for release of a peptide chain it is both necessary and sufficient that the ribosome reach the end of the genetic message. The effect of puromycin on polyribosomes is not a direct one caused by the release of the growing peptide chain by puromycin, for the release can occur under conditions where the polyribosomal structures are unaffected by puromycin. While the synthesis of only a few peptide bonds will suffice for chain release by puromycin, much more extensive peptide bond formation must continue for puromycin to accelerate polyribosome breakdown. The action of puromycin is discussed in relation to modulation of the rate of protein synthesis, and it is proposed that the effect of puromycin on polyribosomes is due to an increased rate of travel of ribosomes along the genetic message.

Homopolynucleotide inhibition of poly U-dependent polyphenylalanine synthesis☆

Abstract Inhibition by homopolynucleotides of polyphenylalanine synthesis and phenylalanyl-RNA binding to ribosomes has been studied in a poly U-dependent, reticulocyte cell-free system. Inhibition by poly A was due to the formation of triple-stranded helical complexes with poly U, whereas poly I inhibited by attaching to ribosomes. Although poly A-poly U complex formation inhibited ribosome attachment to poly U, once attachment had occurred, the subsequent formation of A-U complexes did not inhibit peptide bond formation.