THE NUMBER OF SOLUBLE RNA MOLECULES ON RETICULOCYTE POLYRIBOSOMES

Formylmethionine codon AUG as an initiator of polypeptide synthesis

Oligonucleotide messengers containing the sequence AUG at or near the 5' end of the chain stimulate the incorporation of N-formylmethionine,but not of free (unformylated) methionine. AUG is considerably more active in this regard than UUG, the other N-formylmethionine codon. The coding activity of subsequent in-phase triplets in the 3' direction from AUG is markedly stimulated by this codon.

RNA CODEWORDS AND PROTEIN SYNTHESIS. THE EFFECT OF TRINUCLEOTIDES UPON THE BINDING OF SRNA TO RIBOSOMES

A rapid, sensitive method is described for measuring C(14)-aminoacyl-sRNA interactions with ribosomes which are specifically induced by the appropriate RNA codewords prior to peptide-bond formation. Properties of the codeword recognition process and the minimum oligonucleotide chain length required to induce such interactions are presented. The trinucleotides, pUpUpU, pApApA, and pCpCpC, but not dinucleotides, specifically direct the binding to ribosomes of phenylalanine-, lysine-, and proline-sRNA, respectively. Since 5'-terminal, 3'-terminal, and internal codewords differ in chemical structure, three corresponding classes of codewords are proposed. The recognition of each class in this system is described. The template efficiency of trinucleotide codewords is modified greatly by terminal phosphate. Triplets with 5'-terminal phosphate are more active as templates than triplets without terminal phosphate. Triplets with 3'- or 3' (2')-terminal phosphate are markedly less active as templates. These findings are discussed in relation to the probable functions of terminal codewords. The modification of RNA and DNA codewords, converting sense into missense or nonsense codewords, is suggested as a possible regulatory mechanism in protein synthesis.