Nature of magnesium-induced miscoding: The in vitro synthesis of valine-tyrosine copolypeptide directed by poly (U-G)

Abstract

Poly (U-G) † † In this paper, abbreviations for polynucleotides have been used as recommended by the IUPAC Commission on Macromolecules. For example, poly (U 4,G) random polynuoleotide in which the molar proportions of uridylate to guanylate is 4:1; poly d(T-G) · d(A-C), double-stranded deoxyribonucleotide polymer which contains alternating thymidylate and guanylate units in one strand, and adenylate and cytidylate in the complementary strand; poly (U-G), single-stranded ribonucleotide polymer which contains alternating units of uridylate and guanylate. , containing a repeating dinucleotide sequence, stimulated the incorporation of tyrosine into a polypeptide if the cell-free polypeptide synthesis was carried out in the presence of 0.02 m-magnesium and/or in the presence of purified tyrosine tRNA. Under optimal conditions the valine-tyrosine copolypeptide was synthesized as a major product by poly (U-G), which normally directed the synthesis of the valine-cysteine copolypeptide. It can be concluded that the codon sequence, UGU is ambiguously read as tyrosine, indicating the misrecognition of the second letter. With the magnesium-induced ambiguity, poly (U-G) stimulated the incorporation of serine and arginine in addition to tyrosine. However, other ribopolynucleotides containing repeating dinucleotide sequences such as poly (U-C), poly (A-C) and poly (A-G) had no effect on magnesium-induced ambiguous amino acid incorporation.