Studies on polynucleotides: LVI. Further syntheses, in vitro, of copolypeptides containing two amino acids in alternating sequence dependent upon DNA-like polymers containing two nucleotides in alternating sequence

Abstract

The double-stranded DNA-like polymers, poly d-TC : AG and poly d-TG : AC, which contain, in each strand, the two specified deoxyribonucleotides in alternating sequence, have previously been shown to serve as templates for DNA-dependent RNA polymerase; and the products formed in the presence of sets of two appropriate ribonucleoside 5′-triphosphates, have been characterized as poly UC, poly AG, poly UG and poly AC containing, in every case, the two ribonucleotides in alternating sequence ( Nishimura, Jones & Khorana, 1965 ). The messenger activity of poly UG, poly AC and poly AG in the Escherichia coli cell-free amino acid-incorporating system has now been studied. Poly UG stimulates the incorporation of valine and cysteine, poly AC stimulates the incorporation of threonine and histidine and poly AG stimulates the incorporation of glutamic acid and arginine. The amino acid-incorporations have the following common features: (1) only two amino acids are incorporated; (2) the incorporation of the two amino acids is largely interdependent; and (3) the incorporation of the two amino acids occurs in equimolar amounts. The products of the in vitro syntheses have all been characterized, by chemical and/or enzymic methods, as copolypeptides containing the two amino acids in alternating sequence. From a combination of these and previous results, the following codon assignments can be made unequivocally: UGU, cysteine; GUG, valine; CAC, histidine; ACA, threonine; AGA, arginine and GAG, glutamic acid.