RNA codons and protein synthesis. Xi. Template activity of modified RNA codons

Substituting 5'-, 2'-, 3'-terminal or 2'-internal ribose hydroxyls of oligonucleotides markedly affected their template activity in directing the binding of AA-sRNA to ribosomes. The relative template activity of oligo U preparations was as follows: p-5'-UpUpU > UpUpU > CH3O-p-5'-UpUpU > UpUpU-3'-p > UpUpU-3'-p-OCH3 > UpUpU-2',3'-cyclic phosphate. Trimers with (2'-5') phosphodiester linkages, (2'-5')-UpUpU and also (2'-5')-ApApA, did not serve as templates for phenylalanine- or lysine-sRNA, respectively. The relative template efficiency of oligo A preparations was as follows: p-5'-ApApA > ApApA > ApApA-3'-p > ApApA-2'-p. The hexamer, ApApApApApA was considerably more active as a template than the corresponding pentamer. These data indicate that two adjacent triplets are recognized by two AA-sRNA molecules bound to nearby ribosomal sites. A doublet with 5'-terminal phosphate, pUpC, served as a template for serine-sRNA, whereas a doublet without terminal phosphate, UpC, did not. Although the template efficiency of pUpC was lower than that of the triplet UpCpU the data show that serine-sRNA can recognize pUpC.