Properties of phenylalanine transfer ribonucleic acid with modified 3′-terminal end in protein biosynthesis using a rabbit reticulocyte cell-free system: Effect of the replacement of cytidine residues from the CpCpA end of tRNA by 5-iodocytidine or 2-thiocytidine

Phe-tRNA^phe from yeast containing 2-thiocytidine or 5-iodocytidine in position 75 of the polynucleotide chain or Phe-tRNA^phe in which both positions 74 and 75 were substituted by 5-lodocytidine were investigated in the poly U-dependent polyphenylalanine synthesis on ribosomes from rabbit reticulocytes. Phe-tRNA^phe Cps²CpA was nearly as active as the native Phe-tRNA^phe-CpCpA in the overall process. Phe-tRNA^Phe-Cpi⁵CpA as well as Phe-tRNA^Phe-i⁵Cpi⁵CpA were considerably less active than the native species. In vestigation of individual steps of protein biosynthesis with these modified substrates revealed that the donor activity of peptidyl-tRNAs which contain 5-iodocytidine in their 3′-terminus is strongly impaired suggesting exacting structural requirements for the interaction of the C^pC^pA end of tRNA with the ribosomal P-site.