A Test of tRNA as Amino Acid Adaptor in Hemoglobin Synthesis

In the course of protein synthesis, the amino acids are arranged in a very regular order in the protein molecule. The mechanism by which the sequential order is achieved has been postulated to be the attachment of the amino acid to an adaptor molecule (Crick, 1958; Hoagland, 1959). The sequential order is then determined by the interaction of the adaptor molecule with mRNA. The adaptor hypothesis was confirmed irectly by Chapeville t al. (1962). By catalytic reduction, Cl~-cysteinyl-tRNA cys (E. coli) was converted to C14-alanyl-tRNA cys (E. coli). The incorporation of the C14-alanine into protein was stimulated by poly UG, which normally stimulates the incorporation of cysteine but not alanine. A further test of the adaptor hypothesis was made by use of a naturally occurring messenger RNA by allowing C14-alanyl-tRNA cys (E. coli) to transfer alanine to ribosomes from rabbit reticulocytes. Only one cysteine residue occurs in rabbit hemoglobin and the C14-alanine was detected only in the appropriate cysteine-containing pcptide (yon Ehrenstein, Weisbtum, and Benzer, 1963). The present study is another test of the adaptor hypothesis. Another means for producing an amino acid attached to a tRNA of a different specificity is afforded by one of the phenylalanyl-tRNA synthetases of Neuroslaora crassa. This enzyme interacts with two tRNAs of E. coli to produce phenylalanyl-tRNA TM and phenylalanyl-tRNA Ala (Barnett and Jacobson, 1964). When these were tested for binding to ribosomes, they were bound preferentially in the presence of poly UG and poly UCG, as is the case for the normal binding of valyl-tRNA TM and alanyl-tgNA *la, rather than by poly U, as would be the case for phenylalanyl