VIII. PROTEIN SYNTHESIS AND GENE ACTION 359 



90% pure. It seems likely that individual fractions of T-RNA specific 

 for single amino acids will eventually be isolated. 



A different api)roach which has provided evidence for amino acid- 

 specific RNA chains was first reported by Hecht et al. (1959) and 

 Preiss et al. (1959). These authors showed that all T-RNA chains had 

 the same terminal nucleotides (RNA-pCpCpA) and that amino acids 

 were attached to the terminal adenosine. Since individual amino acids 

 were bound to RNA only to a specific extent and there was no competi- 

 tion for sites among various amino acids, amino acid-specific RNA 

 molecules were assumed to be present (based on an unbranched RNA 

 molecule). This approach was amplified by forming a single amino 

 acyl-RNA (valyl-RNA) and then oxidizing with periodate. The amino 

 acid should have protected only the specific RNA. This was tested 

 (Preiss et al., 19591 by removing the bound amino acid with dilute 

 alkali and showing that incorporation of only a single amino acid (C^*- 

 valine) into the treated RNA was possible. A number of amino acids 

 protected only their own sites against periodate oxidation. 



The interest in separation of amino acid-specific T-RNA molecules 

 stems from the hypothesis (Crick, 1958) that the nucleic acid of the 

 ribosome contains a "code" which specifies the amino acid sequence of 

 the protein being made. This code resides in the sequence of the RNA 

 nucleotides. An amino acid, however, according to this idea, cannot be 

 coded for directly by RNA. An "adaptor" molecule which can hydrogen 

 bond with the ribosomal template coding unit is required to transport 

 the amino acid to its proper position in the peptide chain. This adaptor 

 is T-RNA. Each individual T-RNA must therefore carry the code or 

 more likely the complement of the code for the single amino acid it 

 binds. Further discussion of this concept is given below, but its conse- 

 quences for T-RNA structure and attempts to study these experimentally 

 are appropriate here. 



One consequence of the adaptor hypothesis is that differences in 

 amino acid-specific RNA molecules reside in their base sequence and not 

 in secondary structure or molecular weight. This ajijiears to be correct, 

 since T-RNA is fairly homogeneous in size, and while it has a consid- 

 erable amount of secondary structure, evidence that secondary structure 

 is not required for activity has been reported (see Berg, 1961, for further 

 discussion). Several studies have now appeared which demonstrate 

 considerable variation in the base composition of different T-RNA 

 fractions. An ingenious combination of labeling techniques, enzymatic 

 digestion, and isolation of labeled fragments has been used to study the 

 nucleotide sequences near the adenosine end of T-RNA without prior 



