302 RICHARD .SCinVKKT AM) .lOllX BISHOP 



activating enzyme. It is likely that a (l('<!;('nci-atc code linoi-c Ihaii one 

 code word for a single ainiiio acidj is the exi)lanatiun here. It should Ix; 

 possible to determine whether these RNA chains contain different codes 

 for leucine by study of the transfer system (see below).* 



The synthesis of T-RNA is a problem which is so far unresolved. If 

 a specific sequence of bases is involved in the amino acid transfer 

 function of T-RNA, this information might be expected to come from 

 DNA. However, in contrast to results with "messenger" RNA, no system 

 for synthesizing T-RNA is known and even the site of synthesis in the 

 cell is uncertain. Herbert and Canellakis (1961) have reported experi- 

 ments with soluble RNA which has lost ability to form amino acyl-RNA 

 compounds after treatment with snake venom diesterase. The capacity 

 to accept amino acids was restored by incubation with solul)le enzymes 

 and all four (in some cases only three) ribonucleoside triphosphates. 



Resynthesis of only the CCA end did not explain the restoration 



of activity. However, whether a specific base sequence was synthesized, 

 or whether large oligonucleotides were joined, is uncertain. Further 

 studies of this system may clarify the problem of T-RNA synthesis. 



III. Steps Requiring Participation of the Ribosome 



A great deal of evidence has accumulated which points to the ribo- 

 some as the site of protein synthesis. This evidence consists of kinetic 

 studies where intact cells were incubated with a C^^-amino acid and at 

 various time periods the specific activity of various cell fractions was 

 examined. The ribosomes were the first to be labeled and then reached a 

 steady state of radioactivity consistent with the hypothesis that most 

 soluble proteins were being .synthesized in these particles (Raacke, 1961; 

 Hoagland, 1960). When specific proteins were examined, again ribosomes 

 were found to contain protein of highest specific activity (Rabinowitz 

 and Olson, 1959; Peters, 1957; Siekevitz and Palade, 1960; Morris and 

 Dickman, 1960). Studies of "nascent" enzymes on bacterial and yeast 

 ribosomes indicate a similar situation in microorganisms (Kihara et al., 

 1961; Cowie et al., 1961). The labeling of specific proteins in the cell-free 

 ribosoraal system under discussion has also been reported and constitutes 

 the strongest evidence for protein synthesis in these systems (Schweet 

 et al, 1958b; (;ami)bell et al, 1960; Ogata et al, 1960; Suzuka and 

 Shimura, 1960; Kameyama and Novelli, 1960). 



In this section we shall attempt to review the evidence concerning 

 various steps whirli ](\id to the synthesis of the peptide chain in the 



*Wei.sblum. Bcnzer. and Hollcv Imxc now dfrnonstratod this rosiilt (jirivate 

 communiration). 



