380 RICHARD SCHWEET AND JOHN BISHOP 



mechani:>in would ix'i'iuit a I'citain aiiiouiil of |)e|)ti<lc Ixjiid I'oiniatioii 

 in the presence of puroniycin, which might result in the formation of 

 small peptides or earlier intermediates; Init once the larger pei)tid(' 

 chains in the ribosome were released, no incorporation into TCA- 

 precipitable material would occur. The locus of puromycin action is 

 considered to be at the growing point of the chain, which is interrupted 

 at a specific site, possibly when an aromatic amino acid appears in the 

 sequence. This last result is suggested from the fact that neither the 

 leucine analog of puromycin (Rabinowitz and Fisher, 1962), or the free 

 aminonucleoside (A. Morris and R. Schweet, unpublished data) was 

 active. The former authors have studied the effects of puromycin on 

 intact ascites cells. Incorporation into ribosomes was inhibited, but the 

 formation of labeled soluble protein continued. These results were 

 attributed to release of incomplete proteins. However, no release of 

 protein from the ribosomes was obser\-ed witli intact reticulocytes in this 

 case (Rabinowitz and Fisher, 1962). 



Inhibition of incorporation into i)rotein })y puromycin in the transfer 

 system from E. coli has been studied (Nathans and Lipmann, 1961; 

 Nathans et al., 1962). These authors observed degradation of amino 

 acyl-RNA in the presence of puromycin. This de-acylation was de- 

 pendent on ribosomes, GTP, and soluble enzymes. It is possible that the 

 de-acylation may be a secondary phenomenon resulting from the incor- 

 poration of the C^*-amino acid of the amino acyl-RNA into small 

 peptides and other intermediates in the ribosome which are then released 

 in the presence of puromycin. Material which was not free amino acid 

 was indeed noted by Nathans et al. (1962). Thus, an effect at the first 

 stages of transfer may reflect the primary action of puromycin on an 

 intraribosomal event, as noted earlier in connection W'ith GTP action, 

 where a cofactor required for transjer is believed to act on an intraribo- 

 somal stage. 



To conclude this section on "release," it should be noted that the 

 discussion has been concerned only with the fomiation of the peptide 

 chain sequence. Specific disulfide bond and tertiary structure formation 

 is currently considered to follow spontaneously (Crick, 1958). The 

 evidence for this viewpoint has been discussed by Berg (1961). 



IV. Information Transfer in Protein Synthesis 



In this section, an attempt will be made to review in some detail 

 the question of how^ the information in DNA specifies a particular amino 

 acid sequence. That DNA is the controlling factor, with some possible 

 exceptions to be discussed, has become clear (see Berg, 1961, for a recent 

 discussion). The mechanism bv which DNA transfers this information is 



