37. NUCLEIC ACID AND PROTEIN SYNTHESIS 383 



pH 5. Van der Decken and Hultin 175 found, however, that the hydrolysis 

 of RNA under these conditions reduced the pH 5 precipitability of the 

 activating enzymes so that the effect was a loss of enzyme. This may be 

 part of the explanation of the findings of the Japanese workers. However, 

 they do obtain inhibition of activation (about 15 %) by direct preincubation 

 of the enzyme preparation with ribonuclease (omitting the reprecipitation 

 step) and conclude that the activation of some amino acids may involve 

 RNA. This they further support by preliminary results suggesting that 

 the PP 32 moiety of ATP becomes linked to sRNA and that an amino 

 acid mixture reduces this labeling. The picture evolved is that there may 

 be an activation pathway in which ATP first pyrophosphorylates sRNA, 

 followed by substitution of the PP group by amino acid.] 



Second, it appears highly probable that each amino acid is activated 

 by its own single specific enzyme. Several relatively pure activating enzymes 

 have now been isolated and the work continues. The fact that amino 

 acids do not compete for enzyme sites indicates that in all likelihood the 

 remaining enzymes will be isolated. Third, the activated amino acyl adenyl- 

 ate intermediate is very firmly bound to its specific enzyme and dissociates 

 to only a very small extent in the absence of an acceptor for the amino 

 acid. Thus the activating enzyme and its bound amino acyl adenylate 

 can be thought of as acting effectively as a single unit. 



The formulation of Eq. (5) in Fig. 3 indicates that the transfer of amino 

 acid to sRNA is mediated directly by the activating enzyme without the 

 involvement of other enzymes. This conclusion is strongly supported by 

 the work of Berg's group (cf. Symposium 13 ) which has shown that during 

 the course of a 100-fold purification of an amino acid activating enzyme, 

 there was a parallel increase in the ability of fractions to catalyze acti- 

 vation and transfer of the amino acid to sRNA. It has also been shown 

 by Berg, 13 and by Schweet, 13 that purified activating enzymes will more 

 readily transfer to sRNA that amino acid which they activate. Further- 

 more, there is evidence that synthetic amino acyl adenylates may serve 

 as a source of amino acid on transfer RNA provided they are first bound 

 to activating enzymes. 176 



An anomalous situation has been reported by Berg and his associates. 177 

 They have obtained relatively pure methionine activating enzymes from 

 two sources, E. coli and yeast. The former enzyme will attach over two 

 times more methionine than the latter to E. coli sRNA. This peculiar 

 result is puzzling and has led to the suggestion that perhaps there are 



175 A. van der Decken and T. Hultin, Exptl. Cell Research 17, 188 (1959). 



176 K. K. Wong, A. Meister, and K. Moldave, Biochim. Biophys. Acta 36, 531 (1959). 



177 F. H. Bergmann, P. Berg, J. Preiss, E. J. Ofengand, and M. Dieckmann, Federa- 

 tion Proc. 18, 751 (1959); and unpublished data, (1959). 



