Amino acid adenylate-sRNA incorporation pathway

A, Relevance of Adenylate-sRNA-Ribosome Pathway to Amino Acid Incorporation in the Intact Cell.334... 

Let us consider the first question. Since it has been shown that amino acid-RNA compounds can be formed by highly purified amino acid activating enzymes, and the properties of this reaction and of the reaction product correspond closely to those observed in crude systems or in vivo, it seems to be established beyond reasonable doubt that the mechanism of this reaction is as outlined in Section III, B, 3, d. Furthermore, the demonstration that the amino acid bound to transfer RNA can be transferred to the microsomes and be bound there in the interior of a peptide chain, seems to show that the RNA-amino acid compound can serve as a donor of amino acid for the incorporation reaction however, since not only GTP, but also ATP and soluble fraction (which mi t contribute a large number of other factors besides the transferring enzyme) are required for the transfer, the role of sRNA-amino acid is less clear-cut than it might be. A reversal of the reaction back to the adenylates, however, and incorporation by some other route seems to be excluded by the fact that even a hundredfold excess of nonisotopic amino acid does not interfere with the efficiency of the transfer, which under the right conditions, approaches 100% (14S). The evidence to date, then, indicates that the adenylate-sRNA pathway is a pathway of amino acid incorporation in the microsomal system of mammalian origin. 

The fourth question takes us close to the core of the problem of the mechanism of protein s3uithesis, and if we knew the answer unequivocally, we could really say we knew something about the way proteins are made in vivo. Unfortunately, we do not have this information, and hence we do not know how far our knowledge of the isolated system applies. All we can say at the present time is that in mammalian systems the sRNA seems to be labeled in a manner sufficiently rapid to be consistent with a precursor role 186). The time scale employed may, however, be much too long to warrant any definitive deductions, and, furthermore, it has not been shown in vivo that the RNA is unlabeled as rapidly as it is labeled, which must be the case if it were an obligatory intermediate. Since the adenylate pathway has been studied so far only in relation to amino acid incorporation in the microsomal system in vitro, and not in relation to net protein synthesis, the ultimate answer to this question is, moreover, intimately tied to solvii the problem of the exact relationship between amino acid incorporation and protein synthesis. This will be discussed in the next section. 

Fig. 15. Over-all scbematio representation of the reactions involved in the adenylate-sRNA pathway of amino acid incorporation. Ei represents the amino acid activating enzymes E, the transferring enzymes and Es and E4, the enzymes responsible for the terminal incorporation into sRNA of ATP and CTP, respectively.

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