THE SYNTHESIS OF PROTEINS 2/ 



mined by the host. Thus tobacco mosaic virus with oxidized SH groups 

 (Anson and Stanley, 1941) or the acetyl or phenyluriedo derivative of 

 tobacco mosaic (Miller and Stanley, 1941) all give rise to the original 

 tobacco mosaic protein and not to the modified form which was used 

 for inoculation. 



In these cases the type of virus formed is determined by the host. 

 The changes in yellow fever virus after passage through mouse brain 

 (Lloyd, Theiler, and Ricci, 1936) and of equine encephalitis virus 

 grown in pigeons (Traub and Ten Broeck, 1935) may be other examples 

 in which the nature of the virus is determined by the host. This subject 

 has been reviewed in detail by Stanley (1943). Stanley considers these 

 changes in the virus to be mutations. 



On the other hand, proof of the identity of the virus produced by two 

 different hosts rests in most cases only on immunological and pathologi- 

 cal properties. The same tests fail to differentiate between pepsin formed 

 from cattle or swine pepsinogen (Seastone and Herriott, 1937). The 

 differences between the two enzymes were established only by a mixed 

 solubility determination (Northrop, 1939, p. 32). Such tests have not 

 been made with virus proteins. In any case there is nothing to indicate 

 that the same precursor may not exist in many different species. For 

 example the crystalline lens proteins of many species are indistinguish- 

 able by immunological reactions and may be identical in various species. 



The assumption of a precursor has also been criticized on the grounds 

 that its presence would be detected by immunological tests. This objec- 

 tion sounds very reasonable on a priori grounds, but as a matter of fact 

 only a questionable cross-reaction could be found between pepsin and 

 its precursor, pepsinogen^ (Seastone and Herriott, 1937), or chymo- 

 trypsin and its precursor, even with the classical Dale technique (Ten 

 Broeck, 1934; cf. page 16). These experiments were carried out with 

 concentrated solutions of both enzyme and precursor and it is probable 

 that no cross-reaction would have occurred with crude tissue extracts. 

 If the proteinogen is a denatured protein, such cross-reactions would 

 not be expected. 



Recent work (Knight, 1946) has demonstrated that influenza virus 

 grown on chicken embryos reacts with normal chicken protein antiserum, 



"^ These results, however, are open to the objection that the pepsin was denatured at 

 the /iH of the blood which the pepsinogen was not. Denaturation is known to modify 

 the specificity of proteins, and hence the failure to obtain a cross-reaction in this case 

 is not conclusive. The failure to differentiate bovine and swine pepsin is also complicated 

 by the fact that both proteins were denatured. This objection, however, does not apply 

 to Ten Broeck's results with chymo-trypsin. 



