382 B. MAGASANIK 



the protein in such a manner that its charged groups are not exposed and 

 the charged groups on the surface of the protein are not affected. Such an 

 orderly arrangement of protein and PNA would not be expected to result 

 from the random interaction of the polymers without the influence of a 

 directing force. Nucleotropomyosin does not have a fixed composition.^^ 

 Exposure of this nucleoprotein to low pH results in the liberation of tropo- 

 myosin and in the accumulation of nucleotropomyosin of increased nucleic 

 acid content. Within large limits (10-20% of PNA) the physical properties 

 of these compounds which depend on surface charges are identical. When 

 the PNA content is increased to 30%, a nucleoprotein of lower isoelectric 

 point than ttie original nucleotropomyosin precipitates. 



The properties of nucleotropomyosin are those of a compound in which 

 a core of PNA is completely covered by protein. Apparently a large por- 

 tion of the protein can be peeled off without exposing the charged groups 

 of the PNA core, and therefore without altering those physical properties 

 of the complex which depend on surface charges. 



A similar relationship between protein and PNA has been discovered in 

 one of the crystalline plant viruses, turnip yellow mosaic. ^^ The sap of 

 infected plant leaves contains, in addition to the virus nucleoprotein, 

 another protein free of PNA but of similar size, shape, and crystal struc- 

 ture, and with identical electrophoretic behavior. The two proteins could 

 be separated by ultracentrifugation. They reacted quantitatively in the 

 same manner with antiserum directed against the nucleoprotein. However, 

 only the nucleoprotein could infect plants or cause the formation of anti- 

 bodies when injected into animals. Pancreatic ribonuclease did not attack 

 the intact nucleoprotein, but hydrolyzed the nucleic acid prepared from it 

 in the usual fashion. 



Here again it appears that the PNA contained in the nucleoprotein is 

 completely covered by protein so that the surface properties of the com- 

 plex, such as electrophoretic mobility and reaction with antibodies, are 

 exclusively determined by the nature of the protein component. The 

 stability of the nucleoprotein to the action of pancreatic ribonuclease 

 confirms the assumption that the PNA moiety is not easily accessible. The 

 biological properties of the nucleoprotein, such as infectivity and anti- 

 genicity, depend on the presence of the PNA. However the PNA alone, 

 separated from the nucleoprotein by denaturation of the protein with 

 dilute ethanol, is neither infective nor antigenic. It would seem that the 

 nucleoprotein possesses activities which are not the sum of the activities 

 of its constituent parts; in this sense the nucleoprotein is an ultimate 

 biological unit. Still, it must be borne in mind that we do not know whether 

 the isolated PNA is in its native state; treatment sufficiently drastic to 



" G. Hamoir, Biochem. J. 50, 140 (1952). 



