432 H. FRAENKEL-CONRAT 



For a more detailed discussion of this subject the reader is referred to a 

 review by Steere (1958), to whom this author is indebted for help in the 

 preparation of Table I, 



C. The Nature of Virus-Specific Components in Infected Plants 



Definite evidence has been obtained in recent years for the occurrence of 

 noninfectious virus-specific products in the case of at least two plant virus 

 diseases. The studies of Markham et al. (1948), as extended by Cosentino et 

 al. (1956), have clearly demonstrated that purified turnip yellow mosaic 

 virus (TYMV) isolates can be separated by ultracentrifugation into two com- 

 ponents. The bottom component contained the infectious virus nucleoprotein, 

 while the top component was rich in particles of the same dimensions, yet 

 free from nucleic acid and devoid of infectivity. These protein particles 

 showed the same electrophoretic behavior as the complete virus, and also 

 were serologically almost indistinguishable. 



Similar observations were made for noninfectious proteinaceous com- 

 ponents isolated from TMV-mfected Turkish tobacco plants by Takahashi 

 and Ishii (1952, 1953), Jeener et al. (1954), and Commoner et al. (1953; 

 Commoner and Yamada, 1955). A considerable amount of work performed 

 by these groups has shown that this so-called X-protein resembles the viral 

 protein in its tendency to aggregate below pH 6 to rod-shaped particles of 

 varying lengths, but of the same diameter as TMV. This aggregation, which 

 is also ionic strength-dependent, appears to be accompanied by a consider- 

 able increase in anodic electrophoretic mobility, which then approaches that 

 of the complete virus. Exactly the same behavior has been observed upon 

 reaggregation of the protein obtained by degradation of the viras, the so- 

 called A-protein (see later). It is thus evident that in the case of both TMV 

 and TYMV the electrophoretic mobility of the particle is a function of the 

 protein alone and apparently only of its surface, as it is formed through the 

 aggregation of smaller submiits. This is strong evidence for the interior loca- 

 tion of the nucleic acid in those viruses (Kramer and Wittmann 1958). 



It appears probable that the two or thiee components observed in some 

 preparations of the X-protein represent comparatively stable intermediates 

 in the aggregation process. Each of these, as well as the virus, is serologically 

 closely related. Amino acid and end group analysis have also demonstrated 

 great similarity between the X-protein and the protein isolated from the 

 virus (Newmark and Eraser, 1956). 



Concerning the metabolic significance of the formation of these proteins 

 and their role in the repHcation process, no agreement has been reached 

 among the various groups of workers, and this discussion may be regarded 

 as not within the scope of this chapter. For the understanding of viral function 



