28 S. G. WILDMAN 



F. X-Protein 



An important new development in the study of TMV in its systemic host 

 has been the discovery by Takahashi and Ishii (1952) and confirmed by 

 several other workers (Jeener and Lemoine, 1953; Commoner et al., 1953) 

 that synthesis of TMV nucleoprotein is accompanied by the appearance of 

 other proteins which are closely related in amino acid composition and sero- 

 logical properties to TMV. The X-protein of Takahashi and Ishii does not 

 contain phosphorus and is devoid of nucleic acid, although Bawden and Pirie 

 (1956) claim that similar materials prepared by them do contain small 

 amounts of P. The protein can be aggregated into structures which appear 

 under the electron microscope to be identical in shape with the TMV particle. 

 Discovery of X-protein has provided a decisive clue to the construction of 

 the TMV particle (Frankhn 1955). 



Two possible origins of the X-protein have been considered. One notion, 

 that the X-protein arises through the disruption of originally intact TMV 

 particles due to the harshness of the extraction process, seems to be ehmin- 

 ated by the finding that subjecting purified TMV to the extraction process 

 does not produce X-protein (Takahashi and Isliii, 1953). The other idea, that 

 X-protein represents a stage in the synthesis of TMV finds support from iso- 

 tope experiments. Delwiche et al. (1955), using isotopic nitrogen, indicate 

 that late in the period of infection the isotope is incorporated into X-protein 

 at a faster rate than into TMV, indicatmg that synthesis of X-protein pre- 

 cedes synthesis of TMV. However, it is my opinion that the broad conception 

 now gained of the reproductive behavior of TMV and infectious TMV nucleic 

 acid would logically place the formation of protein submiits after reproduc- 

 tion of the nucleic acid. If X-protein is identical with the subunits, we might 

 imagine a lag in their production at first, and then, perhaps, an acceleration 

 to the point where they are in surplus late in the total infective period. We 

 are left with the vital question, however, of how the nucleic acid manages to 

 engender its own formation as well as that of the protein, about wliich plant 

 virology is still in the dark. 



V. Summary of Steps Involved in Plant Virus Reproduction 



Taking the experimental evidence as a whole, the sequence of events 

 leading to the multiphcation of a plant virus, as represented by TMV, can be 

 summarized in the following way: 



1. TMV gains access to host cell protoplasm only as a result of a wound 

 that exposes the host protoplast and permits the infectious material to sur- 

 mount the otherwise impenetrable barrier of the cell wall. Producing the 

 right kind of wound is a delicate and subtle process whose precise nature is 



