34. THE RIBONUCLEIC ACIDS OF VIRUSES 283 



/. Infectious Ribonucleic Acid and the Host Cell 



TMV produces a general disease in plants of different tobacco races. 

 This disease is expressed in a mosaiclike light and dark green pattern as 

 well as a deformation of the leaves. The virus may be transmitted to plants 

 of different families and, therefore, has a very low host-specificity. For the 

 production of large amounts of TMV, a Samsun strain of Nicotiana tabacum 

 is usually employed. Nicotiana glutinosa plants are best suited for infectivity 

 assays, since the virus produces local necrosis on leaves of these plants 

 rather than general symptoms. The number of lesions produced by a virus 

 preparation increases with the amount of virus over a wide range of virus 

 concentrations and, therefore, can serve as a measure of the biological 

 activity of a virus preparation. The infection is experimentally initiated by 

 mechanically injuring the cells of the leaves so that the virus can enter the 

 cytoplasm directly. 



Cytological studies have shown that the early part of the infection with 

 TMV is characterized by a marked increase in protoplasmic streaming, 

 division, and degeneration of chloroplasts, and the secretion of virus ag- 

 gregates in the form of crystals. 122 By employing ultraviolet microspectro- 

 scopic analysis, it is found that the extinction, Eo^/E^so w , of the various 

 cell constituents is altered in a characteristic fashion after the entry of 

 virus into the hair cells. 123 This ratio remains unchanged in healthy cells. 

 The relative level of the ratio is determined essentially by the nucleic acid 

 and protein components (viz., the aromatic amino acids). Changes in E% / 

 E280 in TMV-infected cells suggest that the ratio of total RNA to protein 

 increases, first in the nucleus and only later in the cytoplasm. Infectious 

 particles may be detected in such infected cells only when the ratio has 

 reached a minimum for the cell nucleus and a maximum for the cytoplasm. 

 However, this method is not suited to determine under which conditions 

 virus specific material is formed. 



Some information concerning the course of virus infection resulting from 

 the inoculation of the intact virus or infectious RNA has been obtained by 

 studying the action of UV irradiation on infected leaves. The course of 

 virus synthesis in the plant cells was also studied by assaying infectious 

 material at various times after infection. On the other hand, studies of the 

 effect of ribonuclease on leaves at various times during the infection have 

 produced no clear picture of the infection process. 



(1) Effect of UV Irradiation on Virus Multiplication. The inactivation of 

 infective centers of Nicotiana glutinosa with UV light after inoculation of 



122 H. Zech, Planta 40, 461 (1952). 



123 H. Zech and L. Vogt-Kohne, Naturwissenschaften 42, 337 (1955). 



