286 HEINZ SCHUSTER 



fectious complexes with ribonuclease under certain conditions. The in- 

 filtration of protamine into infected leaves can also inhibit virus multiplica- 

 tion and, therefore, it is doubtful whether the ribonuclease action is specific. 

 These results cannot be compared with the studies employing UV irradia- 

 tion, since different types of plants were used and the course of infection 

 may be different in different types of cells. 



In studies using epidermal hair cells of Nicotiana langsdorffii, Benda 130 

 was able to show that ribonuclease also prevented lesion formation when 

 introduced into an epidermal hair through which the virus must pass to 

 reach the leaf from the inoculated cell. Since the epidermal hair is a column 

 of cells, one cell in diameter, ribonuclease could be introduced into the 

 basal cell while the distal cell is inoculated with virus. 



(3) Mechanism of Plant Virus Injection. The UV irradiation studies of 

 Siegel et al. on the sensitivity of infective centers lead to the hypothesis of 

 a "lag phase" during which the virus protein is stripped from the nucleic 

 acid. One should, therefore, expect that new infectious material will be 

 synthesized sooner if infection is made with RNA than if infection is made 

 with intact virus. 



Schramm and Engler 131 infected a parallel series of tobacco plants with 

 TMV and TMV-RNA and then determined the amount of infectious ma- 

 terial, at different times after infection, by homogenizing the infected 

 leaves and assaying the homogenate by local lesion test on Nicotiana 

 glutinosa. It is possible to detect 10 -12 -10 -13 gm. TMV/ml. by this assay. 

 A typical experiment is shown in Fig. 11. After a latent period, virus multi- 

 plication is rapid. In both cases, however, it slows down when a concentra- 

 tion of about 10~ 7 gm. virus per gram leaves is reached. If the extrapolation 

 for the two curves is correct, then the latent period for TMV is approxi- 

 mately 30 hours and that for RNA approximately 20 hours. This means that 

 the release of RNA from virus takes several hours, as was also suggested by 

 studies of Siegel et al. However, it is not clear why RNA itself has such a 

 long latent period. This difference in the course of infection with TMV or 

 with TMV-RNA is also observed if one studies the times at which local 

 lesions arise on Nicotiana glutinosa after infection with TMV or with TMV- 

 RNA. 132 



When infected leaves are homogenized, ribonuclease is set free. Although 

 this can have no effect on intact virus particles, infectious virus RNA, 

 which is not yet enclosed in protein, is destroyed. Therefore, infectious 

 RNA which has been synthesized during the latent period cannot be demon- 



130 G. T. Benda, Virology 6, 718 (1958). 



131 G. Schramm and R. Engler, Nature. 181, 916 (1958). 



132 H. Fraenkel-Conrat, B. Singer, and S. Veldee, Biochim. el Biophys. Acta 29, 639 

 (1958). 



