490 A. TSUGITA AND H. FKAKN KP:L-C0NRAT 



(Fraonkcl-Coiirat and Sinf2;cr. 1957, 19')9) ilis])rove^; this explanation. On 

 the other liand, tlie ])resence of rihonucleasc in the leaves of the test 

 plant suggests the possibility of digestion and inactivation of the RNA 

 during the process of infection. The recent finding that the infectivity of 

 the RNA is greatly potentiated by assay in presence of bentonite, an 

 agent which absorbs and inactivates many nucleases, seems to support 

 this interpretation (Singer and Fraenkel-Conrat, 1961). 



B. MECHANISM OF INFECTION 



Our knowledge concerning the inti-accl hilar jirocesses concerned with 

 plant virus replication is as yet quite fragmentary. It seems that these 

 viruses lack the sophisticated infection mechanisms of bacterial and 

 animal viruses, and cellular entry of the vims appears to require a 

 mechanical wounding, which is frequently achieved through an insect 

 vector. Soon after entry into a viable cell the virus appears to shed its 

 protein coat. This, in the case of TMV, requires several hours and 

 accounts for the fact that the infection process is faster when free RNA 

 rather than when the complete virus is used as the inoculum (Siegel 

 et al., 1957; Engler and Schramm, 1959; Fraenkel-Conrat et al., 1959). 

 Subsequently, an eclipse period of a few hours can be observed, during 

 which the RNA is believed to enter the nucleus and initiate its replica- 

 tion (Wildman, 1959; Zech, 1961). The progeny RNA then seems to 

 reenter the cytoplasm and may there initiate virus protein synthesis. 

 While thus infective RNA appears to be formed prior to any complete 

 vims, excess viral protein is often observed at later stages in infected 

 cells. Thus many virus isolates contain virus-like particles lacking 

 nucleic acid and thus void of infectivity. In the case of TMV, this so- 

 called X-protein, first isolated by Takahashi and Ishii (1952), was more 

 I'ccently shown to reconstitute fully active virus when combined with 

 TMV-RNA in vitro (Takahashi, 1959). 



C. CELL-FREE BIOSYNTHESIS OF TMV-PROTEIN 



Nirenberg and ]\Iatthaei recently reported on an amino acid-incorpo- 

 rating system from Escherichia coli which was stimulated by the addition 

 of macromolecular RNA (1961). The system consists of the ribosomal 

 fraction of E. coli, the supei'iiatant (containing S-RNA), an ATP 

 generating system, and amino acids one or all of which are labeled with 

 C'^ When the extent of amino acid incorporation into hot TCA- 

 insoluble material was found greatly stimulated by the addition of 

 TMV-RNA to the reaction mixture, the question arose whether tlie 

 RNA might act as messenger and evoke the syiitliesis of TMV-protein 

 in this cell-free system. This concept schemed to be suppoi'ted when a 



