INFECTIVITY OF TOBACCO MOSAIC VIRUS 451 



tested have also proved to contain the same iV-terminal acetyl-seryl-tyrosine 

 group (Narita, 1958a,b); all appear to contam nucleic acid of the same over- 

 all composition (Knight, 1952; Black and Knight, 1953). That these criteria 

 are not seriously resfcrictivc to the mutation drive of viral genetic material is 

 attested to by the existence of hundreds of mutants of TMV, and to the 

 existence of at least one strain, the Holmes ribgrass (HR), characterized by 

 remarkably wide areas of differences. There are hardly any points of resem- 

 blance between the HR protein and that of common TMV beyond those listed 

 above as required for membership. It is thus not surprising that this strain 

 can be serologically differentiated from common TMV, quite in contrast to 

 most others. HR is also biologically the most segregated strain, detectable 

 even in local lesion appearance (as stated previously). For all these reasons 

 HR has been used more extensively than other strains in the strain recon- 

 stitution studies which will nov/ be discussed. 



A. Mixed Vinises 



Before the infectivity of nucleic acid had been recognized, reconstitution 

 of virus from nucleic acid and protein of two different strains appeared of 

 singular importance and interest. This was possible for quite a number of 

 different pairs; in each case the nature of the disease vfas the same as that of 

 the parent strain supplying the nucleic acid. This appears now not surprising 

 (Fraenkel-Conrat et al., 1957b; Fraenkel-Conrat and Singer, 1957). 



As expected, results were most clear-cut and definitive in the case of the 

 HR and common TMV mixed viruses. These experiments unequivocally 

 proved the reconstitution of active virus rods at a time wlien the yields were 

 still relatively low and contamination with undegraded virus had to be 

 seriously considered. The evidence was as follo\\'s: When HR-NA was recon- 

 stituted with TMV-protein, an HR-like disease was produced, as indicated 

 already by the assay of the test solutions on Nicotiana glutinosa, and further 

 proven by single lesion propagation on N. tabaccmn and N. sylvestris, both 

 of which react very differently to the two parent strains. Yet, addition of 

 anti-TMV serum inhibited this HR-like virus, while anti-HR serum had 

 little effect on it. Thus, its dual nature, i.e., HR-nucleic acid core and TMV- 

 protein coat was clearly demonstrated (Fraenkel-Conrat and Singer, 1957). 

 The expected opposite behavior was noted when HR-protein was combined 

 with TMV- nucleic acid. Similar mixed virus recoustitutions were performed 

 by Bawden (1957) and Commoner (1957), and similar conclusions were 

 reached. 



When the progeny of such mixed virus was isolated and subjected to 

 detailed tests, all its properties naturally corresponded closely to those of 

 the original virus supplying the nucleic acid. This was the case for both 



