450 H. TRAENKEL-CONRAT 



the case was demonstrated by tlie addition of trace amounts of tobacco ribo- 

 nuclease to stable TMV-RNA preparations. It was then found that an enzyme 

 concentration sufficient to inactivate at 0.1 31 salt concentration did not 

 cause inactivation in 0.001 M salt (Fraenkel-Conrat and Singer, 1958a). 

 Thus the probabihty that the lability of TMV-RNA is due to traces of en- 

 zymes is greatly strengthened. Unfortimately, however, no solution of the 

 problem as to how regularly to prepare stable nucleic acid from tlie virus 

 is as yet at hand. 



C. Miscellaneous Reactions 



With any bioactive material the question arises as to how far the entire 

 molecule is required for its functioning and how this is affected by specific 

 cliemical modifications. Little is known about chemical modifications of 

 RNA, but recent studies have indicated that certain "protein reagents" 

 definitely react with nucleic acids. The high toxicity of the mustard gases 

 may well be due to their attack on the nucleic acid (Carpenter et al., 1948). 

 The inactivation of viruses by formaldehyde also is probably due to its 

 affinity for the nucleic acid (Fraenkel-Conrat, 1954). Recent studies with 

 C^''-labeled formaldehyde (Staehelin, 1958) and iodoacetate (unpublished) 

 have shown that the amino groups of the nucleotides combine with the 

 reagents, and that substitution of as little as one such group per 100,000 

 molecular weight unit (20 per particle nucleic acid complement) causes 

 inactivation. 



Inactivation of viruses, or of isolated nucleic acid, by ultraviolet light 

 must also be listed here, although the extent of chemical alteration is as yet 

 not too clearly defined. The mode of inactivation of TMV-RNA by traces of 

 metals (particularly Cu++ and Fe+++) is also uncertain. The slow rate of 

 this effect, and its irreversibility by later addition of chelating agents indi- 

 cates that changes more extensive than metal binding are involved (Fraenkel- 

 Conrat, 1958). 



VII. Reconstitution of Viruses from Different Strains 



TMV, like most viruses, represents a family of related viruses, usually 

 referred to as strains or mutants. Only in rare instances are the genetic 

 relationships of plant virus strains clearly established. These aspects will be 

 discussed elsewhere. In the case of TMV strains, chemical criteria have become 

 estabhshed in recent years which clearly define "membership in that club." 

 Thus, all TMV strains seem to be composed of protein subunits of a molecular 

 weight of about 18,000, containing one cysteine SH group and C-terminal 

 prolyl-alanyl-threonine (Niu and Fraenkel-Conrat, 1955); all that were 



