X. TMV STUDIES IN GENETIC CODING 495 



1 ►|2h- 3— h* 



TMV ——J 



Y-TAMV ■ 



*|-^6— 1*|— 8-*|-9-|-H0*|-M-|— 12— 



G-TAMV H r- 



HR 



r 



Fig. 4. Location of differences in protein structure of natural strains of TMV. 

 The peptide chain of common TMV, subdivided at the sites of trypsin attack into 

 12 peptides is illustrated (A), as well as segments of this structure in which differ- 

 ences have been observed (thin line) in strains of groups B, C, and D (see text). 

 It may be noted that peptides 5, 7, and 9 are the same throughout. 



Val.Ala.Ileu.Arg (112-122) are not changed throughout the four groups, 

 which indicates that these sequences might play an important role in the 

 biological activity of the protein molecule. These data are as yet cjuite 

 fragmentary, but it is believed that they will ultimately supply the basis 

 for important genetic and protein structural considerations. 



C. MIXED RECONSTITUTION 



Although DNA has long been regarded as the sole genetic material 

 on the molecular level, it has been unequivocally proven in recent years 

 that RNA fulfills this function for the RNA viruses. This was first 

 clearly demonstrated by means of the reconstitution technique (Fraen- 

 kel-Conrat and Singer, 1957). Three kinds of natural strains and the 

 normal TMV strain were chosen for this purpose. These strains could be 

 distinguished from the normal strain by either their symptoms on plants 

 or their amino acid compositions, or both. Both the protein and the 

 nucleic acid components were isolated from common TMV and from the 

 natural strains, and the various possible combinations of pairs of these 

 were reconstituted and tested. For instance, with the HR strain the 

 following couples of reconstitution experiments were made: 



HR-protein + HR-nucleic acid (a) 



HR-protein + TMV-nucleic acid (b) 



TMV-protein + HR-nucleic acid (c) 



TMV-protein + TMV-nucleic acid (d) 



Each mixed reconstituted virus was allowed to infect tobacco plants 

 and the resultant symptoms were observed, and the amino acid composi- 

 tion of the progeny protein determined. The results showed clearly that 

 the nucleic acid determined both the symptoms and the amino acid 

 composition, so that the progeny from (a) and (c) seemed to be HR 

 and those from (b) and (d) seemed to be TMV both chemically and 

 biologically. In other words, the nucleic acid of TMV was experimentally 

 proved to be the sole genetic material carr>'ing the complete information 



