150 C. A. KNIGHT 



uridylic acid found were large, accounting for from approximately 42 to 

 71 % of tlie total amount of these nucleotides in the nucleic acids. This 

 means that in a high proportion of cases two or more pyrimidine nucleotides 

 are linked together in the polynucleotide chains without intervening purine 

 nucleotides. Conversely, one can conclude that purine nucleotides must also 

 frequently occur ui clusters in the polynucleotide chains. However, the most 

 significant fact revealed by the data in Table VI is the significantly greater 

 amounts of cytidylic and uridylic acids in digests of M nucleic acid than in 

 digests of the nucleic acids of TMV, HR, and YA. This is taken to be a clear 

 indication of a larger proportion of pyrimidine polynucleotide segments in M 

 nucleic acid than in the nucleic acids of TMV, HR, and YA, and constitutes 

 the first evidence for structural differences between the nucleic acids of 

 strains of a virus. Subsequently (Reddi, 1958), it has been found that 

 ribonuclease digests of the nucleic acids of strains TMV, M, and HR 

 contain different quantities of certain di- and tri-nucleotides. This provides 

 additional evidence for the occurrence of different sequences of nucleotides 

 in the strain nucleic acids. 



h. Summary of Structural Features of Strain Nucleic Acids. Nothing has 

 been found so far to distinguish viral ribonucleic acids from others of this 

 type. Hence, it can be assumed that strain nucleic acids possess a common 

 sugar-phosphate backbone that achieves unique quahty by variation of the 

 sequence of attached purine and pyrimidine bases, i.e., by different nucleo- 

 tide sequences. Evidence for actual differences in nucleotide sequences has 

 been found with three strains of TMV. This type of investigation is just 

 beginning and should yield much more information in the next few years. 



IV. Production of Variants by Chemical and Physical 

 Treatments 



The knowledge that treatment with certain chemicals or exposure to 

 radiations could cause genetic changes in higher organisms suggests that 

 similar effects might be obtained with viruses. This seemed particularly 

 likely with plant viruses, since they are chemically and morphologically so 

 simple that they have often been considered raw genetic material akin to 

 chromosomes. Consequently, the chemical structure of TMV (on which most 

 of this type of experimenting has been done) has been modified by more or 

 less specific treatments and the effects on biological activity observed. Like- 

 wise, the virus has been irradiated under a variety of conditions and the 

 effects noted. None of these treatments has given results which could be 

 unequivocally construed as production of mutants, but some of the treat- 

 ments are briefly summarized here as a guide to further investigation along 

 these lines. 



