364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



of the Virus Laboratory has been unsuccessful. The failure to find 

 sufficient N-terminal groups to match the C-terminal groups of threo- 

 nine has been interpreted by Fraenkel-Conrat to indicate the pres- 

 ence of a cyclic structure in which the N-terminal groups are linked 

 by peptide bonds to w-carboxyls of glutamic or aspartic acids. It 

 is, of course, not known how important such a structure, if it actually 

 exists, may prove to be in the virus field. 



It must be remembered that Knight has found that carboxypep- 

 tidase treatment of other viruses has resulted in the release of several 

 different amino acids and not, as in the case of tobacco mosaic virus, 

 exclusively of threonine. In any event, considerable information 

 regarding the chemical structure of an important subunit of the 

 protein component of tobacco mosaic virus has been obtained and 

 the complete elucidation of the chemical structure of this subunit 

 does not seem impossible. As indicated earlier, the nucleic acid com- 

 ponent of the virus was found to consist exclusively of ribonucleic 

 acid. Despite extensive investigation of this component, nothing 

 new or unusual has been found to date, and any interest that may 

 develop in this area must await further development of nucleic acid 

 chemistry. Tobacco mosaic virus provides a unique source of a very 

 pure ribonucleic acid preparation, and this material is being studied 

 by a variety of newly developed techniques. Fortunately there is 

 great activity in nucleic-acid chemistry, and it does not seem unrea- 

 sonable to expect that important structural developments may result. 



It has been known for a long time that viruses can mutate to form 

 new strains which cause different disease manifestations. Now, one 

 may well inquire as to the nature of nucleoproteins isolated from a 

 given kind of host diseased with different strains of the same virus. 

 As a result of an immense amomit of very painstaking work, the most 

 important and significant discovery has been made that, when tobacco 

 mosaic virus mutates, the strains that are formed possess characteristic 

 differences in chemical structure. In some cases there are differences 

 in the relative amounts of certain amino acids present in the virus 

 strains, and in other cases there are differences in the kind of amino 

 acids that are present. Thus, sometimes, at least, mutation is accom- 

 panied by the incorporation of a new amino acid into the virus struc- 

 ture. The well-known tendency for isolable mutants to form when a 

 virus is grown in an unnatural host may result from a selection process 

 due to the new environment, but it might also result from the absence 

 of certain building blocks in the new host cells which forces the use 

 of available substitute building blocks and that this results in the 

 new strain. 



The transformation in virus or biological activity that may accom- 

 pany mutation can be most dramatic. For example, ordinary tobacco 



