206 HUBERT S. LORING 



somes, or in a particular tissue presents a number of difficulties depending 

 on the amounts of non-nucleic acid purine or pyrimidine or other similar 

 ultraviolet-absorbing compounds that may be present and the variability 

 of the respective materials. Of the many types of biological materials avail- 

 able for study, the plant viruses approach most closely substances that can 

 be described as relatively homogeneous, soluble nucleoproteins. Because of 

 their high molecular weight, it is relatively simple to separate them by dif- 

 ferential ultracentrifugation from the many low-molecular-weight com- 

 pounds which are associated with living cells and might interfere in the 

 analysis for the purine and pyrimidine components of the ribonucleic acid. 

 Analyses have been made on the nucleic acid prepared from purified tobacco 

 mosaic virus*^-^^ and many of its strains,^* by heat or alkali treatment as 

 well as on the trichloroacetic acid-extracted nucleoprotein (TCA nucleo- 

 protein)** and on the purified virus itself. [Cf. Magasanik, Chapter 11.] The 

 average results expressed as percentage molar proportions of the four bases 

 are summarized in Table III under the respective host species used for the 

 cultivation of the virus. While examination of the combined analyses re- 

 veals some differences in the composition of tobacco mosaic virus nucleic 

 acid as reported by the three laboratories, it is also clear that most of the 

 results are in good agreement and the differences within the experimental 

 error. Examination of Table III reveals that quite similar results were found 

 for both the TCA nucleoprotein and its isolated nucleic acid by the spectro- 

 photometric method described above. These results are also in good agree- 

 ment with those found by Knight for the isolated nucleic acid if the high 

 adenine value reported by him is corrected for a small amount of cytidine 

 likely to be present in the same area of the chromatogram and if a similar 

 correction is made for small losses of unrecovered uridine.^'- ^^ The results 

 from the two laboratories are thus in essential agreement for the composi- 

 tion of tobacco mosaic virus nucleic acid when the virus is produced in 

 Turkish tobacco plants. Knight in studies of a number of strains derived 

 from tobacco mosaic virus found a relatively constant composition for the 

 respective nucleic acids, the results suggesting, in fact, that the nucleic 

 acids of different strains of the same virus may have identical composi- 

 tions. ^^^ ^* 



The fractionation of broken-cell preparations of tissues into large and 

 small granule fractions as begun by Bensley^^ and modified and improved 



" R. Markham and J. D. Smith, Biochem. J. 46, 513 (1950). 



«2 C. A. Knight, J. Biol. Chem. 197, 241 (1952). 



*' W. D. Cooper and H. S. Loring, J. Biol. Chem., in press. 



6* F. L. Black and C. A. Knight, J. Biol. Chem. 202, 51 (1953). 



«6 R. Markham and J. D. Smith, Biochem. J. 49, 401 (1951). 



«6 C. A. Knight, /. Biol. Chem. 171, 297 (1947). 



" R. R. Bensley, Biol. Symposia 10, 323 (1943). 



