492 J. BRACKET 



For instance, H. and R. Jeener**^ have been able, in the case of Thermo- 

 bacterium acidophilus, to interfere selectively with either PNA or DNA 

 synthesis by the removal from the culture medium of uracil and DNA, 

 respectively. In the absence of DNA, the cells still grow as elongated fila- 

 mentous forms, but the number of bacterial nuclei remains small. In 

 cultures deprived of uracil, growth is inhibited and both nuclei and cyto- 

 plasm are affected. These findings indicate that while protein synthesis is 

 dependent on PNA synthesis, it is much less directly related to DNA 

 synthesis. 



Another indirect line of evidence can be found in Swenson's^"'' interesting 

 discovery that, when ultraviolet light inhibits the adaptive synthesis of 

 galactozymase in yeasts, the action spectrum is very similar to a nucleic 

 acid absorption spectrum; it bears no relation to the absorption spectrum 

 of an unconjugated protein. As pointed out by Swenson,'"'' "the results 

 strongly indicate that nucleic acid is the cellular constituent affected by 

 the light." Swenson's results, however, do not allow us to decide whether 

 the light-sensitive substance is PNA rather than DNA. Indeed, recent 

 work by Kelner^"^ shows that while ultraviolet light at 253.7 mju has little 

 immediate effect on growth and PNA synthesis, both phenomena being 

 affected in the same way, it seems to inhibit DNA synthesis immediately 

 and in a specific manner. It is therefore possible that the adaptive synthesis 

 of galactozymase is under direct nuclear control, although such a conclusion 

 is hard to reconcile with a number of facts which will be discussed later. 



A recent observation by Jeener^"- may also prove of great importance. 

 It was mentioned earlier that Jeener and Rosseels^" found labeled thiouracil 

 to be incorporated into the PNA of tobacco mosaic virus, when the leaves 

 are treated with this radioactive chemical analogue of uracil. When tobacco 

 leaves showing a systemic infection by tobacco mosaic virus are treated 

 with ordinary thiouracil, it is found that there is a considerable and parallel 

 decrease in the turnover rates of the glutamic acid present in the proteins 

 of the leaves and of the virus, and in the purines and pyrimidines from 

 normal and virus PNA's. The fact that the turnovers of both the PNA and 

 the proteins are affected simultaneously constitutes still more evidence in 

 favor of an intimate relationship between PNA and protein synthesis. 



It should finally be mentioned that, according to Okamoto^"^ and Bern- 

 heimer,''''* addition of PNA from several but not all sources greatly stimu- 



99 H. Jeener and R. Jeener, Exptl. Cell Research 3, 675 (1952). 

 loo P. A. Swenson, Proc. Natl. Acad. Sci. U. S. 36, 699 (1950). 

 i»> A. Kelner, J. Bacteriol. 65, 252 (1953). 

 '"'^ R. Jeener, personal communication. 



103 H. Okamoto, Japan. J. Med. Sci. IV, Pharmacol. 12, 167 (1939). 

 1"^ A. W. Bernheimer, in "Phosphorus Metabolism" (McElroy and Glass, eds.), Vol. 

 2, p. 358. Johns Hopkins Press, Baltimore, 1952. 



