30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 



10 (5 20 



Pose (ergs /mm. 2 ) 



Fig. 15. Ultraviolet inactivation of transforming DNA from H . influenzae for: (1) 

 capsule 6 ore formation, (2) capsuled formation, (3) streptomycin resistance in type 

 b, (4) streptomycin resistance in type d, (5) streptomycin resistance in type a [from 

 S. Zamenhof, G. Leidy, S. Greer, and E. Hahn, J. Bacteriol. 74, 194 (1957)]. 



It is as yet not feasible to define the reasons for this differential stability 

 of different markers, especially since TDNA preparations are by no means 

 homogeneous. Ravin 179 objects to the use by Zamenhof et al. m of the 

 phrase "differential stability of heredity determinants" on the grounds 

 that it presupposes the mechanism of the effect, and suggests there may be 

 no difference in stability of various markers but only a variation in the 

 ability of different markers in a given irradiated TDNA to be incorporated 

 into the host genome. It is consequently worth noting that the ability to 

 transform is apparently much more sensitive to radiation than the ability 

 to be taken up irreversibly, from which it has been concluded that ultra- 

 violet inactivated determinants can at least enter the bacterial cell. 172 



c. RNA Core 



The ability of the ribonuclease-resistant "core" of RNA (Volume I, 

 Chapter 15) to promote streptolysin S formation in streptococci (Volume 



179 A. W. Ravin, Ann. Rev. Microbiol. 12, 309 (1958). 



