82 



D. SHUGAR 



pected if it is assumed that only the uridylic and cytidylic acid residues 

 are involved. It is also rather striking that the dose required for total 

 inactivation is only about 1 % of that required to depolymerize the RNA 

 molecule. 168 



Finally, attention should be drawn to the fact that samples of infectious 

 RNA, isolated from two different TMV strains which differed more than 

 five-fold in radiation sensitivity, were equally sensitive to ultraviolet ir- 

 radiation, 167 a finding which has proved useful in studies on the nature of 

 interaction between the protein and nucleic acid components of viruses and 

 to which we shall return below. 



b. Transforming DNA 



The inactivation at 253.7 nuz of transforming DNA (TDNA) for capsule 

 b formation in Haemophilus influenzae has been reported to follow a "multi- 



200 



1000 



400 600 800 



Dose (ergs/mm. 2 ) 



Fig. 13. Course of ultraviolet inactivation of H. influenzae transforming DNA 

 (a) for type b capsule formation [from S. Zamenhof, G. Leidy, E. Hahn, and H. E. 

 Alexander, J . Bacteriol. 72, 1 (1956)] and (b) for streptomycin resistance in type d 

 [from R. Pakula, E. Hulanicka, and W. Walczak, Bull. acad. polon. sci., Classe IF, 7, 

 217 (1959)]. 



