84 D. SHUGAR 



tically "thick" so that complete absorption of the incident light must have 

 prevailed 171 — this requires re-examination. 



Latarjet and Cherrier 175 have emphasized the remarkable resistance of 

 pneumococcal TDNA to irradiation and calculate a quantum yield of 

 about 10~ 5 (molecular weight of DNA not given). Some idea regarding the 

 differences in sensitivity of different TDNA's may be gained by comparing 

 the doses in Fig. 14 with results for H. influenzae DNA (Fig. 13) which 

 requires about 500 ergs/mm. 2 for 99% destruction of activity for capsule 

 transformation and about 100 ergs/mm. 2 for destruction of activity for 

 streptomycin resistance. 



For H. influenzae TDNA the dose required to produce a noticeable de- 

 crease (3%) in viscosity is 500-fold greater than that required for 90% 

 inactivation. 171 In conjunction with analogous observations on infectious 

 RNA from TMV 168 (Section VI, 2, a) it follows that the doses required to 

 provoke important biological effects in nucleic acids are much lower than 

 those resulting in degradation of nucleotide chains and that the biological 

 effects are highly localized in character and most likely involve the pyrimi- 

 dine nucleotide residues. It is of some interest in this connection that the 

 RNA core is highly resistant to biological inactivation (Section VI, 2, c). 



Attempts to demonstrate differences in stability of several pneumococcal 

 transforming factors towards ionizing radiations were unsuccessful. 176 Nor 

 is there any difference in sensitivity of various TDNA's if loss of viscosity 

 is used as a criterion of radiation sensitivity. 



Different markers of the same transforming DNA, on the other hand, 

 may exhibit different sensitivities to ultraviolet 172 ' 177 and, of a variety of 

 agents possessing the ability to produce differential inactivation of more 

 than one marker, ultraviolet is by far the most effective. 177 Figure 15 il- 

 lustrates the differences in stability of different heredity determinants in H. 

 influenzae TDNA. 



The high radiation resistance of the 1 % residual activity in curve 3 of 

 Fig. 15 suggests that some molecules might be more stable than others, 

 possibly owing to the presence of several heredity determinants. This was 

 excluded by preparing new TDNA from receptor cells transformed with 

 the residual activity; the new TDNA exhibited similar radiation sensi- 

 tivity. It should be recalled that irradiated infectious RNA from TMV also 

 exhibits a residual activity highly resistant to radiation and the same is 

 true for many viruses (Section IX). A similar phenomenon has been ob- 

 served for pneumococcal TDNA inactivated by X-rays. 178 



175 R. Latarjet and N. Cherrier, 2nd Intern. Congr. Pholobiol., Turin, 1957 p. 131 

 (1957).. 



176 J. Marmur and D. J. Fluke, Arch. Biochem. Biophys. 57, 506 (1955). 



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



178 H. Ephrussi-Taylor and R. Latarjet, Biochim. et Biophys. Acta 16, 183 (1955). 



