334 Essays in Biochemistry 



in a standard buffer) ; on the other hand, the activities were irreversibly 

 reduced by exposure to lower (but not to higher) ionic strength. This 

 permanent damage could be due to the breakage of a few vital bonds, 

 such as hydrogen bonds, during stretching caused by repulsion of anions 

 in the DNA molecule in the absence of salts. 



4. Deamination. The chemicals whose action on the transforming 

 principle was studied 21 were chosen for two reasons: (1) because the 

 nature and the extent of reaction with DNA could be determined; 

 (2) because the agents themselves have important biological activity, 

 either mutagenic, carcinogenic, or carcinostatic. 



Nitrous acid belongs to the first group of reagents. Incubation of 

 the transforming principle with 2 M NaN0 2 at pH 5.3 (veiy mild 

 deaminating conditions) results in a very rapid inactivation. However, 

 the viscosity remains constant indicating that the average DNA mole- 

 cule is but slightly altered. The extent of deamination corresponding 

 to a 1000-fold decrease of activity is found to be of the order of 0.1%. 

 Thus it seems that practically all the primary amino groups are essen- 

 tial for activity. 



5. Mutagenic agents. The agents in this very heterogeneous class 

 are grouped together, although the mechanisms of their action may be 

 entirely different; they all seem to prime a phenomenon of great 

 importance and of unknown nature, namely, mutation. It has been 

 suggested 26 that the processes leading to mutation and to death are 

 essentially the same, with the exception that the latter is accompanied 

 by more extensive molecular changes. If this is indeed so, then the 

 inactivation of the transforming principle by these agents could be a 

 demonstration of a "too strong mutation." 



6. Ultraviolet irradiation. The ultraviolet irradiation of the trans- 

 forming principle (transformation to type specificity) has been studied 

 in H. influenzae. 27 The inactivating dose was found to be of the same 

 order of magnitude as the one necessary to inactivate bacterial viruses. 

 The action of ultraviolet irradiation on DNA is now believed to be 

 due to free radicals H- and OH-, and to peroxides formed during 

 irradiation of organic molecules. McCarty 28 observed reversible in- 

 activation of pneumococcal transforming principle by ascorbic acid and 

 several other self-oxidizing agents. 



7. Ferrous ion. Ferrous ion self-oxidizes even in the absence of H 2 2 . 

 In concentrations as low as 10 -5 M, Fe++ alone causes a 10-fold 

 inactivation of the transforming principle of H. influenzae ; 21 again, no 

 change in viscosity is observed. The exact nature of the damage is still 



