30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 



89 



5 fO (5 



Time of illumination (Wns.) 



Fk;. 17. Increase in activity of H. influenzae transforming DNA (previously in- 

 activated by exposure to ultraviolet) in presence of E. coli extract under influence 

 of visible light; during period indicated by arrows the source of illumination was 

 removed [from C. S. Rupert, S. H. Goodgal, and R. M. Herriot, J. Gen. Physiol. 41, 

 451 (1958)]. 



It has now been demonstrated 182 that ultraviolet inactivated H. influenzae 

 transforming DXA may be partially reactivated by visible light in the 

 presence of a cell-free E. coli B extract (Fig. 17). The degree of reactivation 

 is proportional to the concentration of the extract, light intensity, and 

 temperature, and varies from 10-50% of the activity of a nonirradiated 

 control. The extract consists of a dialyzable, heat-stable fraction and a non- 

 dialyzable thermolabile component. 



It appears most likely from the evidence thus far presented that the 

 reaction system involves a light-activated enzymic reaction and it is tempt- 

 ing to speculate about the possibility of enzymic removal of a water mole- 

 cule from the saturated 4,5 double bond of pyrimidine residues in the 

 ultraviolet inactivated DXA molecule. 



It is instructive that the active extract is derived from E. coli B, an 

 organism which itself is photoreactivable. whereas an extract from H. 

 influenzae (an organism not susceptible of photoreactivation following 

 ultraviolet inactivation) is ineffective in the above in vitro system. 



An illuminated E. coli B extract has been reported capable of reactivating to a 



182 C. S. Rupert, S. H. Goodgal, and R. M. Herriot, J. Gen. Physiol. 41, 451 (1958). 



