OSWALD T. AVERY, COLIN M. MACLEOD, AND MACLYN MCCARTY 



151 



optically visible boundary. Thus in both the electrical and centrifugal fields, 

 the behavior of the purified substance is consistent with the concept that bio- 

 logical activity is a property of the highly polymerized nucleic acid. 



Ultraviolet absorption curves showed maxima in the region of 2600 A and 

 minima in the region of 2350 A. These findings are characteristic of nucleic 

 acids. 



Quantitative Determination of Biological Activity. — In its highly purified state 

 the material as isolated has been found to be capable of inducing transformation 

 in amounts ranging from 0.02 to 0.003 fig. Preparation 44, the purification of 

 which was carried out at low temperature and which had a nitrogen-phosphorus 



TABLE IV 



Titration of Transforming Activity of Preparation 44 



* Solution from which dilutions were made contained 0.5 mg. per cc. of purified material. 

 0.2 cc. of each dilution added to quadruplicate tubes containing 2.0 cc. of standard serum 

 broth. 0.05 cc. of a 10~ 4 dilution of a blood broth culture of R36A is added to each tube. 



ratio of 1.58, exhibited high transforming activity, 

 of this preparation is given in Table IV. 



Titration of the activity 



A solution containing 0.5 mg. per cc. was serially diluted as shown in the protocol. 

 0.2 cc. of each of these dilutions was added to quadruplicate tubes containing 2.0 cc. 

 of standard serum broth. All tubes were then inoculated with 0.05 cc. of a IO" 4 

 dilution of a 5 to 8 hour blood broth culture of R36A. Transforming activity was 

 determined by the procedure described under Method of titration. 



The data presented in Table IV show that on the basis of dry weight 0.003 

 Hg. of the active material brought about transformation. Since the reaction 

 system containing the 0.003 fig. has a volume of 2.25 cc, this represents a final 

 concentration of the purified substance of 1 part in 600,000,000. 



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