134 NITROGEN METABOLISM 



more complex forms of life, the inheritance of specific char- 

 acters is controlled by the deoxypentose nucleic acids, and 

 in this respect a group of natural substances, known as trans- 

 forming factors, are of particular interest. A transforming 

 factor induces a susceptible cell to acquire a particular here- 

 ditable and characteristic property of the cell from which 

 the factor emanates. Once a cell has been 'transformed', the 

 acquired feature is transmitted through all subsequent 

 generations. The most thoroughly investigated example of 

 this phenomenon is provided by the pneumococci whose 

 virulence is associated with the possession of a capsule of 

 polysaccharide material. Differences in the composition of 

 the latter have enabled the pneumococci to be grouped into 

 more than thirty serologically distinct types. Griffith ob- 

 served that living non-encapsulated avirulent type II pneu- 

 mococci were changed into virulent encapsulated type III 

 pneumococci by passage of the former together with heat 

 killed cells of the latter through mice. Such transformations 

 can be brought about in vitro in certain well-defined condi- 

 tions and later Avery and his colleagues obtained convincing 

 evidence that the agents responsible for the transformation 

 of pneumococcal types were deoxypentose nucleic acids, 

 each acid being specific for one type of transformation 

 [see i]. More recently, other transformations dealing with 

 capsulation, resistance to penicillin and the ability to fer- 

 ment particular sugars have been demonstrated with certain 

 strains of Haem. influenzae, Esch. coli, Shigella paradysen- 

 teriae, B. anthracis and Pr. vulgaris; as in the pneumococci, 

 the factors accomplishing these transformations appear to be 

 deoxypentose nucleic acids [i]. Pentose nucleic acids have 

 been implicated in the formation of stretolysin S, the O2- 

 stable haemolytic exotoxin of Strep, haemolyticus [7]. 



Enzymic degradation of nucleic acids [31] 



The enzymic degradation of nucleic acids commences 

 with disruption of the internucleotide linkages by ribo- 

 nuclease or deoxyribonuclease, enzymes specific for the 

 pentose and deoxypentose nucleic acids respectively: the 

 nucleic acid is thus reduced to a mixture of mono-, di- and 



