The Biochemistry of the Bacterial Viruses 105 



phage particles are implicated in the phenomena of transduction in 

 which two different autotropic mutants of Salmonella typhimurium, 

 when separated by means of a ground-glass filter, exchange genetic 

 characteristics by way of a filterable agent. In both cases phage appears 

 to be associated with non-lethal changes in the genetic characteristics 

 of the bacterial cell. Along with the bacterial viruses, representing a 

 group of biologically active nucleic acid-protein complexes secreted or 

 manufactured by the cell itself, we must recognize also the existence 

 of other biologically active particles, formed by the bacterial cell, 

 which are either DNA or protein. The transforming factors which 

 are capable of altering the genetic material of the infected bacterial 

 cell appear to be entirely DNA. On the other hand, the colicines are 

 DNA-free proteins capable of killing bacterial cells but unable to 

 reproduce themselves, although their formation can be induced in bac- 

 terial strains by the action of ultraviolet light. It seems quite certain 

 that the biology of the nucleoproteins involves a whole spectrum of 

 phenomena of which, as yet, we have recognized only a portion. 



