CHAPTER XIII 



ISOTOPIG STUDIES ON THE FATE OF THE 

 INFECTING PHAGE PARTIGLES 



The first use of the expression, "fate of the infecting virus par- 

 ticle," occurs in the title of a paper by Putnam and Kozloff 

 (1950). The expression is usually understood in the sense of the 

 fate of the chemical constituents of which the infecting virus 

 particle is composed. However it could with equal justification 

 be used to mean the fate of the genetic factors with which the in- 

 fecting virus particle is endowed. In the present chapter we will 

 be concerned primarily with the fate of the material substance. 



The principal experimental approach to this problem calls for 

 the use of isotopes, because it is only by the use of such specific 

 labels that one is able to distinguish phage material from host 

 material. By using appropriate differential labels it is possible 

 to trace phage protein and phage nucleic acid during the infec- 

 tious process. The bacteriophage particles are labeled by grow- 

 ing the host bacteria in a nutrient medium containing the ap- 

 propriate labeled ingredient, infecting with phage, and per- 

 mitting lysis to take place in the labeled medium. The phage 

 is then isolated and purified by differential centrifugation until 

 the isotope content becomes constant. Putnam and Kozloff 

 (1950) found no evidence that extracellular phage can exchange 

 isotope labeled compounds with the environment. This is in 

 agreement with the generally accepted principle that bacterio- 

 phages have no metabolic activity of their own. The isotope 

 composition of a labeled phage preparation which has been prop- 

 erly purified remains constant until infection, except for changes 

 due to radioactive decay. 



207 



