274 G. S. STENT 



and we shall state here only that once the prophage has given rise to a vegeta- 

 tive phage, the sequence of events in the induced lysogenic bacterium is 

 quite similar to that in a sensitive bacterium infected with the same bacterio- 

 phage strain (Lwoff, 1953). 



VII. The Vegetative Phage 



We have had frequent occasion to speak of vegetative phage in the preceding, 

 without having bothered to bring this concept into focus. But some difficulties 

 present themselves now in connection with this notion, which are not unlike 

 those which eventually developed with the meaning of the term gene. After 

 gene had long served as the unit of recombination, mutation, and function, it 

 became necessary to re-examine the idea of the unit of inheritance, when the 

 study of genetic processes revealed that these three aspects of the hereditary 

 substance are by no means inseparable (Pontecorvo, 1952; Benzer, 1957). 

 Similarly, after "vegetative phage" had first represented simply the non- 

 infective form in which the bacteriophage multipHes within the host ceU, it 

 later transpired, as we have seen, that there is no one "form," but rather that 

 a variety of different phage-related precursor and nonprecursor structures 

 make their appearance within the infected bacterial cell. Hershey (1957a) 

 proposed, therefore, that the meaning of the term vegetative phage should be 

 restricted to "the physical structure with which the new germinal substance 

 is associated during vegetative reproduction," or to what Lwoff (1953) had 

 already called the "gonophage." This restriction eliminates the incomplete 

 phage proteins, such as the precipitable and serum-blocking antigens or the 

 doughnuts discussed in Section III, from the vegetative phage concept, 

 since these materials pertain to the somatic rather than germinal substance 

 of the virus, and leaves the field clear for the phage precursor DNA, the 

 presumptive carrier of the hereditary information of the extracellular phage 

 particle. Nevertheless, even this more precise definition is not unambiguous, 

 because the general nature of the mechanism by which the phage DNA is 

 synthesized is not yet understood. The ambiguity becomes apparent if we 

 consider two possible alternative DNA replication models. 



Model 1. The DNA molecules injected into the host cell by the parent phage 

 reduplicate by serving directly as the "template" for the ordered copoly- 

 merization of rephca DNA molecules of nucleotide sequence identical to the 

 parental molecules, possibly by means of the mechanism of complementary 

 base pairing suggested by "Watson and Crick (1953) or one of its numerous 

 subsequent modifications (cf. Delbriick and Stent, 1957). Each rephca DNA 

 molecule in turn serves as the template in further replication cycles, and 

 hence the entire process is geometric. Under this mode, the vegetative phage 

 is the phage precursor DNA. 



