INTRACELLULAR MULTIPLICATION OF BACTERIAL VIRUSES 275 



Model 2. At the outset of the infection process, the parental DNA molecules 

 transfer their hereditary information to some other type of macromolecules, 

 which then in turn act as templates in the synthesis of replica phage precursor 

 DNA molecules. If only the parental but not the rephca DNA molecules act 

 in the estabhshment of these templates, then the entire process of DNA 

 replication is hnear. If, on the other hand, there is a turnover of templates 

 and both parental as well as replica DNA molecules continue to transfer their 

 information and establish templates, even at late stages of intracellular 

 phage gro\\i:h, then the process of DNA rephcation is quasi-geometric. Under 

 this model the germinal substance of the phage is "associated" with both the 

 phage precursor DNA and the non-DNA templates during vegetative 

 reproduction, and hence it is not clear upon which structure the name 

 vegetative phage should be conferred. 



That the process of replication of the genetic substance, and, by inference, 

 of the precursor DNA of the bacteriophage is geometric is indicated by an 

 experiment of Luria (1951), which showed that the distribution of spon- 

 taneous phage mutants in single infected bacteria is clonal (cf. Chapter 8). 

 This finding, however, does not distinguish between the two models. Model 2 

 finds support from two different directions. If highly P^^-labeled bacteria are 

 infected with equally highly labeled T-even phages and the lethal effects of 

 P^^ decay followed at various stages of the latent period, it is found that 

 radioactive disintegrations can no longer prevent the appearance of infective 

 progeny if phage development has already proceeded for 7 or 8 minutes, 

 even though both the infecting DNA as well as any of its vegetative rephcas 

 which may have already been formed are highly radioactive and ought to be 

 subject to destruction by radioactive decay (Stent, 1955). Model 2 explains 

 this unexpected observation, if the non-DNA templates to which the heredi- 

 tary information is transferred are not sensitive to disruption by disintegra- 

 tion of incorporated P^^ atoms, either because they contain no phosphorus 

 (protein?) or because they possess a structure of greater inherent stabihty 

 than the free DNA duplex (nucleoprotein?). Second, the role of the essential 

 nonprecursor protem discussed in Section IV, B, whose synthesis during the 

 first few minutes after infection must be allowed to occur before the formation 

 of phage precursor DNA can take place is also readily accounted for by Model 

 2, if this protein forms an integral part of the non-DNA template. In fact, a 

 Model 2-type molecular scheme of the rephcation and genetic recombination 

 of the hereditary material of the phage has been conceived in which the 

 templates are ribonucleoprotein molecules made up of the postinfection RNA 

 and the essential nonprecursor protein of Section IV (Stent, 1958). The 

 experimental resolution of the question of the chemical nature of the 

 vegetative phage and its mode of replication and genetic exchange is one 

 of the immediate goals of present research efforts. 



