254 G. S. STENT 



first infective progeny make their intracellular appearance, already 40 to 80 

 phage equivalents of HMC-containing DNA are present within the infected 

 cells. The synthesis of this DNA continues throughout the remainder of the 

 latent period, there being always an excess of the number of phage equivalents 

 of phage DNA over the total number of mature progeny viruses which 

 accumulate during this time. The excess phage DNA which does not form 

 part of intact progeny particles is "free" in the sense that after artificial lysis 

 of the infected cell, it is sensitive to the action of deoxyribonuclease and does 

 not sediment in centrifugal fields capable of sedimenting mature phages 

 (Hershey, 1953a). 



It is thus apparent that the formation of both the protein and the nucleic 

 acid of the descendants of the infecting bacteriophage commences during the 

 eclipse. Protein and nucleic acid moeities, however, first appear in separate 

 forms within the infected cell prior to a process of maturation by which these 

 two principal components of the virus are united to constitute the infective 

 unit. 



B. The Precursor Pool 



In order to account for his discovery of muJtipHcity reactivation after UV 

 irradiation, Luria (1947) proposed that the genome of the T-even phages is 

 comprised of a number of subunits, which multiply independently of one 

 another and generate an intrabacterial subunit pool from which a full genetic 

 complement is subsequently withdrawn for each progeny particle. Some 

 years later, Visconti and Delbriick (1953) were similarly led to propose, on 

 the basis of their study of the mechanism of genetic exchange of bacterio- 

 phages, that, prior to their maturation into infective progeny, the genetic 

 structures of the vegetative phage find themselves in a mating pool, in w^hich 

 they undergo recombination and from which they are withdrawn at random 

 for maturation into resting, infective progeny (cf. Chapter 8). Contemporan- 

 eously with these genetic investigations, isotopic tracer studies of the kinetics 

 of synthesis and assembly of the viral components bad commenced. These 

 studies, as we shall see now, likewise led to the idea that the incomplete viral 

 components, phage DNA and phage protein, exist in pools prior to their being 

 put together into mature phage particles at the termination of the eclipse. 



1. Origin of Progeny Nucleic Acid 



Some theories of the nature of phage multiplication envisioned that there 

 are already present within the normal host bacterium bacteriophage 

 precursors, whose metamorphosis into mature bacteriophages is merely 

 triggered off by the infecting phage particle (Krueger and Scribner, 1939). 

 This view was dispelled by an experiment of Cohen (1948b), which has since 



