INTEACELLULAR MULTIPLICATION OF BACTERIAL VIRUSES 259 



are phenotypically like T4, inasmuch as they have the T4 host range, but are 

 genotypically like T2, inasmuch as their progeny have the T2 host range. The 

 process by which phage particles arise possessing the host range genome of 

 one but the host range phenotype of another parent is called phenotypic 

 mixmg (Hershey et ai., 1951b). Phenotypic mixing has been observed not only 

 for the host range character but also for neutrahzing antigens and adsorption- 

 cofactor requirements (Streisinger, 1956; Brenner, 1957). From detailed 

 studies of the mechanism of phenotypic mixing, Streisinger (1956) concluded 

 that among the issue of a mixed infection, host range and neutralizing 

 antigen phenotypes are randomly associated with the corresponding geno- 

 types. Since the proteinaceous phage tail is the site of adsorption to the 

 sensitive host (Anderson, 1953; Kozloff and Henderson, 1955; KeUenberger 

 and Arber, 1955) and antiserum neutralization (Lanni and Lanni, 1953), it is 

 evident that the phenotypes involved in phenotypic mixing represent 

 certain structural features of the phage protein. The genotype of these traits, 

 on the other hand, probably resides in the phage DNA molecules, which, as 

 shall soon be considered, bid fair as the germinal substance of the extracellular 

 virus particle. One may envision, therefore, that the union of the viral DNA 

 with the viral (tail) protein proceeds by random association of materials from 

 the phage DNA and phage protein precursor pools. 



2. Proflavine 



One other hne of experimentation relevant to the maturation process has 

 shown that it is possible to interfere with maturation by means of drugs. 

 Foster (1948) observed that in the presence of the acridine dye, 'proflavine, 

 phage-infected bacteria lyse after the normal latent period but release no 

 infective progeny. If proflavine is removed from the gro\vth medium a few 

 minutes before the onset of lysis, however, a considerable burst of infective 

 progeny is released, suggesting that proflavine blocks a rather late step of 

 phage reproduction. Electron optical, immunological, and chemical studies 

 support this view, since noninfective proflavine lysates do contain the 

 "incomplete" phage constituents mentioned in an earher section. Thus, it 

 has been foimd that, in the presence of proflavine, empty phage heads or 

 doughnuts, complement-fixing (CF), serum-blocking (SBP) phage antigens, 

 and phage (i.e., HMC-containing) DNA make their intracellular appearance 

 in amounts and with kinetics not unlike those characteristic of infected 

 bacteria growing in the absence of the drug. No structures are present in 

 proflavine lysates, however, which exliibit the morphology of intact phage 

 particles (DeMars et al, 1953; DeMars, 1955; KeUenberger and KeUenberger, 

 1957). One would thus like to beHeve that whUe proflavine aUows the synthesis 

 of all known major constituents of the phage, it inliibits in some way their 

 assembly into the mature virus. The observation of KeUenberger and Sechaud 



