238 ROBERT L. SINSHEIMER 



prevents an unknown but apparently irrevocable step toward lysis. If, 

 at this time of inhibition, there are many vegetative phage components 

 present in the cell, the chance of a successful lysogenization is apparently 

 enhanced. 191 



A final decision between a lytic or lysogenic development is apparently 

 not made until some time after infection. Thus with PI the frequency 

 of lysogenization is 0.8 if the cells are kept at 20° and 0.2 if the cells are 

 kept at 37°. 7 If the cells are infected at 37°, however, they may be trans- 

 ferred to 20° at any time up to 20 minutes after infection and the 80 % 

 lysogenization will be obtained. 



It is generally assumed, in the absence of direct evidence, that the 

 DNA of temperate phage is injected into the host cell as with T2. Good- 

 gal 192 has shown that the phosphorus of labeled temperate phage PI is 

 released in phage-free form if infected cells are lysed shortly after adsorp- 

 tion. He has also shown using phosphorus-labeled PI, that under condi- 

 tions providing 80 to 90 % lysogenization, over 75 % of the phosphorus 

 taken into the cells in the first few minutes remains in the cells for up 

 to 8 generations of growth. He states that this phosphorus is present in 

 the form of DNA. 



Presuming that the DNA of the infecting phage enters the cell, various 

 lines of evidence suggest that replication of the DNA occurs before a defi- 

 nite transformation to prophage is made. Levine 193 has presented evi- 

 dence that genetic recombination may occur before formation of a defi- 

 nite prophage. Luria et al. 191 have shown by careful pedigree studies that 

 a stable lysogenic state may not be achieved for several cell generations 

 during which replication of the phage genome must occur. These studies 

 indicate that after the "decision not to lyse" 188, 191 has been made, a loose 

 association is established between one or two phage genomes and the bac- 

 terial genome. Then this replicates, occasionally throwing off either stable 

 lysogenic cells, or nonlysogenic cells, or persisting for a protracted period 

 as an unstable clone. 



Stent and Fuerst 194 have used heavily P 32 -labeled lambda phage and 

 studied the sensitivity to phosphorus decay of phage-bacterium complexes 

 at various times after infection. They demonstrated that both lytic and 

 lysogenic centers are equally sensitive, indicating that comparable amounts 

 of phage DNA are necessary to produce either response. They also demon- 

 strated that by 15 to 25 minutes after infection both types of complex 

 were completely resistant to decay of the P 32 of the parental phage. This 



191 S. E. Luria, D. K. Fraser, J. H. Adams, and J. W. Buras, Cold Spring Harbor 

 Symposia Quant. Biol. 23, 71 (1958). 



192 S. H. Goodgal, Biochim. et Biophys. Acta 19, 333 (1956). 



193 M. Levine, Virology 3, 22 (1957). 



194 G. S. Stent and C. R. Fuerst, Virology 2, 737 (1956). 



