102 Essays in Biochemistry 



under the proper conditions conversion of the prophage to the vege- 

 tative phase occurs, followed by practically complete lysis of the 

 culture and liberation of virus. The phage produced under these cir- 

 cumstances is identical with the original infecting particles producing 

 the prophage. Although the yields obtained here are somewhat smaller 

 than those from the virulent process, it is possible, by use of the 

 induction process, to obtain sufficient quantities of virus for chemical 

 work. 



As one might expect, this general pattern can be modified by changes 

 in the variables involved in each phase of the process. Even if the 

 phage is of the necessary genetic strain, it is possible, by changing 

 the temperature at which the cells are maintained or by altering the 

 multiplicity of the infection, to cause the virus to behave in a virulent 

 rather than temperate fashion. Moreover, reduction to the prophage 

 state may be affected by changes in environmental conditions. For 

 example, in E. coli strain K v2 exposed to the temperate phage A, reduc- 

 tion requires something less than 1 hour at 37°C. During this period 

 the intracellular phage is more easily heat-inactivated than free phage, 

 so that it is possible during the reduction period to rid cells of their 

 potential prophage by subjecting them to higher temperatures. On 

 the other hand, once established, the prophage is even more resistant 

 to heat-inactivation at 43°C. than is free phage. Even when lysog- 

 enization has been accomplished, this can in some cases be reversed 

 by changing the medium in which the material is cultured, e.g., if 

 lysogenic Bacillus megatherium is cultured in a synthetic medium 

 containing citrate, lysogenicity is lost after 61 subcultures. 



The nature of the conversion of phage to prophage, i.e., the process 

 of reduction, is not known. There is evidence, however, that during 

 the period when reduction is occurring the phage particle behaves as a 

 cytoplasmic unit, whereas after the lysogenic status has been estab- 

 lished, the prophage is firmly associated with or bound to a specific 

 chromosomal site. 



At present we have no precise information as to the chemical nature 

 of the prophage. Attempts have been made to look for the specific 

 protein of the infecting phage in a lysogenized strain, but without 

 success, and it appears that viral protein is synthesized only after the 

 lysogenic bacteria are induced. In experiments comparing the fate 

 of DNA from a virulent phage with that from a temperate phage, 

 it has been observed that with the virulent phage a considerable portion 

 of the phosphorus of the viral DNA is found in the acid-soluble 

 phosphate fraction of the bacterial cell immediately after infection, 



