A. D. HERSHEY 



inhibited by streptomycin, and that even under these conditions 

 there is no transfer of isotope from superinfecting phage to the 

 offspring of the primary infection. 



We have recently studied superinfection a Httle further, in- 

 hibiting bacterial deoxyribonuclease when desired by reducing 

 the concentration of magnesium in the cultures to 10~^ M (29). 

 At this low magnesium concentration, injection and growth of 

 the primary infecting phage, and the transfer of its phosphorus 

 to viral offspring, are normal. Independently of magnesium 

 concentration, superinfecting phage injects only half its DNA 

 (or half the particles inject), and both genetic and isotopic 

 markers derived from it are excluded from the viral yield. The 

 half that is injected may or may not be broken down, depending 

 on the magnesium concentration. When the bacterial de- 

 oxyribonuclease is inhibited, superinfecting phage is excluded 

 from genetic participation in growth and half its DNA enters 

 the cells and remains intact without metabolic participation in 

 viral growth. Unfortunately, the significance of this parallel is 

 doubtful because the mechanism of exclusion is unknown. 



RADIOGENETIG STRUCTURE ANALYSIS 



Viral radiogenetics began some years ago when Luria (42) 

 proposed that phage particles inactivated by ultraviolet light 

 could be revived by a process of genetic substitution. His idea 

 was appealing, first, because it promised a test of the much older 

 idea that radiations produce localized genetic damages to biologi- 

 cal materials and, second, because it led to rather special as- 

 sumptions about the mechanism of viral growth. His idea sub- 

 sequently lost favor without really being disproved, partly be- 

 cause Dulbecco (13) showed that the inactive particles could 

 also be revived by something resembling repair, partly because 

 the quantitative predictions of the substitution hypothesis failed 

 at high radiation doses (14), partly because the early radiogenetic 

 experiments suffered from what is best described as lack of 

 faith. 



Stent (57) and Doermann (12) have recently revived the 



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