CHEMISTRY AND VIRAL GROWTH 



and Maal0e (65) and French et al. (18) described similar results 

 without, however, subscribing to KozlofF's conclusion. 



Subsequent experiments (29) have shown that the earlier 

 results can be questioned on strictly technical grounds. For 

 this purpose it is necessary to distinguish between the effects of 

 ultraviolet light and ionizing radiations. 



Phage inactivated by ionizing radiations are found to be 

 largely incapable of injecting their DNA into bacterial cells, 

 although their ability to attach to the cells is not affected. 

 Following mixed infection with irradiated and unirradiated 

 virus, the viral yield after cellular lysis is heavily contaminated 

 with the original particles of irradiated virus. For this reason it 

 is not possible to measure isotopic contribution from the ir- 

 radiated virus to the progeny of the mixed infection (29). 



The early experiments with phage inactivated by ultra- 

 violet light are questionable for two reasons. First, the genetic 

 potency of ultraviolet-killed phage in mixed infections is only 

 now being investigated in a quantitative fashion (12) and proves 

 to be much greater than previously supposed. Second, the 

 isotopic contribution is not independent of the radiation dosage, 

 as Kozloff believed, but falls slowly and continuously with in- 

 creasing doses (29). This effect cannot be attributed solely to 

 loss of the ability to inject ; the latter property is extremely re- 

 sistant to ultraviolet light. The careful experiments that will be 

 required to assess the significance of the correlation between the 

 behavior of genetic and isotopic tracers remain to be done, but 

 it is clear that a correlation exists. 



A very clear qualitative correlation can be demonstrated for 

 the following special situation. As previously mentioned, phage 

 particles attaching to bacteria that have already been infected 

 2 minutes or so earlier fail to contribute genetic markers to the 

 progeny, and half the DNA of the superinfecting particles is 

 quickly split into acid-soluble material. At first it was thought 

 that the chemical breakdown might be the cause of the genetic 

 exclusion. However, French et al. (18) found that no chemical 

 breakdown occurs when the bacterial deoxyribonuclease is 



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