FATE OF INFECTING PHAGE PARTICLES 231 



mately half of it is transmitted as pieces containing perhaps 20 

 per cent each of the DNA in one particle, the remainder in con- 

 siderably smaller pieces. 



The transfer as large and small pieces without apparent inter- 

 conversion suggests to Levinthal a functional division into two 

 kinds of DNA which, however, must be transmitted with equal 

 efficiency. The large pieces, at least, may have genetic function 

 since they seem, in the experiments of Hershey and Burgi, to be 

 transmitted in association with radiation damage and authentic 

 genetic markers. At present, however, these ideas are rather 

 precariously based on preliminary results that are by no means 

 free of inconsistencies. 



An unexpected finding of the transfer experiments is the phe- 

 nomenon of superinfection breakdown. Shortly after infection 

 with T2, the receptivity of the cell to virus is changed in several 

 important ways. Superinfecting T2 injects only half its DNA, 

 and this half is quickly broken down to low molecular weight 

 material and excreted into the culture medium. If the cell is 

 superinfected with some other phages no breakdown occurs, but 

 the superinfecting phage particles are nevertheless excluded from 

 participation in growth. It is clear that the cellular deoxyribo- 

 nuclease brings about the breakdown of superinfecting T2, and 

 that an intracellular inhibitor of this enzyme is partly destroyed 

 shortly after infection. The fact remains, however, that the DNA 

 of the primary infecting phage is subject to little or no breakdown 

 at the time of infection, and remains immune afterwards. It must 

 be protected either by a change in state, or by a favored location 

 in the cell that the DNA of superinfecting phage fails to reach. 

 The second alternative is perhaps favored since injection by super- 

 infecting T2 is clearly abnormal, and may fail entirely for other 

 phages whose DNA is not subject to breakdown. In any case it 

 is clear that the breakdown by deoxyribonuclease is not the 

 primary cause of genetic exclusion of superinfecting phages. 



