554 S. E. LUEIA 



non-DNA molecules, which will then carry, specified in their own chemical 

 language, the whole specificity of the phage heredity (Delbriick and Stent, 

 1957). If so, some such non-DNA intermediate may be able to take over tlie 

 control of DNA synthesis when the DNA itself is incapacitated, for example, 

 by radioactive decay of P^- atoms in its nucleotides (Stent, 1955), 



Tlie evidence concerning the DNA nature of prophage comes from isotope 

 experiments using coliphage A. For this and other phages it has been possible 

 to determine by bacterial crosses and by transduction (Lederberg and 

 Lederberg, 1953; Jacob and Wollman, 1957) the presence and location of the 

 corresponding prophages within the linear sequence of genetic determinants 

 of the bacterial cell chromosome. The A prophage can be inactivated in the 

 lysogenic cell by the radioactive disintegration of P^^ atoms incorporated 

 into the cell. The rate of this inactivation is the same as the rate of inactiva- 

 tion of the infectivity of similarly labeled mature phage A (Stent et al., 1957). 

 This provides a remarkable proof of the similarity of the content of essential 

 DNA in the mature phage and in the prophage. 



B. Infectious DNA from Phaxje Particles 



These studies make it possible to identify the genetic material of the phage 

 in its various states — mature, vegetative, and prophage — with a specific 

 piece of DNA, which, at least in the form introduced into the ceU by the 

 mature particle, is probably not associated with genetically significant 

 protein. Direct evidence has been obtained with phage T2 about the exist- 

 ence and size of this "master piece" of DNA and about its behavior and con- 

 servation in the process of replication (Levinthal and Thomas, 1957a,b; 

 Hershey and Burgi, 1956). 



Assuming that the nongenetic components of the mature virus particle are 

 a protective and injecting device for the essential phage DNA, it can reason- 

 ably be expected that the DNA portion, extracted either from mature 

 particles or from infected bacteria, may be able to initiate mfection, if a 

 system is available that permits penetration of the DNA into susceptible 

 cells. At least for DNA from mature phage, the expectation seems to have 

 been reahzed by the use of "protoplasts", that is, of cells deprived of part 

 of their cell wall (Spizizen, 1957; Fraser and Malder, 1957). According to these 

 reports (which may not have excluded all possible pitfaUs) the naked 

 protoplast can be infected by disrupted phage, albeit with very low effici- 

 ency. We may recall in this connection that transformation phenomena with 

 bacteria have estabhshed that fragments of bacterial DNA may be trans- 

 mitted even to intact cells (Avery et al., 1944; Hotchkiss, 1956). We may 

 also mention here the phenomenon of zygotic induction (Jacob and WoUman, 

 1956), in which vegetative phage multiplication is initiated by the 



