RADIOBIOLOGY OF BACTERIOPHAGE 383 



all these cases, the possibility of homology between the phage and host cell 

 renders ambiguous the origin of the observed mutations. It seems most 

 probable that many of the mutations result from genetic recombination 

 between the phage and a non-wild-type portion of the phage homolog carried 

 by the host cell. 



VI. Summary and Principal Conclusions 



The inactivation by radiation of T-even bacteriophage is primarily a 

 result of discrete lesions produced in the genetic structure of the particle. 

 These lesions have effects in several types of experiments which are closely 

 restricted to the region around the hit. Thus, in a genetic cross between 

 inactive and nonirradiated phage, a genetic marker which happens to be 

 remote from any lesion will appear in the progeny. A marker which happens 

 to be close to a lesion is less likely to do so. The probability of such a marker 

 rescue is like the probability of ordinary genetic exchange in that it depends 

 on distance. The biological function of a hit region of the genome is in- 

 activated by the hit. Other unhit regions in the same particle retain their 

 functions. These conclusions provide a basis for understanding the two 

 oldest and most provocative experiments in phage radiobiology — multi- 

 plicity reactivation and the Luria-Latarjet Experiment. 



Multiphcity reactivation occurs whenever an injected ceU contains at least 

 one complete unhit set of genes whose function is necessary for the process of 

 duplication and recombination between genomes. Recombination between 

 particles then occurs with a sufficiently high efficiency to assure in most 

 cases the reconstitution of an undamaged genome. 



The Luria-Latarjet effect is to be understood in similar terms. During the 

 first 6 minutes after infection the preduplicational gene functions are being 

 performed. The onset of duplication at 6 minutes creates essential immunity 

 to radiation, since multiphcity reactivation is limited then only by the degree 

 to which recombination between damaged genomes falls short of being 

 perfectly efficient. 



The picture as presented is at present primarily a formal one. It will 

 achieve more meaning upon the identification of the genes (cistrons) which 

 perform preduplicational functions. It seems likely that two of these have 

 been identified for the special case of T4 gro-wnng in K12(A). One would anti- 

 cipate that the gene(s) required for the synthesis of hydroxymethyl cytosine 

 will turn out to be another. 



It is likely that the radiobiology of all phages cannot be forced into this 

 picture. It is equally likely that the experiments and ideas appHed to T-even 

 radiobiology will supply guidance for future studies with other systems. 



