136 BENZER ET AL. 



present in the yield should measure the inactivation of the unit carry- 

 ing the r locus. On this basis a survival curve (see 11) for the r locus 

 has been determined; this curve has a curvature with a downward 

 concavity. However, in the mixed bursts the yield of particles belong- 

 ing to the same type as the radiated parent is strongly reduced (yield 

 reduction effect): this shows that other factors besides the activity of 

 the individual units interfere with reactivation. 



(d) Mixed infection with one active and one inactive particle 

 marked at two genetic loci (Dulbecco and Luria, unpublished). 



The same technique is used as in the previous section. The phage 

 combinations used were T2hr and T2h+r+, T2hr+ and Tah+r (r and 

 h not linked). Each member of each pair was used in the inactive 

 form, combined with the other in the active form. As mentioned in 30, 

 if a similar experiment is performed with active phages every burst 

 contains on the average 30% recombinants. In the experiments with 

 active and inactive phages every burst yielded the active parental type, 

 whereas the inactive parental type appeared in a very few bursts; the 

 recombinant types were not present in every burst, but bursts contain- 

 ing recombinants were much more frequent than bursts containing 

 the parental irradiated type. Yield of phage carrying any character 

 belonging to the irradiated parent was always strongly reduced. 



VIII. UV Method for Following Early Stages of 

 Intracellular Development 



35. Bacteria which have been exposed to considerable doses of 

 UV irradiation can still support phage growth (Anderson, 1948c). 

 This enabled Luria and Latarjet (1947) to study the inactivation by 

 UV of phage in the intracellular state. Bacteria are infected with T2 

 and incubated to the desired point in the latent period. A sample is 

 taken, exposed to UV, and plated (before bursting). The survival of 

 infective centers is thus determined as a function of the dose. 



Immediately after infection, the survival curve obtained is ap- 

 proximately the same as for free phage, but as intracellular develop- 

 ment proceeds, the resistance to UV becomes progressively much 

 higher, although the survival curves remain exponential. About mid- 

 way in the latent period, the curves develop a multiple-hit character. 

 Similar observations, using X-rays, were made by Latarjet (1948). 



The following method (Benzer, unpublished) makes use of the 

 large change in resistance as a tool in studying phage development. If 

 washed bacteria are infected in buffer and incubated, there is no change 



