50 



Essays in Biochemistry 



that inaptitude is not caused by screening we know nothing more about 

 it. However, the preoccupation with the mechanism of excretion led 

 to the next working hypothesis. We considered the possibility that 

 inaptitude is the result of the loss during starvation, either by excretion 

 or by enzymatic disposal, of some radiation-sensitive locus within the 



Fig. 9(«). Infectious centers in E. coli K 12 incubated with irradiated leucovorin 

 at a dilution of~2 X 10" 4 . 



cell. That there is a specific site, an Achilles' heel, as it were, upon 

 which the inducers fall in a lysogenic organism has a compelling plausi- 

 bility. How else could we account for the homogeneity of the lyso- 

 genic response to induction? It must be recalled that close to 100% 

 of an inducible organism will, in logarithmic growth phase, consistently 

 respond to the small inducing irradiation by the proliferation of bac- 

 teriophage. The phenomenon is quite different from the mutagenesis 

 induced by irradiation. Irradiation for mutagenesis must be, by com- 

 parison to that for induction, prodigious, killing over 99.9% of the 



