82 Essays in Biochemistry 



their killing action by provoking unbalanced growth. For example, 

 the nitrogen mustards will kill E. coli and in so doing produce filamen- 

 tous cells in which DNA synthesis, but not RNA synthesis, is inhibited. 

 Penicillin kills only when cells are actually dividing. The treated cells 

 become filamentous and accumulate uracil-containing compounds. 



E. coli exposed to low levels of ultraviolet irradiation become fila- 

 mentous as a consequence of cytoplasmic synthesis and inhibited DNA 

 synthesis. Kanazir and Errera have recently shown that such cells 

 accumulate thymidylic acid. ^'e have compared the killing action 

 of ultraviolet irradiation with thymineless death in 15 T -. 5 The two 

 processes appear to be entirely analogous, although some complicated 

 phenomena have been found following irradiation. If irradiated cells 

 are incubated in a liquid medium for 20 minutes, a majority of the 

 cells are restored to life, as determined by plating on a thymine- 

 containing nutrient agar before and after incubation. This restoration 

 has the characteristics of the decay curve for a toxic product. At 

 maximal restoration in liquid medium it has been observed that the 

 restored cells die again whether thymine is present in the medium or 

 not. This may be observed in Fig. 1. Restoration has left a residual 

 lesion, which is expressed in the time necessary for one division, since 

 by this time all restored cells left in the liquid medium have completed 

 their second death. Plating on the solid medium appears to interrupt 

 the second death. 



As can be seen in Fig. 1, the second death occurs at the same rate 

 as thymineless death. We have also found that this may be prevented 

 by eliminating utilizable carbohydrate or nitrogen from the medium 

 and from the cells. In a complete medium, the presence of 5-methyl- 

 tryptophan, at a concentration which inhibits growth, also prevents 

 death. 5 It is a matter of some interest that restoration is not sig- 

 nificantly impeded by any of these treatments, and it becomes possible 

 to restore and conserve almost all of the cells of an irradiated popula- 

 tion, which was originally killed to the extent of 90% . It would appear 

 that in ultraviolet irradiation it is the second death, dependent on 

 unbalanced growth, which is really dangerous. In extending these 

 studies we have found identical phenomena in organisms without exog- 

 enous thymine requirements. 



It is my feeling at this moment that induction of unbalanced growth 

 describes the common mechanism of action of most of the major anti- 

 tumor agents in use at the present time. The proof of this hypothesis 

 calls for a great deal of work, but the case is so strong with bacteria 

 as experimental materials that it should be possible to use the hypoth- 



