MANNER OF PRODUCTION OF MUTATIONS 549 



alent to the hydrogen peroxide which had been produced, but now prob- 

 ably in organic form, was demonstrated by the amount of evohition of 

 oxygen brought about by the addition of catalase. At the same time, 

 this degradation of the peroxide by catalase, whether carried out on a 

 medium which had been treated with hydrogen peroxide or on one 

 treated with ultraviolet, rendered the medium mutagenically inactive. 

 Moreover, it is highly significant that strains of the organisms which 

 contained an unusually high amount of catalase within their cells were 

 found to be virtually immune to the mutagenic action of either irradiated 

 or hydrogen peroxide-treated medium (Wyss et al., 1950). Thus the 

 proof is convincing that, in these experiments, peroxides, derived from 

 amino acids and vulnerable to catalase, constituted an intermediary 

 substance in the production of mutations by ultraviolet. 



Subsequently Laterjet (cited by Howard, 1950) found that administra- 

 tion of catalase to E. coli before ultraviolet treatment increased their 

 survival. In fact, with strain K12 its administration even 2 hours (at 

 37°C) after ultraviolet treatment greatly increased the influence of 

 visible light in promoting their survival, a finding which tied in the 

 reparative effect of catalase with that of light. In these experiments, 

 however, only the rate of killing, not that of mutation, was studied. 



As had been found in experiments on the inactivation of enzymes and 

 viruses by radiation in cases in which "protective" substances were 

 absent from the medium, the ultraviolet-treated medium proved, both 

 with Staphylococcus and with Neurospora, to be more effective in so far as 

 the proportion of altered particles (mutated organisms) was concerned, 

 when these particles were present in dilute concentration than when they 

 were more crowded. This well-known "dilution effect" is caused by the 

 limited number of active particles (in this case the peroxide molecules) 

 relative to those which are to be acted upon (the microorganisms). It 

 shows that the active particles travel about so that, when they do not 

 encounter a recipient particle nearby, they can eventually meet with and 

 affect one that was originally more distant. Here then is a further 

 result which is incompatible with the idea of a narrow spatial limitation 

 of the mutagenic pathway. 



Direct irradiation of the cells was, however, found to exert a dispropor- 

 tionately great mutagenic effect, as compared with irradiation of only the 

 medium (Wagner et al., 1950). This is especially true if the longer muta- 

 genic wave lengths, such as 2700 A, which are ineffective in activating 

 water, are used; for it has not been found possible to obtain mutations by 

 irradiating the medium with them (Haas et al., 1950). This shows that, 

 in addition to the long-distance effect discussed, there is a more potent 

 short-distance effect (although not necessarily different in principle from 

 the other) by which the greater part of the mutations are ordinarily pro- 

 duced. These two effects are differently modifiable, as shown for 



