134 MUTATIONS 



priming for certain new processes is made easy and for others it is 

 made difficult. 



This idea was the starting point for experiments which Dr. K0h"nark 

 and I (4) did on the adenine phis inositol requiring strain of Neu- 

 rospora. As I said already, the adenine locus responds strongly to 

 diepoxybutane, the inositol locus hardly at all. Both respond well to 

 UV, the inositol locus rather better than the adenine locus. In a cell 

 that has been pretreated with diepoxybutane, UV produces many more 

 adenine reversions than in a cell that has not been so pretreated. In 

 some way, treatment with diepoxybutane sensitizes the adenine locus 

 — but not the inositol locus — to a subsequent treatment with UV. 

 This interaction need not occur at the stage when the mutated gene 

 has to prime the cell for making adenine; it may occur earlier at the 

 stabilization stage, and there are in fact reasons for thinking this more 

 likely. Westergaard and his co-workers (50) found a similar inter- 

 action between formaldehyde and UV in the same strain. Again, 

 formaldehyde sensitized the adenine locus, but not the inositol locus 

 to mutation by UV, and this is particularly striking because formal- 

 dehyde itself is a very much weaker mutagen than diepoxybutane. In 

 addition, these experiments showed another kind of interaction that 

 has not been stressed by Westergaard, but that was quite pronounced. 

 In a cell that had been pretreated with formaldehyde, UV not only 

 produced more adenine reversions than in control cells, it also pro- 

 duced fewer inositol reversions. Whatever the final explanation may 

 be, all these cases cannot be explained simply by assuming that there 

 is a specific reaction of a mutagen with a gene. 



Next the mutated cell has to form a mutant clone in competition 

 witli the nonmutant auxotrophic cells with which it is plated together. 

 To guard against spurious specificity at this stage, one carries out 

 reconstruction experiments, that is to say, one treats together the 

 original auxotrophs and prototrophs of the kind that would have arisen 

 by mutation. If the treatment, instead of producing mutations, selects 

 for or against spontaneous mutants, it will cause an increase or de- 

 crease in the ratio of prototrophs to auxotrophs. Such reconstruction 

 experiments are valid only if they are carried out under the same 

 conditions as the mutation experiments, that is to say, with very small 

 admixtures of prototrophs and very high plating densities. Even so, 

 they do not really show the effect of competition between nonmutant 

 cells and incipient mutants, for in several cases these latter have been 

 shown to react differently from established ones. Ryan (63), for in- 

 stance, has shown that incipient reversions at the histidine locus in E. 



