in nature with respect to their nutritional requirements, we began our work 

 with the mold Neurospora crassa. 



I shall not renumerate the factors involved in our selection of this organism 

 for the production of chemical or nutritionally deficient mutants, but must 

 take this opportunity of reiterating our indebtedness to the previous basic 

 findings of a number of investigators. Foremost among these, to B. O. Dodge 

 for his establishment of this Ascomycete as a most suitable organism for genetic 

 studies (6); and to C. C. Lindegren (7), who became interested in Neurospora 

 through T. H. Morgan, a close friend of Dodge. 



Our use of Neurospora for chemical genetic studies would also have been 

 much more difficult, if not impossible, without the availability of synthetic 

 biotin as the result of the work of Kogl (8) and of du Vigneaud (9). In addi- 

 tion, the investigations of Nils Fries on the nutrition of Ascomycetes (10) 

 were most helpful, as shown by the fact that the synthetic minimal medium 

 used with Neurospora for many years was that described by him and supple- 

 mented only with biotin, and has ordinarily since been referred to as "Fries 

 medium". It should also be pointed out that the experimental feasibihty of 

 producing the desired nutritionally deficient mutant strains depended on the 

 early pioneering work of Roentgen, with X-Rays, and on that of H. J. Muller, 

 on the mutagenic activity of X-Rays and ultraviolet light on Drosophila. All 

 that was needed was to put these various facts and findings together to produce 

 in the laboratory with irradiation, nutritionally deficient (auxotrophic) mutant 

 strains of Neurospora, and to show that each single deficiency produced was 

 associated with the mutation of a single gene (11). 



Having thus successfully tested with Neurospora the basic premise that the 

 biochemical processes concerned with the synthesis of essential cell constituents 

 are gene controlled, and alterable as a consequence of gene mutation, it then 

 seemed a desirable and natural step to carry this approach to the bacteria, 

 in which so many and various naturally occurring growth-factor requirements 

 were known, to see if analogous nutritional deficiencies followed their exposure 

 to radiation. As is known to all of you, the first mutants of this type were 

 successfully produced in Acetohacter and in E. coli (12), and the first step 

 had been taken in bringing the bacteria into the fold of organisms suitable for 

 genetic study. 



Now to point out some of the curious coincidences or twists of fate as in- 

 volved in science: One of the first series of mutants in Neurospora which was 

 studied intensily from the biochemical viewpoint was that concerned with the 

 biosynthesis of tryptophan. In connection with the role of indole as a precursor 



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