168 



to be based on systems of biochemical 

 reactions so complex as to make analy- 

 sis exceedingly difficult. 



Considerations such as those just 

 outlined have led us to investigate the 

 general problem of the genetic control 

 of developmental and metabolic reac- 

 tions by reversing the ordinary proce- 

 dure and, instead of attempting to 

 work out the chemical bases of known 

 genetic characters, to set out to deter- 

 mine if and how genes control known 

 biochemical reactions. The ascomycete 

 Neiirospora offers many advantages 

 for such an approach and is well suited 

 to genetic studies.^ Accordingly, our 

 program has been built around this 

 organism. The procedure is based on 

 the assumption that x-ray treatment 

 will induce mutations in genes con- 

 cerned with the control of known 

 specific chemical reactions. If the or- 

 ganism must be able to carry out a 

 certain chemical reaction to survive 

 on a given medium, a mutant unable 

 to do this will obviously be lethal on 

 this medium. Such a mutant can be 

 maintained and studied, however, if it 

 will grow on a medium to which has 

 been added the essential product of the 

 genetically blocked reaction. The ex- 

 perimental procedure based on this 

 reasoning can best be illustrated by 

 considering a hypothetical example. 

 Normal strains of Neurospora crassa 

 are able to use sucrose as a carbon 

 source, and are therefore able to carry 

 out the specific and enzymatically 

 controlled reaction involved in the 

 hydrolysis of this sugar. Assuming this 

 reaction to be genetically controlled, 

 it should be possible to induce a gene 

 to mutate to a condition such that the 

 organism could no longer carry out 

 sucrose hydrolysis. A strain carrying 



BEADLE AND TATUM 



this mutant would then be unable to 

 grow on a medium containing sucrose 

 as a sole carbon source but should be 

 able to grow on a medium containing 

 some other normally utilizable carbon 

 source. In other words, it should be 

 possible to establish and maintain such 

 a mutant strain on a medium contain- 

 ing glucose and detect its inability to 

 utilize sucrose by transferring it to a 

 sucrose medium. 



Essentially similar procedures can 

 be developed for a great many meta- 

 bolic processes. For example, ability to 

 synthesize growth factors (vitamins), 

 amino acids and other essential sub- 

 stances should be lost through gene 

 mutation if our assumptions are cor- 

 rect. Theoretically, any such meta- 

 bolic deficiency can be "by-passed" if 

 the substance lacking can be supplied 

 in the medium and can pass cell walls 

 and protoplasmic membranes. 



In terms of specific experimental 

 practice, we have devised a procedure 

 in which x-rayed single-spore cultures 

 are established on a so-called "com- 

 plete" medium, i.e., one containing as 

 many of the normally synthesized con- 

 stituents of the organism as is prac- 

 ticable. Subsequently these are tested 

 by transferring them to a "minimal" 

 medium, i.e., one requiring the organ- 

 ism to carry on all the essential syn- 

 theses of which it is capable. In prac- 

 tice the complete medium is made up 

 of agar, inorganic salts, malt extract, 

 yeast extract and glucose. The minimal 

 medium contains agar (optional), in- 

 organic salts and biotin, and a disac- 

 charide, fat or more complex carbon 

 source. Biotin, the one growth factor 

 that wild-type Neurospora strains can- 

 not synthesize,^ is supplied in the form 



« Dodge, B. O., Jour. Agric. Res. 35:289- 

 305, 1927; and Lindegren, C. C, Bull. Torrey 

 Bot. Club 59:85-102, 1932. 



"^ In so far as we have carried them, our 

 investigations on the vitamin requirements 

 of Neurospora corroborate those of Butler, 

 E. T., Robbins, W. J., and Dodge, B. O., 

 Science 9'i:262-26i, 1941. 



