172 



son, most of these failed to germinate. 

 From seven asci, however, one or more 

 spores germinated. These were grown 

 on a medium containing glucose, malt 

 extract and yeast extract, and in this 

 they all grew normally. The normal 

 and mutant cultures were differen- 



BEADLE AND TATUM 



tiated by growing them on a Be defi- 

 cient medium. On this medium the 

 mutant cultures grew very little, while 

 the non-mutant ones grew normally. 

 The results are summarized in table 

 2. It is clear from these rather limited 

 data that this inability to synthesize 



Table 2 



Results of Classifying Single Ascospore Cultures from the Cross of 

 Pyridoxinless and Normal N. sitophila 



N, normal growth on Bg-free medium, pdx, slight growth on Bg-free medium. Failure 

 of ascospore germination indicated by dash. 



• Spores 2, 3 and 4 isolated but positions confused. Of these, two germinated and 

 both proved to be mutants. 



vitamin B^ is transmitted as it should 

 be if it were differentiated from nor- 

 mal by a single gene. 



The preliminary results summarized 

 above appear to us to indicate that the 

 approach outlined may offer consider- 

 able promise as a method of learning 

 more about how genes regulate de- 

 velopment and function. For example, 

 it should be possible, by finding a 

 number of mutants unable to carry 

 out a particular step in a given syn- 

 thesis, to determine whether only one 

 gene is ordinarily concerned with the 

 immediate regulation of a given spe- 

 cific chemical reaction. 



It is evident, from the standpoints 

 of biochemistry and physiology, that 

 the method outlined is of value as a 

 technique for discovering additional 

 substances of physiological signifi- 

 cance. Since the complete medium 

 used can be made up with yeast ex- 



tract or with an extract of normal 

 Neurospora, it is evident that if, 

 through mutation, there is lost the 

 ability to synthesize an essential sub- 

 stance, a test strain is thereby made 

 available for use in isolating the sub- 

 stance. It may, of course, be a sub- 

 stance not previously known to be 

 essential for the growth of any organ- 

 ism. Thus we may expect to discover 

 new vitamins, and in the same way, it 

 should be possible to discover addi- 

 tional essential amino acids if such 

 exist. We have, in fact, found a mutant 

 strain that is able to grow on a me- 

 dium containing Difco yeast extract 

 but unable to grow on any of the 

 synthetic media we have so far tested. 

 Evidently some growth factor present 

 in yeast and as yet unknown to us is 

 essential for Nenrospora. 



Sznmiiary—A procedure is outlined 

 by which, using Neurospora, one can 



