VITAMIN-SyNTHESIZING DEFICIENCIES 21-9 



the weak synthesizers were always able to grow eventually. An 

 even more important bit of confirmatory evidence was obtained 

 from the stock incapable of synthesizing uracil. The uracil -defi- 

 cient cultures responded quickly to large amounts of uracil added 

 to Bukholder's medium, but in the medium without uracil they died. 

 Transfer from uracil deficient medium to a complete medium a 

 few days after inoculation, proved that the cells had died. 



Lindegren and Raut's results indicate that all so-called "non- 

 synthesizers" probably require qualification because synthesis 

 might have been discovered if observation were made over a longer 

 period. Many reversions of Beadle and Tatum's "vitaminless" mu- 

 tants in the deficient medium have been reported by Bonner, Tatum, 

 and Beadle (1943) and other workers in this field, but they are usual- 

 ly regarded as exceptional cases rather than the standard expected 

 behavior while the reverse is apparently the case with yeasts. The 

 above results suggest the possibility that some other improvements 

 of the medium than the addition of the vitamin for which the stock 

 appears to be deficient might often lead to a higher frequency of 

 reversions. 



There are two important differences in the vitamin deficiencies 

 described by us and those described by Beadle and Tatum: (1) Our 

 variants were derived from naturally occurring forms while theirs 

 were induced by radiation. (It is possible that deficiencies induced 

 by radiation may be more complete than natural ones and have less 

 capacity for reversion.) 



(2) Our stocks were selected after a series of mating in which 

 irregular segregations had occurred. Beadle and Tatum did not 

 begin work with a treated culture unless it segregated regularly at 

 the first trial. 



