ADAPTATION TO PANTOTHENATE DEFICIENCY 



22-18 



volved in the new route of synthesis. It is difficult to conceive of 

 the process being established by a spontaneous chance one -step gene 

 mutation, unless most of the route were previously established. 

 It seems most likely that the complete mechanism for pantothenate 

 synthesis was operative in some of the progenitors of culture 2154, 

 but that a step was blocked in the non- synthesizer and this block 

 was overcome by the adaptation. 



MUTATION IN THE PRESENCE OF PANTOTHENATE 



An attempt was made to discover why adaptation occurs at 2> 

 per liter or lower, but not above this point. The first question is 

 whether mutations occur at all in high concentrations of pantothen- 

 ate. To determine this two groups of test tubes containing 4 ml, of 

 complete medium were inoculated with around 500-1000 cells of 

 2154 and allowed to grow to completion. The yeast was then washed 

 five times with sterile water and the entire cultures plated out on 

 pantothenate -free agar. The plates were incubated at 30°C and ob- 

 served for appearance of colonies. The results obtained are given 

 in table 22-2. 



Table 22-2, 



Occurrence of Mutants in the 

 Presence of Pantothenate, 



Experiment I 



Average number of cells seeded on each plate — 5.78 x lo''', 



