ADAPTATION TO PANTOTHENATE DEHCIENCY 22-16 



This analysis indicates that adaptation to pantothenate indepen- 

 dence in 2154 resulted from a mutation of dependent locus b to in- 

 dependent locus B. If the original unadapted 2154 carried the genes 

 ab, and its synthesizing sisters 2 15 land 2153 Ab, then the mutation 

 during adaptation changed ab to aB. A mating of aB (2154 +, adapted) 

 by Ab 2152 (+) can produce three types of asci containing two, three 

 and four independent spores respectively. Those containing three 

 independent spores would be like the ascus 4175, 4176, 4177, 4178 

 which is of the constitution AB ab Ab aB. Cultures 4175 and 4178 

 are both single mutants, either Ab or aB. 



The two genes can be roughly distinguished by their growth rates. 

 The original (Fig. 22-4) synthesizers (Ab) usually begin the rapid 

 observable phase of growth in pantothenate free medium before the 

 adapted 2154 genotype (aB). Among the asci in which 3:1 segrega- 

 tions occurred, two of the clones usually grew more rapidly than 

 the third. Hence, apparently, the clone of the genotype AB and that 

 of the genotype Ab generally grow at the same rate while the geno- 

 type aB grows more slowly. Fig. 22-5 shows growth curves for 

 the B gene (2154 adapted) on a series of concentrations of panto- 

 thenate; as compared with the original unadapted 2154 (ab), the 

 adapted strain (aB) grows more slowly when sufficient pantothenate 

 is supplied than the original. The ab clones showed no evidence of 

 growth for over a month, but all were subject to the adaptation 

 phenomenon described in detail for culture 2154. 



SYNTHESIS OF PANTOTHENATE BY GENE B 



To determine if the B gene controls the sjmthesis of pantothenate, 

 culture 2152 (Ab), which synthesizes pantothenate, and adapted 2154 

 (aB), were assayed. 



The cultures were prepared for assay by growing them on Burk- 

 holder's synthetic medium minus pantothenate, washing, centrifuging 

 and digesting in 10 ml. distilled water at pH 5.5 in the autoclave at 

 20 pouiids pressure for 15 minutes. An aliquot was dried and weighed 

 to determine the dry weight of the sample. 



The extracts thus prepared were assayed with culture 2154. Ex- 

 tracts from both cultures produced growth, indicating the presence 

 of free pantothenate in both of the cultures. The presence of panto- 

 thenate in the adapted strain indicates that the new gene B does not 

 function by allowing the yeast to grow without pantothenate, but in- 

 stead makes possible the synthesis of pantothenate. 



ALTERNATE ROUTE OF SYNTHESIS 



Since pantothenate is S5mthesized rapidly in the presence of ei- 

 ther A or B, or both, the new gene must enable synthesis to take 

 place over an alternate route, unless both A and B affect the same 

 step. 



