21-8 THE YEAST CELL 



plied with inositol and pantothenate. There were probably consid- 

 erable amounts of biotin in the agar, for removal of biotin did not 

 usually reduce growth greatly. Removal of biotin and inositol sim- 

 ultaneously was serious. When either inositol or pantothenate were 

 removed singly no serious effect occurred, but when both were re- 

 moved together there was considerably less growth. The cells could 

 synthesize both inositol and pantothenate easily when only one was 

 absent but in the absence of both they synthesized poorly. These 

 facts indicate that ability to grow in the absence of vitamins may 

 involve simply lack of capacity to grow, or begin to grow, under 

 the prescribed conditions rather than inability to synthesize the 

 absent vitamins under all conditions (Williams, 1941). 



THE RELATION BETWEEN VIABILITY OF CELLS AND 

 INABILITY TO GROW ON DEFICIENT MEDIUM 



The ability or inability to grow in a deficient medium may 

 merely involve inviability in the new medium rather than absolute 

 inability to synthesize the vitamin in question. If the cells are in- 

 viable in the deficient medium they will be unable to begin synthe- 

 sis of the required vitamin. The fact that most yeasts begin to syn- 

 thesize when they are permitted to stand a sufficiently long time 

 suggests that continued examination of cultures is necessary. This 

 points up an important difference between Neurospora and yeasts. 

 A conidium of Neurospora can put out a germ tube and begin growth 

 in distilled water but if it is to continue growth it must be immedi- 

 ately supplied with the substances which it requires. It is relative- 

 ly difficult to establish a dormant mycelium capable of awaiting 

 future developments. Probably one of the most important advan- 

 tages of a single -celled form over a filamentous fungus is that the 

 single -celled form can go through one or two divisions and settle 

 down to dormancy while a filamentous organism with specialized 

 spores must continue to grow once the filament is produced or it 

 will perish. This may give the appearance of absolute deficiencies 

 to many mutants which die shortly after producing a filament in a 

 medium lacking a specific vitamin. This suggests that many "vita- 

 minless" mutants which are incapable of producing the enzyme com- 

 pleting one step in the synthesis of the specific vitamin, might be 

 able to synthesize the vitamin if the medium were adequate. 



These results have been supported by further work on three 

 other mutants differentiated by ability to synthesize p-aminoben- 

 zoic acid, pyridoxine and uracil. So-called nonsynthesizers of py- 

 ridoxine eventually grew in a medium containing no added pyridox- 

 ine. Diploids heterozygous for a gene controlling synthesis of p-a- 

 minobenzoic acid produced two classes of offspring, one which grew 

 rapidly and one which grew slowly in the absence of p-aminoben- 

 zoic acid. Although these two classes were clearly differentiated. 



