Wild-type Neurospora has modest food requirements.
It can grow in the laboratory on a simple solution containingminimal nutrients for growth—inorganic salts, glucose,
and the vitamin biotin—incorporated into agar, a support medium. From this so-called minimal medium, wild-type mold cells use their metabolic pathways to produce all the other molecules they need for growth, dividing repeatedly and forming visible colonies of genetically identical cells. As shown in Figure 17.2, Beadle and Tatum generated differ- ent “nutritional mutants” of Neurospora cells, each of which was unable to synthesize a particular essential nutrient. Such cells could not grow on minimal medium but could grow
on complete medium, which contains all nutrients needed for growth. For Neurospora, the complete medium consists of the minimal medium supplemented with all 20 amino acids and a few other nutrients. Beadle and Tatum hypothesized that in each nutritional mutant, the gene for the enzyme that synthesizes a particular nutrient had been disabled.
This approach resulted in a valuable collection of mutant strains of Neurospora, catalogued by their defect in a particular pathway. Two colleagues of theirs, Adrian Srb and Norman Horowitz, used a collection of arginine-requiring mutants to investigate the biochemical pathway for arginine synthesis in Neurospora . Srb and Horowitz pinned down each