One Gene-one Enzyme Hypothesis - Origin

Origin

Although some instances of errors in metabolism following Mendelian inheritance patterns were known earlier, beginning with the 1902 identification by Archibald Garrod of alkaptonuria as a Mendelian recessive trait, for the most part genetics could not be applied to metabolism through the late 1930s. Another of the exceptions was the work of Boris Ephrussi and George Beadle, two geneticists working on the eye color pigments of Drosophila melanogaster fruit flies in the Caltech laboratory of Thomas Hunt Morgan. In the mid-1930s they found that genes affecting eye color appeared to be serially dependent, and that the normal red eyes of Drosophila were the result of pigments that went through a series of transformations; different eye color gene mutations disrupted the transformations at a different points in the series. Thus, Beadle reasoned that each gene was responsible for an enzyme acting in the metabolic pathway of pigment synthesis. However, because it was a relatively superficial pathway rather than one shared widely by diverse organisms, little was known about the biochemical details of fruit fly eye pigment metabolism. Studying that pathway in more detail required isolating pigments from the eyes of flies, an extremely tedious process.

After moving to Stanford University in 1937, Beadle began working with biochemist Edward Tatum to isolate the fly eye pigments. After some success with this approach—they identified one of the intermediate pigments shortly after another researcher, Adolf Butenandt, beat them to the discovery—Beadle and Tatum switched their focus to an organism that made genetic studies of biochemical traits much easier: the bread mold Neurospora crassa, which had recently been subjected to genetic research by one of Thomas Hunt Morgan's researchers, Carl C. Lingegren. Neurospora had several advantages: it required a simple growth medium, it grew quickly, and because of the production of ascospores during reproduction it was easy to isolate genetic mutants for analysis. They produced mutations by exposing the fungus to X-rays, and then identified strains that had metabolic defects by varying the growth medium; if the synthesis of a particular nutrient (such as an amino acid or vitamin) was disrupted by mutation, that mutant strain could be grown by adding the necessary nutrient to the medium. Following their first report of three such auxotroph mutants in 1941, Beadle and Tatum used this method to create series of related mutants and determined the order in which amino acids and some other metabolites were synthesized in several metabolic pathways.

The nutritional mutants of Neurospora also proved to have practical applications; in one of the early, if indirect, examples of military funding of science in the biological sciences, Beadle garnered additional research funding (from the Rockefeller Foundation and an association of manufacturers of military rations) to develop strains that could be used to assay the nutrient content of foodstuffs, to ensure adequate nutrition for troops in World War II.