b. The accumulation of intermediates prior to blocked reactions. 



c. The ability of the mutant blocked in the first reaction to make use of an 

 intermediate accumulated as a result of a genetic interruption of the second 

 reaction. The principle involved is the same as that employed in the cross- 

 feeding technic later so much used in detecting biosynthetic intermediates in 

 microorganisms. 



What was later called the one gene-one enzyme concept was clearly in our 

 minds at this time although as I remember, we did not so designate it. 



Ours was a scheme closely similar to that proposed by Garrod for alcap- 

 tonuria, except that he did not have genes that blocked an adjacent reaction 

 in the sequence. But at the time we were oblivious of Garrod's work, partly 

 because geneticists were not in the habit of referring to it, and partly through 

 failure of ourselves to explore the literature. Garrod's book was available in 

 many libraries. 



We continued the eye-color investigations at the California Institute of 

 Technology, Ephrussi having returned there to spend part of 1936. Late 

 in the year, Ephrussi returned to Paris and I went for a year to Harvard, 

 both continuing to work along similar lines. We identified the source of dif- 

 fusible substances — fat bodies and malpighian tubercules — and began to 

 devise ways of determining their chemical nature. In this I collaborated to 

 some extent with Professor Kenneth Thimann. 



In the fall of 1937 I moved to Stanford, where Tatum shortly joined me to 

 take charge of the chemical aspects identifying the eye-color substances. Dr. 

 Yvonne Khouvine worked in a similar rcle with Ephrussi. We made progress 

 slowly. Ephrussi and Khouvine discovered that under certain conditions 

 feeding tryptophane had an effect on vermilion eye color. Following this lead, 

 Tatum found — through accidental contamination of an asceptic culture 

 containing tryptophane and test flies — an aerobic Bacillus that converted 

 tryptophane into a substance highly active in inducing formation of brown 

 pigment in vermilion flies. He soon isolated and crystallized this, but its final 

 identification was slowed down by what later proved to be a sucrose molecule 

 esterified with the active compound. 



Professor Butenandt and co-workers (6) in Germany who had been col- 

 laborating with Professor Kuhn on an analogous eye-color mutant in the 

 meal moth Ephestia, and Amano et al. (i), working at Osaka University, 

 showed that t;+ substance was kynurinine. Later, Butenandt and Hallmann 

 (5), and Butenandt et al. (7) showed that our original cw+ substance was 

 3-hydroxy kynurenine . 



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