Beadle & Tatum (1941)

Genetic Control of Biochemical Reactions in Neurospora:

Beadle and Tatum (1941)

Introduction and Background:

Biochemical genetics was a "coming field with a glowing future"
Many biochemical reactions are controlled in specific ways by specific genes
Beadle and Tatum had begun work in area

Drosophila eye pigment experiments

Prior work done by attempting to determine biochemical basis of already known hereditary traits

one-gene-one enzyme hypothesis suspected but not stated
Innovation! -reversed general procedure

Purpose: "to determine if and how genes control known biochemical reaction"
Work with bread mold Neurospora

easy to grow and maintain
easy to induce mutation
easy to identify and isolate mutants
biochemistry already known (better than Drosophila)

Experimental Method:

Procedure Part A:

X-ray single asexual spore colonies on complete medium
transfer to minimal medium
observe : Growth on minimal=no mutation ; No Growth=nutritional mutation induced
subsequent tests on mutants in systematic manner (XFig 14.11)
repeat approximately 2000 strains

Results Part A:

3 mutants were created and tested
#1 unable to synthesis Vitamin B6 ( pyridoxine)
#2 unable to synthesis Vitamin B1 (thiamine- thiazole half)
#3 unable to synthesis para-aminobenzoic acid

At the time the paper was sent to print further testing had been completed only on Mutant #1-unable to synthesize Vitamin B6. Further testing showed similar results for the remaining mutants.

Procedure Part B:

Tests of Growth Rates of Neurospora strains:

grew mutant B6 with varying concentrations of B6 in medium
observed length of growth in test tubes of varying dimensions and air flow
compared to control tubes of normal in similar conditions

Results Part B:

mutant grown on medium with 1mg B6 hydrochloride per 25cc of medium closely approaches rate and characteristics of normal strain grown on similar medium without B6
lower concentrations of B6 give intermediate rates (bioassay possible?)

Procedure Part C:

crosses between normal and mutant strains were made using techniques for hybridization and ascospore isolation as developed by Dodge and Lindergren
germinated examined for mutation present or absent

Results Part C:

ascospores from 24 asci of cross isolated and position in asci recorded
most failed to germinate- reasons unknown
7 asci in which 1 or more spores germinated
results of number mutations present not provided in paper

Conclusions:

proved that x-ray treatment can induce mutations in genes concerned with control of known biochemical reactions, and in sufficient frequency for study
studies comparing mutant and normal strains showed only difference is ability to synthesize specific metabolite (difference removed if metabolite added)
* showed genes do affect specific reactions (assumed by affecting enzymes)
crosses of mutant and normal followed pattern of single gene- single gene

* showed single genes affect specific (single?) reactions

Relation to Modern Genetics:

Major Innovation= reversing the process
able to study essential 'characters'- past methods confined to superficial
able to study characters without visible manifestations- which were often very complex, making analysis difficult

First sound scientific evidence for one-gene-one-enzyme hypothesis
this hypothesis not stated until 1945, though alluded to by Garrod in 1902, and behind reasoning of this experiment
"genes may control or regulate specific reactions in system, either acting directly as enzymes or determining the specificities of enzymes"
attempting to understand how genes affect phenotype knowing nothing about genes!
experiment shows specific mutation, which appear to be single gene, affects a specific rnx

suspected that pathway of B6 synthesis more than one step and that mutation affects one specific single step (=single enzyme) - further tests to identify mutant to specific step
proposed that their research method gave considerable promise to method to learning more about how about how genes regulate development and function

ie. By finding a number of mutants unable to carry out particular step in given synthesis (missing same enzyme), to determine whether only one gene is ordinarily involved

Applications:

1. Beadle and Tatum suggested within the original paper the possibility that growth rate of mutant unable to synthesize metabolite X could be used as bioassay for presence of metabolite X

2. By testing mutants unable to synthesize various intermediates metabolic pathways can be determined. (XFig 14.12)

3. This experiment serves as a stepping stone towards understanding of the relationship between genes and biochemical reactions.