Page 90 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

7.5 What Mutations Tell Us About Gene Function 249

(a) Isolation of arginine auxotrophs (b) Growth response if nutrient is added to minimal medium
X-rays Supplements
Arginino-
Mutant strain Nothing Ornithine Citrulline succinate Arginine
1. Asci Wildtype: Arg + + + + + +
ARG-E – – + + + +
ARG-F – – – + + +
ARG-G – – – – + +
Mutagenized Fruiting bodies –
conidia Ascospores ARG-H – – – – +
Wild type Crossed with dissected and
opposite wild type transferred;
one to each (c) Inferred biochemical pathway
culture tube
Genes:
2. Tubes of complete
medium inoculated ARG-E ARG-F ARG-G ARG-H
with single ascospores
Complete medium Enzymes:
Acetylornithinase Ornithine Argininosuccinate Argininosuccinate
Germination, transcarbamylase synthetase lyase
production of conidia
Reactions:
N-Acetylornithine Ornithine Citrulline Argininosuccinate Arginine
3. Conidia from Carbamyl Aspartate
each culture tested phosphate
on minimal medium. Minimal medium
No growth = nutritional mutant
Figure 7.27 Experimental support for the one gene, one
4. Conidia from enzyme hypothesis. (a) Beadle and Tatum mated an X-ray-
cultures that fail mutagenized strain of Neurospora with another strain, and they
to grow on minimal isolated haploid ascospores that grew on complete medium.
medium are tested Cultures that failed to grow on minimal medium were nutritional
on minimal medium mutants. Nutritional mutants that could grow on minimal medium
supplemented Glycine Leucine Valine Proline Serin plus arginine were Arg auxotrophs. (b) The ability of wild-type and
−
with individual Arginine Tyrosine Glutamic acid Cysteine mutant strains to grow on minimal medium supplemented with
Asparagin
amino acids. intermediates in the arginine pathway. (c) Each of the four ARG
Addition of arginine restores growth, genes specifies an enzyme needed to convert one intermediate to
reveals arginine auxotroph the next in the pathway.

pathway for arginine synthesis. They named the four genes constituents of minimal medium, each intermediate is
ARG-E, ARG-F, ARG-G, and ARG-H. both the product of one step and the substrate for the next.
They next asked whether any of the mutant Neuros- Each reaction in the precisely ordered sequence is cata-
pora strains could grow in minimal medium supple- lyzed by a specific enzyme, and the presence of each en-
mented with any of three known intermediates (ornithine, zyme depends on one of the four ARG genes.
citrulline, and argininosuccinate) in the biochemical A mutation in one gene blocks the pathway at a par-
pathway leading to arginine, instead of with arginine ticular step because the cell lacks the corresponding en-
itself. This test would identify Neurospora mutants able zyme and thus cannot make arginine on its own.
to convert the intermediate compound into arginine. Bea- Supplementing the medium with any intermediate that oc-
dle and Tatum compiled a table describing which argi- curs beyond the blocked reaction restores growth to the
nine auxotrophic mutants were able to grow on minimal mutant because the organism has all the enzymes required
medium supplemented with each of the intermediates to convert the intermediate to arginine. Supplementation
(Fig. 7.27b). with an intermediate that occurs before the missing en-
zyme does not work because the mutant cell cannot convert
the intermediate into arginine.
Interpretation of results: Genes Each mutation abolishes the cell’s ability to make an
encode enzymes enzyme capable of catalyzing a certain reaction. By infer-
On the basis of these results, Beadle and Tatum proposed ence, then, each gene controls the synthesis or activity of
a model of how Neurospora cells synthesize arginine an enzyme, or as stated by Beadle and Tatum: one gene,
(Fig. 7.27c). In the linear progression of biochemical re- one enzyme. Of course, the gene and the enzyme are not
actions by which a cell constructs arginine from the the same thing; rather, the sequence of nucleotides in a