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Modern Genetic Analysis Chapter 3. Gene Function Protein Function and Malfunction in Cells
Genetics In Process3-2: Charles Yanofsky and the colinearity of gene and protein structure

In 1941, George Beadle and Edward Tatum originated the one-geneone-enzyme hypothesis. When James Watson and Francis Crick deduced the structure of DNA in 1953, it seemed likely that there must be a linear correspondence between the nucleotide sequence in DNA and the amino acid sequence in protein (such as an enzyme). However, it was not until 1963 that an experimental demonstration of this colinearity was obtained, from two research groups, one of which was led by Charles Yanofsky at Stanford University.

Yanofsky had induced 16 mutant alleles of the tryp A gene of Escherichia coli, whose wild-type allele was known to code for the a protein subunit of the enzyme tryptophan synthetase. Tryptophan synthetase is a tetramer composed of two a and two b subunits. It catalyzes the following reactions:

It is now known that the subunits act sequentially without letting go of the indole.

Yanofsky showed that the mutant tryp A alleles, which produced an inactive enzyme, were structurally all substantially similar to the wild-type allele but differed at 16 different mutant sites. He went on to map the position of the mutant sites. In 1963 DNA sequencing had not been invented, but the mutant sites could be mapped by crossover analysis (Chapter 5). The closer together two mutant sites are, the less frequently will they recombine to produce wild-type recombinants. In the following example, crossing-over of homologous chromosomes occurs between the mutant allele tryp A-1 on one chromosome and tryp A-2 on the other. The resulting chromosome (shown in the center) combines the "good" sections from each mutant gene to produce a gene free of mutation (a wild type).

Through biochemical analysis of the tryp A protein, it was shown that each of the mutants resulted in an amino acid substitution at different positions in the protein. By comparing the relative positions of the mutant sites in the gene with the relative positions of the amino acid changes in the protein, it was shown that the two were colinear.

In the summary of the 1966 paper, Yanofsky wrote:

The concept of colinearity of gene structure and protein structure was examined with 16 mutants with alterations in one segment of the A gene and the A protein [now called a ] of tryptophan synthetase. The results obtained demonstrate a linear correspondence between the two structures and further show that genetic recombination values are representative of the distances between amino acid residues in the corresponding protein.

The results subsequently have been shown to be of general application to mutations in other proteins.

© 1999 by W. H. Freeman and Company. All rights reserved.