Page 114 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

8.1 The Genetic Code 273

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large number of Trp auxotrophic mutants in E. coli that acid (Glu) at the same position. In another example, muta­
carried mutations in the trpA gene for a subunit of the en­ tion 78 changed the glycine at position 234 to cysteine
zyme tryptophan synthase. He next made a fine structure (Cys), while mutation 58 produced aspartic acid (Asp) at
recombination map of these mutations analogous to Ben­ the same position. These are all missense mutations that
zer’s fine structure map for the rII region of bacteriophage change a codon for one amino acid into a codon that speci­
T4, which was discussed in Chapter 7. Yanofsky then puri­ fies a different amino acid.
fied and determined the amino acid sequences of the mutant In both cases, Yanofsky found that recombination could
tryptophan synthase subunits. occur between the two mutations that changed the identity
As Fig. 8.3a illustrates, Yanofsky’s data showed that of the same amino acid; such recombination would produce
the order of mutations mapped within the DNA of the gene a wild­type tryptophan synthase gene (Fig. 8.3b). Because
by recombination was indeed colinear with the positions of the smallest unit of recombination is the base pair, two
the amino acid substitutions occurring in the resulting mu­ mutations capable of recombination—in this case, in the
tant proteins. By carefully analyzing his results, Yanofsky same codon because they affect the same amino acid—must
deduced two other key features of the relationship between be in different (although nearby) nucleotides. Thus, a codon
nucleotides and amino acids. must contain more than one nucleotide.

Evidence that a codon is composed Evidence that each nucleotide is part
of more than one nucleotide of only one codon
Yanofsky observed that point mutations altering different As Fig. 8.3a illustrates, each of the point mutations in the
nucleotide pairs may affect the same amino acid. In one tryptophan synthase gene characterized by Yanofsky al­
example shown in Fig. 8.3a, mutation 23 changed the ters the identity of only a single amino acid. This is also
glycine (Gly) at position 211 of the wild­type polypeptide true of the point mutations examined in many other genes,
chain to arginine (Arg), while mutation 46 yielded glutamic such as the human genes for rhodopsin and hemoglobin

Figure 8.3 Mutations in a gene are colinear with the sequence of amino acids in the encoded polypeptide. (a) The
relationship between the genetic map of E. coli’s trpA gene and the positions of amino acid substitutions in mutant tryptophan synthase
proteins. (b) Codons must include two or more base pairs. When two mutant strains with different amino acids at the same position were
crossed, recombination could produce a wild-type allele.
(a) Colinearity of genes and proteins

1 m.u.
Genetic map for trpA mutation

N C
Position of altered amino acid
in TrpA polypeptide
1 15 22 49 175 177 183 211 213 234 235 243 268
Amino acid in wild-type polypeptide Lys Phe Glu Tyr Leu Tyr Gly Gly Gly Ser Gln

Amino acid in mutant polypeptide STOP Leu Val Gln Cys Arg Ile Arg Glu Val Cys Asp Leu STOP
(mutant number) (23) (46) (78) (58)

(b) Recombination within a codon

0.001 m.u. 0.001 m.u.
codon for codon for
aa 211 aa 234
–
–
trpA mutant (Arg) trpA mutant (Cys)
23 78
–
–
trpA mutant (Glu) trpA mutant (Asp)
46 58
+
+
trpA wild-type recombinant (Gly) trpA wild-type recombinant
(Gly)
codon for codon for
aa 211 aa 234