12. Gene Regulation in Prokaryotes

Tryptophan Operon and Attenuation

Open Question
Attenuation of trp operon transcription is controlled by the formation of stem-loop structures in mRNA. The attenuation function can be disrupted by mutations that alter the sequence of repeat DNA regions 1 to 4 and prevent the formation of mRNA stem loops. Describe the likely effects on attenuation of each of the following mutations under the conditions specified.

Mutated Region Tryptophan Level
a. Region 1 Low
b. Region 1 High
c. Region 2 Low
d. Region 2 High
e. Region 3 Low
f. Region 3 High
g. Region 4 Low
h. Region 4 High
Open Question
A repressible operon system, like the trp operon, contains three genes, G, Z, and W. Operon genes are synthesized when the end product of the operon synthesis pathway is absent, but there is no synthesis when the end product is present. One of these genes is an operator, one is a regulatory protein, and the other is a structural enzyme involved in synthesis of the end product. In the table below, '+' indicates that the enzyme is synthesized by the operon and '−' means that no enzyme synthesis occurs. Use this information to determine which gene corresponds to each operon function. <>
Open Question
Figure 16.13 depicts numerous critical regions of the leader sequence of mRNA that play important roles during the process of attenuation in the trp operon. A closer view of the leader sequence, which begins at about position 30 downstream from the 5' end, is shown below, running along both columns. Within this molecule are the sequences that cause the formation of the alternative hairpins. It also contains the successive triplets that encode tryptophan, where stalling during translation occurs. Take a large piece of paper (such as manila wrapping paper) and, along with several other students from your genetics class, work through the base sequence to identify the trp codons and the parts of the molecule representing the base-pairing regions that form the terminator and antiterminator hairpins shown in Figure 16.13.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning.

The entire trpL region is deleted.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning.

Region 4 is deleted.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning.

Region 3 is deleted.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning. The eight uracil nucleotides immediately following region 4 are deleted.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning. The entire polypeptide coding sequence of trpL is deleted.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning. Two nucleotides are inserted immediately following the polypeptide start codon.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning. Ten nucleotides are inserted between regions 2 and 3 of trpL.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning. Twenty nucleotides are inserted into the trpL region immediately after the polypeptide stop codon.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning. Two nucleotides are inserted into the trpL region immediately after the polypeptide stop codon.
Open Question
What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning. The start (AUG) codon of the trpL polypeptide is deleted.
Open Question
Section 9.4 describes the function of tRNA synthetases in attaching amino acids to tRNAs (see Figure 9.16). Suppose the tRNA synthetase responsible for attaching tryptophan to tRNA is mutated in a bacterial strain with the result that the tRNA synthetase functions at about 15% of the efficiency of the wild-type tRNA synthetase. Would formation of the 3–4 stem-loop structure in mRNA be more frequent or less frequent in the mutant strain than in the wild-type strain? Why?
Open Question
Section 9.4 describes the function of tRNA synthetases in attaching amino acids to tRNAs (see Figure 9.16). Suppose the tRNA synthetase responsible for attaching tryptophan to tRNA is mutated in a bacterial strain with the result that the tRNA synthetase functions at about 15% of the efficiency of the wild-type tRNA synthetase. How would this mutation affect attenuation of the tryptophan operon? Explain your answer.