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Exam 16: Control of Gene Expression in Prokaryotes

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Question 1

Multiple Choice

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RNA molecules that are complementary to particular sequences on mRNA are called:

A) complementary RNA.
B) sense RNA.
C) antisense RNA.
D) riboswitches.
E) ribozymes.

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Question 2

Multiple Choice

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Which parts of the DNA region shown in the diagram encode proteins? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

A) P
B) I, P, O
C) P, O, Z, Y, A
D) I, Z, Y, A
E) I, P, O, Z, Y, A

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D

Question 3

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You have isolated two mutations linked to the lac operon, which you designate Lac1^- and Lac2^- that cause constitutive expression of the operon. You construct strains carrying a lac operon with a mutant lacY gene on an F'. You test both ß-galactosidase activity and Lac permease activity in the strains you constructed using the artificial inducer IPTG. $\begin{array}{l}~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\text { B-galactosidase activity Lac permease activity }\\\begin{array} { ccccc } & \text { - IPTG}&{+ IPTG } & \text { - IPTG } & \text { + IPTG } \\\mathrm { Lac1 } ^ { - } \operatorname { lac } Z^- \operatorname { lac } Y ^ { + } / \mathrm { F } ^ { \prime } \operatorname { lac } Z ^ { + } \operatorname { lac } Y ^ { - } & - & + & + & + \\\mathrm { Lac2 } ^ { - } \operatorname { lac } Z^- \operatorname { lac } Y ^ { + } / \mathrm { F } ^ { \prime } \operatorname { lac } Z ^ { + } \operatorname { lac } Y ^ { - } & + & + & + & +\end{array}\end{array}$ Are Lac1^- and Lac2^- dominant or recessive? Do they act in cis or in trans? Indicate how you can determine both of these properties for each mutation. What type of lac mutations best fit the properties of Lac1^- and of Lac2^-?

When a structural gene is under negative inducible control, what would be the result of a mutation that eliminates the repressor protein?

Which of the following is NOT a common DNA-binding motif?

The lysC gene in Bacillus subtilis encodes the first enzyme in the metabolic pathway that generates lysine from its precursor molecule. In the presence of high concentrations of lysine, lysC expression is reduced. Researchers have generated mutant bacteria that produce the LysC enzyme constitutively, even in the presence of lysine. A mutant identified in this manner had two nucleotide changes in the 5' UTR of the lysC gene itself. These nucleotide changes are thought to affect the function of a riboswitch regulating transcription of lysC. Describe two possible ways in which the mutations might affect the function of this riboswitch.

A mutation occurs in the trp operon DNA of E. coli and results in the change to the two UGG tryptophan codons in the 5' UTR of the RNA to UAG stop codons. What effect will this mutation be expected to have on the regulation of this mutant trp operon compared to a wild-type operon?

What is the difference between negative control operons that use induction and systems that use repression?

Fill in the blanks in this table with "yes" or "no" for each condition of lac operon regulation. The strain is wild type with no partial diploidy. The first line is filled in for reference. $\begin{array} { | l | c | c | c | c | c | } \hline \text { Condition } & \begin{array} { c } \text { Allolactose } \\\text { levels high? }\end{array} & \begin{array} { c } \text { lac repressor } \\\text { bound to } \\\text { operator? }\end{array} & \begin{array} { c } \text { cAMP } \\\text { levels } \\\text { high? }\end{array} & \begin{array} { c } \text { CAP bound to } \\\text { CAP-binding } \\\text { site? }\end{array} & \begin{array} { c } \text { Transcription } \\\text { at highest } \\\text { level? }\end{array} \\\hline \begin{array} { l } \text { high glucose } \\\text { no lactose }\end{array} & \text { no } & \text { yes } & \text { no } & \text { no } & \text { no } \\\hline \begin{array} { l } \text { no glucose } \\\text { high lactose }\end{array} & & & & & \\\hline \begin{array} { l } \text { high glucose } \\\text { high lactose }\end{array} & & & & & \\\hline \begin{array} { l } \text { no glucose } \\\text { no lactose }\end{array} & & & & & \\\hline\end{array}$

Transcriptional control that acts by regulating the continuation of transcription is called:

A promoter that affects only genes that are on the same piece of DNA is referred to as a(n) _____-acting promoter.

A lac operon of genotype lacI⁺ lacP⁺ lacO⁺ lacZ^- lacY⁺ will not produce $\beta$ -galactosidase but will produce permease when:

If a mutation prevents the formation of the antiterminator 2+3 loop in the trp operon, what would be the effect?

What would happen to the lac operon in the absence of allolactose?

A deletion occurs in the trp operon DNA of E. coli and results in the loss of the attenuation region in the 5'UTR of the RNA. The DNA sequences of the structural genes and the operator/promoter region are not affected by deletion. What effect will this deletion be expected to have on the regulation of this mutant trp operon compared to a wild-type operon?

Where would the lac repressor be bound in a (nonmutant) E. coli cell that is growing in low glucose and high lactose? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

The 5' UTR of the trp operon RNA contains several UGG tryptophan codons. What would be the effect on transcription-attenuation gene regulation if the trp codons were converted to UGC cysteine codons?

If there are mutations that inactivate lacP and lacI, which of the following statements is true? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

A mutant E. coli strain, grown under conditions that normally induce the lac operon, does not produce ß-galactosidase. What is a possible genotype of the cells?

It is possible for a repressor to negatively regulate the expression of an operon because: