Question 1

Which of the following best describes the function of the lacI gene in the regulation of the lac operon in E. coli?

Accepted Answer

It encodes a repressor protein that binds to the operator and inhibits transcription of the lac operon.

Question 2

A mutation in the operator site of the lac operon (Oc) results in which of the following outcomes?

Accepted Answer

The lac operon is constitutively expressed because the repressor cannot bind to the mutated operator.

Question 3

Which of the following scenarios will result in the highest level of lac operon transcription in E. coli?

Accepted Answer

Lactose present, glucose absent, high cAMP

Question 4

In the trp operon, what is the role of tryptophan in gene regulation?

Accepted Answer

It acts as a corepressor that binds to the repressor, enabling it to bind to the operator and block transcription.

Question 5

Which of the following best explains why the lacI- mutation leads to constitutive expression of the lac operon?

Accepted Answer

The mutation prevents the synthesis of a functional repressor, so the operator is never blocked.

Question 6

A bacterium has a mutation in the trpL leader peptide such that the two Trp codons are replaced with codons for alanine. What is the most likely effect on attenuation of the trp operon?

Accepted Answer

Attenuation will occur regardless of tryptophan levels, leading to reduced expression of the structural genes.

Question 7

Which of the following statements about catabolite repression of the lac operon is correct?

Accepted Answer

Glucose inhibits adenylyl cyclase, lowering cAMP levels and reducing CAP activation.

Question 8

Which of the following is a key difference between inducible and repressible operons in bacteria?

Accepted Answer

Inducible operons are usually turned on by the presence of a substrate, while repressible operons are turned off by the product of the pathway.

Question 9

A mutation eliminates the O1 operator site of the lac operon but leaves O2 and O3 intact. According to experimental data, what is the expected effect on repression of the operon?

Accepted Answer

Repression will be minimal, dropping to 1-2 fold, because O1 is the primary operator.

Question 10

Which of the following mechanisms allows the trp operon to respond rapidly to changes in tryptophan levels in the environment?

Accepted Answer

Attenuation, which couples transcription and translation to terminate transcription early when tryptophan is abundant

Question 11

In the context of feedback inhibition, which of the following best describes the role of the final product in a metabolic pathway?

Accepted Answer

It binds to the allosteric site of the first enzyme, inhibiting its activity when product concentration is high.

Question 12

Which of the following best explains how antisense RNA regulates the translation of the ompF gene in E. coli under high osmolarity conditions?

Accepted Answer

Antisense RNA binds to the ompF mRNA, forming a double-stranded region that blocks ribosome binding and translation.

Question 13

A bacterium is exposed to high levels of thiamin pyrophosphate (TPP). How does the TPP riboswitch in B. subtilis regulate the thi operon under these conditions?

Accepted Answer

TPP binds to the aptamer domain, causing the formation of a terminator stem-loop and premature termination of transcription.

Question 14

Which of the following is a common reversible covalent modification that regulates protein function posttranslationally in bacteria?

Accepted Answer

Phosphorylation, which adds a $$-PO_4$ group to the protein

Question 15

Which of the following best describes the mechanism of translational repression by a regulatory protein in bacteria?

Accepted Answer

The protein binds to the mRNA near the Shine-Dalgarno sequence, blocking ribosome binding and translation initiation.

Question 16

A mutation in the lacI gene produces a super-repressor (Is) that cannot bind allolactose. What is the expected phenotype of a bacterium with this mutation?

Accepted Answer

The lac operon is always repressed, even in the presence of lactose.

Question 17

Which of the following equations correctly represents the effect of an inducer on a repressor protein at an inducible gene?

Accepted Answer

$$	ext{Repressor} + 	ext{Inducer} ightarrow 	ext{Transcription occurs}$$

Question 18

Which of the following best explains why attenuation of the trp operon only occurs in bacteria and not in eukaryotes?

Accepted Answer

Bacteria have coupled transcription and translation, allowing the ribosome to influence mRNA secondary structure during transcription.

Question 19

A mutation in the CAP binding site of the lac operon prevents CAP-cAMP from binding. What is the most likely effect on lac operon expression when lactose is present and glucose is absent?

Accepted Answer

Transcription will be low because CAP-cAMP cannot enhance RNA polymerase binding.

Gene Regulation in Bacteria: Mechanisms and Examples

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