Chapter 14: Gene Regulation in Bacteria

If a gene is repressible and under positive control, what kind of effector molecule and regulatory protein are involved in its regulation? Explain how the binding of the effector molecule affects the regulatory protein.

- an inhibitor molecule and an activator protein are involved - the binding of the inhibitor molecule to the activator molecule would prevent it from binding to the DNA and thereby inhibit its ability to activate transcription

What is part of the lac operon?

- lacO - lacZ - lacA - lacP

What would be considered a reversible posttranslational modification?

- phosphorylation - acetylation - methylation

Transcription of the lac operon results in he production of how many different mRNA transcripts?

1

Determine if each of the following statements describes regulation at the transcription, translation, or posttranslation level: 1. Genetic regulatory proteins bind to the DNA 2. A transcription terminator forms in the RNA 3. Repressor proteins bind to an mRNA and prevent ribosome binding. 4. Riboswitches produce an RNA conformation that prevents ribosome binding. 5. Antisense RNA binds to the mRNA. 6. The product of a metabolic pathway inhibits the first enzyme in a pathway. 7. A protein is covalently modified.

1. Transcription 2. Transcription 3. Translation 4. Translation 5. Translation 6. Posttranslation 7. Posttranslation

T/F: 1. The LacI gene makes a repressor protein. 2. The structural proteins encoded by the lac operon are involved in lactose synthesis 3. the CAP protein is an activator that binds to the operator 4. The product of LacI is an allosteric protein that can undergo a conformational change when allolactose binds to it. 5. CAP binding causes the DNA to bend facilitating RNA polymerase binding.

1. True 2. False 3. False 4. True 5. True

How many binding sites are there in the lac operon?

3

Bacteria use riboswitches extensively; about ____% of bacterial genes are regulated this way

4%

How can antisense RNA inhibit translation?

An antisense RNA binds to a transcript and inhibits translation

T/F: In the absence of glucose and presence of lactose, allolactose combines with the repressor and allows RNA polymerase to proceed with transcription.

True

The lac repressor is inactivated by binding to which of the following?

allolactose

The trpR gene encodes a ______________.

repressor

One form of gene expression control found frequently in prokaryotes involves a ______________, which occurs when an RNA can exit in two different secondary conformations.

riboswitch

One conformation of the RNA requires a small molecule binding which stabilizes the ______________ structure.

secondary

the RNA forms stem loops as it is being ___________.

transcribed

In Bacilus subtilis, a riboswitch controls the _________________ of the thi operon involved in thiamin pyrophosphate synthesis.

transcription

The trp operon is also regulated by ___________ attenuation

transcription

When both glucose and lactose are present...

transcription of the lactose operon is turned off

Riboswitches can regulate transcription, ____________, and RNA stability

translation

Translation of mRNA transcribed from the trp operon can be regulated by formation of a secondary structure within the mRNA. The mRNA that forms this secondary structure is transcribed from which gene in the trp operon?

trpL

What is the difference between a constitutive gene and a regulated gene?

- A constitutive gene is unregulated (its expression level is relatively constant) - In bacteria, the regulation of genes often occurs at the level of transcription by combinations of regulatory proteins and small effector molecules (gene expression can be regulated at the level of translation or the function of a protein can be regulated after translation is completed)

In general, why is it important to regulate genes? Discuss examples in which it would be advantageous for a bacterial cell to regulate genes.

- In bacteria, gene regulation enhances the efficiency of cell growth. - it is more efficient and the cell is better at competing in its environment if it expresses a gene only when the gene product is needed example: a bacterium will express only the genes that are necessary for lactose metabolism only when the bacterium is exposed to lactose. (similarly, when tryptophan levels are high within, the cytoplasm, the genes required for tryptophan biosynthesis are repressed)

T/F: An inhibitor regulates an inducible gene.

False

How is the lac operon regulated?

It is inducible, under negative and positive control

If a cell has a mutation in the lacY gene that produces a non-functional product, how is regulation of the lac operon affected?

The lac operon cannot be induced, because lactose cannot enter the cell

T/F: The lac repressor protein is active in the absence of lactose within the cell

True

T/F: The presence of a small effector molecule may inhibit transcription

True

Attenuation of the trp operon requires which conditions to be met?

Tryptophan levels must be high, and regions 3 and 4 must pair

Transcriptional regulation often involves protein that binds to a segment of DNA and a small effector molecule that binds to the regulatory protein

a. repressor - regulatory protein b. inducer - effector molecule c. operator site - DNA segment d. corepressor - effector molecule e. activator - regulatory protein f. attenuator - DNA segment g. inhibitor - effector molecule

The absence of lactose results in the lac repressor protein _______________.

binding to DNA

The lac repressor..........

binds to the operator and prevents transcription of the lactose operon.

When glucose is present,

cyclic AMP is low, CAP does not bind to the activator binding site, and transcription of the lactose operon is turned off

The repressor is bound to the operator when tryptophan levels are _______.

high

The attenuator is similar to rho- ___________ termination.

independent

The lac repressor binds to what sire within the lac operon?

lacO

Which gene is responsible for producing the protein that converts lactose to allolactose?

lacZ

What would result in high levels of gene transcription from the lac operon?

low glucose, high lactose

The cAMP-CAP binding site is between operators O1 and O3. When cAMP-CAP is bound it facilitates binding of the repressor. Under what conditions would this occur?

low glucose, low lactose

How would the interpretation of the Jacob, Monod, and Pardee experimental results change if lacO was mutated instead of lacI?

the lac operon has constitutive expression and O has a cis-effect

T/F: Polycistronic mRNA is transcribed from the lac operon when lactose is present in the cell

true