Chapter 14 Gene regulationHow is the lac operon regulated?

Which of the following sets of conditions would result in high levels of gene transcription from the lacoperon?

Low glucose, high lactose The lac operon needs to turn on when lactose is present and when little glucose is available to the cell.

True or False The lacI gene makes a repressor protein.

True

In Jacob, Monod, and Pardee's experiment, how many functional copies of lacI were there in the merozygote?

1

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

1

What stem-loop conformations favor attenuation?

1-2 and 3-4

Each statement describes a type of gene regulation. Determine if it describes regulation at the transcription, translation or post-translation 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. post-translation 7. post-translation

How many binding sites are there in the lac operon for the lac repressor?

3

Bacteria use riboswitches extensively; about____ of bacterial genes are regulated this way.

4%

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

Allolactose

What would be the result if the U-rich sequence after the 3/4 stem loop in the trp operon was replaced by a UG-rich sequence?

Attenuation would not occur if tryptophan was high Attenuation occurs when tryptophan levels are high. The U-rich sequence after the 3/4 stem loop is crucial for attenuation because it is sequence that allows rho-independent termination to occur. Recall that in rho-independent termination, after the RNA polymerase pauses, the weakly bound U-rich sequence cannot hold the RNA-DNA hybrid together. If a UG sequence were present instead, it would not be as weakly bound. This would likely prevent attenuation from occurring.

True or false: An inducer binds to an activator protein and inhibits transcription.

False

True or false: The CAP protein is an activator that binds to the operator.

False

How is the lac operon regulated?

It is inducible, under negative and positive control

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 cAMP-CAP is only bound under low glucose conditions; the lac repressor binds under low lactose conditions. Only when both of these two conditions are met will there be no transcription of the lac operon.

You are studying riboswitches in different bacterial strains and you mix up two of your samples! One sample is from E. coli and the other is from B. subtilis. You have an assay in the lab that allows you measure the levels of the enzymes encoded by the thi/thiMD operon (assume the enzymes from the two organisms are similar). Would this assay allow you to distinguish between the two strains?

No - it would be difficult to distinguish between the strains using this assay. Unfortunately, this assay would not be sufficient to distinguish between the two strains. In both cases, when TPP is low, transcription and translation occur, which would produce the enzymes. When TPP is high, the enzymes are not made, either because transcription is halted or translation is halted.

Which of the following is an example of post translational regulation in prokaryotes?

Phosphorylation of an enzyme

Which of the following would not be considered a reversible posttranslational modification?

Proteolytic processing

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

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

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 if lacO was mutated, the lac operon would be constitutively expressed, because the repressor couldn't bind the operator. This effect would be a cis-effect, because the operator on an F' factor would still be functional, and could be bound by the repressor.

A deletion in an operon removes the terminator. How will that affect the transcript that is produced from the operon?

The transcript will be produced, but longer than normal A terminator sequence is found at the end of an operon, and signals the end of transcription. If the terminator of an operon were removed, we would still expect transcription to occur, but it would not terminate correctly, and would be longer than normal.

You identify a strain of bacteria that has a mutated trp operator that cannot bind the repressor. Under what circumstances will the trp operator be transcribed?

The trp operon will be transcribed when levels of tryptophan are in the cell are low. The trp repressor binds to the trp operator when levels of tryptophan in the cell is high. Even if this mechanism is not functional, attenuation also occurs when tryptophan levels are high. Attenuation stops transcription at the attenuator sequence. Therefore, even if the operator is mutated, transcription will still only occur when levels of tryptophan in the cell are low.

If CAP could not bind to its CAP site, then what would be the result? Assume lactose is present in each scenario.

Transcription would be difficult to activate in the absence of glucose When glucose is low, cAMP is high, and cAMP binds to CAP, which then binds to the CAP site and enhances transcription. If CAP could not bind to the CAP site, it would be difficult to achieve a high rate of transcription in the absence of glucose.

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

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

Allosteric regulation is accomplished by __________.

a small molecule that fits into a site on the enzyme that is not the active site

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

True or false: The structural proteins encoded by the lac operon are involved in lactose synthesis.

false

true or false: When both glucose and lactose are absent, transcription of the lac operon occurs.

false

Enzymes involved in metabolism are most likely regulated via __________.

feedback inhibition

Which of the following is not part of the lac operon?

is not- lacI is: lacO lacZ lacA lacP

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

lacO

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

lacZ

An activator is present and results in the increase in transcription of the target gene. This is an example of _______________.

positive control

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

riboswitch

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

secondary

In Bacillus subtilis, a riboswitch controls the _______of the thi operon involved in thiamin pyrophosphate synthesis.

transcription

When both glucose and lactose are present __________.

transcription of the lactose operon is turned off

Riboswitches can regulate

transcription, translation, and RNA stability.

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 trpoperon?

trpL

True or false: In the absence of glucose and presence of lactose, allolactose combines with the repressor and allows RNA polymerase to proceed with transcription.

true

True or false: The product of lacI is an allosteric protein that can undergo a conformational change when allolactose binds to it.

true

true or false Polycistronic mRNA is transcribed from the lac operon when lactose is present in the cell.

true

true or false The lac repressor protein is active in the absence of lactose within the cell. Group starts

true

true or false: CAP binding causes the DNA to bend facilitating RNA polymerase binding.

true