Biology Lecture 36 Control of Gene Expression in Bacteria

Repressor

(1) In bacteria, a protein that binds to an operator sequence in DNA to prevent transcription when an inducer is not present and that comes off DNA to allow transcription when an inducer binds to the prepressor proteins. (2) In eukaryotes, a protein that binds to a silencer sequence in DNA to prevent or reduce gene transcription.

Negative control

A type of regulation that works by slowing an ongoing process. In the context of gene transcription, the action of a regulatory protein to shut down transcription by binding to DNA in or near the gene.

Positive control

A type of regulation that works by speeding up a process. In the context of gene transcription, the action of a regulatory protein to trigger expression by binding to DNA in or near the gene.

Constitutive

Always occurring or always present. Commonly used to describe proteins that are synthesized continuously or mutants in which one or more genetic loci are constantly expressed due to defects in gene control.

Constitutive mutant

An abnormal (mutated) strain that produces a product at all time, instead of under certain conditions.

Which of these is a regulatory gene?

D

Activators bind to regulatory sequences in ______ and enhance the binding of _____ polymerase to the promoter.

DNA; RNA

The operon model of the regulation of gene expression in bacteria was proposed by ______.

Jacob and Monod

Which mechanism of gene expression regulation is the fastest but most energy demanding of the cell? Translational control Post-translational control of the protein activity Production of multiple proteins by a single mRNA Transcriptional control

Post-translational control of the protein activity

IPTG is a molecule with a structure very similar to lactose. IPTG can be transported into cells by galactoside permease and can bind to the lac repressor protein. However, unlike lactose, IPTG is not broken down by B-galactosidase. Which of the following would occur if IPTG was added, but no glucose or lactose? The lac operon would be induced constitutively The lac operon would remain repressed The lac operon would be induced, but less than if lactose were present

The lac operon would be induced constitutively

Global gene regulation

The regulation of multiple bacterial genes that are not part of one operon.

Regulatory proteins bind to _______.

the operator

In the lac operon, the repressor inhibits transcription when the repressor is bound to the inducer the repressor is not bound to the inducer the repressor is bound to glucose the repressor is not bound to the operator

the repressor is not bound to the inducer

In the lac operon, the repressor inhibits transcription when _____. the repressor is not bound to the operator the repressor is bound to glucose the repressor is not bound to the inducer the repressor is bound to the inducer

the repressor is not bound to the inducer

Compare and contrast mechanisms of gene regulation in bacteria

1. Transcriptional control The cell could make mRNAs only for proteins it needs. If genes for unneeded proteins are NOT transcribed into mRNA, then ribosomes cannot make these proteins. This form of control occurs when regulatory proteins affect RNA polymerase's ability to bind to a promoter and initiate transcription. 2. Translational control The cell could prevent the mRNAs for unneeded proteins from being translated. This form of control occurs either through regulation of the mRNA's life span or ability to be translated. 3. Post-translational control After translation, many proteins have to be activated by chemical modification, such as the addition of a phosphate group, in order to function. These modifications can be controlled to regulate gene expression. Which type of control is most efficient (saves energy)? transcriptional Which type of control provides the most rapid response? post-translational Transcriptional control is particularly important due to its efficiency- it saves the most energy for the cell because it controls gene expression before the cell expends many resources. Translational control allows more rapid changes than transcriptional control in the amounts of different proteins because the mRNA has already been made and is available for translation. Post-translational control provides the most rapid response of all three mechanisms because only one step is needed to activate or inactivate an existing protein.

Catabolite activator protein (CAP)

A bacterial protein that activates the transcription of operons involved in the use of sugars other than glucose.

Regulon

A large set of genes distributed in bacterial DNA that are controlled by a single type of regulatory molecule. Regulon genes are transcribed in response to environmental cues and allow cells to respond to changing environments.

Corepressor

A molecule that binds to a bacterial repressor protein to induce an allosteric (shape) change in the repressor, thus allowing the repressor to bind to its operator and exert negative control of transcription.

Activator

A protein that binds to a DNA regulatory sequence to increase the frequency of transcription initiation by RNA polymerase.

Operon

A region of bacterial DNA that codes for a series of functionally related genes and is transcribed from a single promoter into one mRNA.

Negative feedback

A self-limiting, corrective response in which a deviation in some variable (e.g., concentration of some compound) triggers responses aimed at returning the variable to a target value. Represents a means of maintaining homeostasis.

lac operon

A set of three genes in E. coli that are transcribed into a single mRNA and required for metabolism of the sugar lactose.

Operator

In bacterial DNA, a binding site for a repressor protein; located near the start of an operon.

What does it mean to say a gene is under both positive and negative regulation?

In negative control, a regulatory protein shuts down transcription and in positive control, an activator switches on transcription.

Explain regulation of the lac operon (what does it look like off vs on? How is transcription turned on?)

Lac operon regulation: 1. Cotranscription: lacZ, lacY, and lacA genes are adjacent and are transcribed into one mRNA initiated from the single promoter of the lac operon. Results in the coordinated expression of the three genes. 2. Repressor protein encoded by lacI binds to a specific sequence in DNA and prevents transcription of the lac operon genes (lacZ, lacY, and lacA) 3. The inducer (lactose) binds to the repressor. When it does, the repressor changes shape: allosteric regulation. The shape change causes the repressor to come off the DNA. When the inducer binds to the repressor, the repressor can no longer bind to DNA, and transcription can proceed. Transcription can be regulated in two ways: Negative control - repressor protein binds to DNA and shuts down transcription Positive control - activator protein binds to DNA and triggers transcription Lac operon: negative control -default state is 'off' lacI codes for the repressor protein repressor binds to the operator to prevent transcription operator: DNA sequence that overlaps with the promoter region -to turn 'on' the operon: lactose (inducer) binds to the repressor repressor changes shape, can no longer bind to DNA -lactose absent inside cell - lac operon 'off' repressor is bound to DNA and transcription is blocked -lactose present inside cell - lac operon 'on' lactose (inducer) bound to repressor; repressor releases from DNA; transcription occurs -If lacI is mutated then no functional repressor is synthesized and expression is constitutive. The presence of lactose does not matter. Lac operon: positive control -CAP (catabolite activator protein) when bound to cAMP (second messenger), binds to DNA to increase transcription cAMP is only produced when glucose concentration is low high glucose = low cAMP = low transcription of lac operon

Compare and contrast the lac operon with the trp operon

Like the lac operon, the trp operon contains an operator that overlaps the promoter, and control of transcription is mediated by a repressor protein. Lac operon has a default state of 'off' while the trp operon has a default state of 'on' Trp operon: genes need to be turned on when something-tryptophan- is absent lac operon: genes should be transcribed only when something-lactose-is present. The trp repressor binds to its operator only when it is bound by its regulator, tryptophan. The lac repressor only binds to the lac operator when it is not bound to its regulator, lactose. In both the lac and trp operons, a small molecule binding to the repressor changes the repressor's shape and activity.

Explain regulation of the trp operon

Negative control of an anabolic pathway (synthesis of the amino acid tryptophan) Transcription needs to happen when Trp is absent Default state is 'on' Repressor cannot bind to the operator To turn 'off' the operon Trp (co-repressor: works with repressor to make it active) binds to the repressor Repressor changes shape, now able to bind DNA

Allosteric regulation

Regulation of an enzyme's (or other protein's) activity by binding of a regulatory molecule at a site distinct from the active site. Binding often results in a change in the protein's shape that affects the function of the active site.

Post-translational control

Regulation of gene expression by modification of proteins (e.g., addition of a phosphate group of sugar residues) after translation.

Transcriptional control

Regulation of gene expression by various mechanism that change the rate at which genes are transcribed to produce messenger RNA. In negative transcriptional control, binding of a regulatory protein to DNA represses transcription; in positive transcriptional control, binding of a regulatory protein to DNA promotes transcription.

Translational control

Regulation of gene expression by various mechanisms that alter the life span of messenger RNA or the efficiency of translation.

Why are the genes involved in lactose metabolism considered to be an operon? They are all controlled by the sample promoter They occupy adjacent locations on the E. coli chromosome They have a similar function They are all required for normal cell function

They are all controlled by the same promoter

When the lac repressor protein binds to lactose, why does it fall off its binding site at the operator?

When bound to lactose, the lacl protein changes its shape

The product of lacl gene in the Lac operon is _______.

a repressor protein always expressed in the cell a DNA binding protein

Describe how glucose affects lac operon expression

high glucose = low cAMP = low transcription of lac operon glucose inhibits the activity of galactoside permease: prevents lactose from entering cell Under what conditions is expression greatest? lactose and cAMP are high concentrations glucose is low concentration

In the lac operon, a mutation in the operator that prevents repressor binding will cause which of the following? less expression of the lac operon when lactose is present more expression of the lac operon when lactose is present less expression of the lac operon when lactose is absent more expression of the lac operon when lactose is absent

more expression of the lac operon when lactose is absent

In the presence of a regulatory protein the lac operon is _______.

not transcribed

Which of these is NOT a component of the lac operon? lactose-utilization genes only promoter only regulatory gene only operator only promoter and operator

regulatory gene only