Genetics Exam 3: Gene Regulation in Bacteria

Class 17

What does the term “gene regulation” mean?

the level of gene expression can vary under different conditions

What are genes that are unregulated called?

constitutive genes

How are constitutive genes expressed?

constant levels of expression

What do constitutive genes frequently encode?

proteins that are continuously necessary for the survival of organism 

What is the benefit of gene regulation?

encoded proteins will be produced only when required

Which cellular processes is gene regulation important in?

metabolism, response to environmental stress, cell division

At what points can gene regulation occur?

at any points on the pathway to gene expression 

When can regulation of gene expression occur in bacteria?

as the gene is copied into mRNA during transcription

What do genetic regulatory proteins do?

bind to DNA and control the rate of transcription

What is attenuation?

when transcription terminates soon after it has begun due to the formation of a transcriptional terminator

When else can regulation of gene expression occur besides transcription?

as the mRNA is translated into protein during translation

What do translational repressor proteins do?

bind to the mRNA and prevent translation from starting 

How do riboswitches regulate translation?

they produce an mRNA confirmation that prevents translation from starting

What does antisense RNA do?

bind to mRNA and prevent translation from starting

When can regulation of gene expression occur after translation?

as the protein undergoes posttranslational changes to make it functional 

What is feedback inhibition?

the product of a metabolic pathway inhibits the first enzyme in the pathway

How can covalent modifications to the structure of a protein regulate proteins?

it alters the protein's function

What is the most common way to regulate gene expression in bacteria?

influencing the initiation of transcription 

How can the rate of RNA synthesis be controlled in bacteria?

it can be increased or decreased

What does transcriptional regulation involve?

the actions of two main types of regulatory proteins

What is the role of repressors in transcriptional regulation?

binds to DNA and inhibits transcription

What is the role of activators in transcriptional regulation?

binds to DNA and increases transcription

What does “negative control” refer to in transcriptional regulation?

transcriptional regulation by repressor proteins 

What does “positive control” refer to in transcriptional regulation?

transcriptional regulation by activator proteins 

What are small effector molecules in gene regulation?

molecules that affect transcription regulation by binding to regulatory proteins but not directly to DNA

How can small effector molecules increase transcription?

by acting as inducers

What are inducers?

small effector molecules that either bind activators and cause them to bind to DNA or bind repressors and prevent them from binding to DNA

What are genes regulated by inducers called?

inducible genes

How can small effector molecules inhibit transcription?

by acting as corepressors or inhibitors

What do corepressors do?

bind to repressors and cause them to bind to DNA

What do inhibitors do?

bind to activators and prevent them from binding to DNA

What are genes regulated by corepressors or inhibitors called?

repressible genes

What happens in the absence of an inducer in an inducible system?

the repressor protein blocks transcription

What happens when the inducer is present in an inducible system?

a conformational change that inhibits the repressor protein from binding to DNA, allowing transcription to proceed

What happens to the activator protein in the absence of an inducer?

the activator protein cannot bind to the DNA

How does the inducer affect the activator protein when it is bound to it?

it enables the activator protein to bind to DNA and activate transcription

What happens in the absence of a corepressor in a repressible system?

the repressor protein cannot bind to DNA so transcription does occur

What happens when a corepressor is bound to the repressor protein in a repressible system?

a conformation change that allows the repressor to bind to DNA and inhibits transcription

What happens to the activator protein in the absence of an effector molecule?

it binds to DNA and activates transcription

How does the presence of an inhibitor affect the activator protein?

a conformational change that prevents the activator protein from binding to DNA, inhibiting transcription

What did scientists observe about certain enzymes at the turn of the 20th century?

a particular enzyme appears in the cell only after the cell has been exposed to the enzyme’s substrate

What is the observation term for the phenomenon where an enzyme appears only in the presence of its substrate?

enzyme adaptation

François Jacob and Jacques Monod were interested in the enzyme adaptation phenomenon. What did they focus their attention on?

lactose metabolism in E. coli

What is an operon?

a regulatory unit consisting of a few structural genes under the control of one promoter

What type of mRNA does an operon encode, and what does it contain?

polycistronic mRNA that contains the coding sequence for 2+ structural genes

What is the advantage of having an operon?

it allows a bacterium to coordinately regulate a group of genes that encode proteins with a common functional goal

What are the important DNA sequences found in an operon?

promoter, terminator, structural genes, operator

What are the two transcriptional units involved in lactose utilization in E. coli?

the lac operon and lacl gene

What are the main DNA elements of the lac operon?

promoter, operator, CAP site

What is the function of the promoter in the lac operon?

binds RNA polymerase to initiate transcription

What is the function of the operator in the lac operon?

binds the lac repressor protein to block transcription

What is the function of the CAP site in the lac operon?

binds the catabolite activator protein to regulate transcription

What are the structural genes of the lac operon?

lacZ, lacY, lacA

What does the lacZ gene encode?

B-galactosidase

What does the lacY gene encode?

lactose permease

What does the lacA gene encode?

galactoside transacetylase

Is the lacI gene part of the lac operon?

not considered part of lac operon 

What promoter controls the lacl gene?

has its own promoter, the i promoter

How is the lacI gene expressed?

constitutively expressed at fairly low levels

What does the lacl gene encode?

the lac repressor 

What is the structure of the lac repressor protein?

it functions as a tetramer

How much lac repressor protein is needed to repress the lac operon?

only a small amount of protein

What are the two main ways the lac operon can be transcriptionally regulated?

by a repressor and activator protein 

What type of control mechanism involves the lac repressor protein?

an inducible, negative control mechanism

What molecule acts as the inducer in the lac operon system?

allolactose

How does the allolactose inducer regulate the lac operon?

it binds to the lac repressor and inactivates it

What happens to the lac operon when the inducer allolactose is absent?

the repressor protein is tightly bound to the operator site, which blocks RNA polymerase from transcribing the operon

What happens when allolactose is present in the cell?

it binds to the repressor and prevents it from binding to the operator site, so transcription can proceed

What happens first when lactose becomes available to E. coli?

a small amount of lactose is taken up and converted to allolactose by B-galactosidase

How does allolactose affect the lac repressor?

it binds to the repressor causing it to fall off the operator site

What is the result of the lac repressor detaching from the operator?

the lac operon proteins are synthesized which promotes the efficient uptake and metabolism of lactose

What happens after lactose is depleted?

allolactose levels decrease and is released from the repressor, allowing it to bind to operator site

What happens to the proteins involved in lactose utilization after repression resumes?

most proteins involved with lactose utilization are degraded

What did Jacob, Monod, and their colleague Arthur Pardee discover in the 1950s?

a few rare mutant strains of bacteria with abnormal lactose adaptation

What was the mutant type that involved a defect in the lacl gene?

lacl-

What is the result of an lacI- mutation?

constitutive expression of the lac operon even in the absence of lactose

Where do lacI⁻ mutations map relative to the lac operon?

very close to the lac operon 

What did Jacob, Monod, and Pardee hypothesize that the lacI- mutation results in?

the synthesis of an internal inducer

According to the internal inducer hypothesis, what would happen if it were correct?

the inducer protein produced from the chromosome can diffuse and activate the lac operon on the F’ factor

What was the alternative hypothesis to the internal inducer idea?

lacl- mutation eliminates the function of a lac repressor that can diffuse throughout the cell

According to the alternative hypothesis, what would happen if it were correct?

the repressor from the F’ factor can diffuse and turn off the lac operon on the bacterial chromosome

Why did mutant strains show 100% enzyme activity in the Jacob-Monod-Pardee experiment?

constitutive expression in the lacl- strain

What happened in the absence of lactose in the merozygote strain?

both lac operons are repressed, showing less than 1% enzyme activity 

What happened in the presence of lactose in the merozygote strain?

both lac operons are induced, showing 220% of enzyme activity

What is a trans-effect in gene regulation?

genetic regulation that can occur even though DNA segments are not physically adjacent

What mediates a trans-effect?

genes that encode regulatory proteins

What is an example of a trans-effect?

the action of the lac repressor on the lac operon 

What is a cis-effect or cis-acting element?

a DNA sequence that must be adjacent to the gene(s) it regulates

How is a cis-effect mediated?

by sequences that bind regulatory proteins

What is an example of a cis-acting element?

the lac operator

What happens when a mutation occurs in a trans-acting factor?

it is complemented by the introduction of a second gene with normal function

What happens when a mutation occurs in a cis-acting element?

it is not affected by the introduction of another normal cis-acting element

What is the second way the lac operon can be transcriptionally regulated?

by catabolite repression 

What happens when E. coli is exposed to both lactose and glucose?

E. coli uses glucose first, and catabolite repression prevents the use of lactose

What happens when glucose is depleted?

catabolite repression is alleviated, and the lac operon is expressed

What is diauxic growth?

the sequential use of two sugars by a bacterium

Is glucose the small effector molecule in catabolite repression of the lac operon?

no, the effector molecule is not glucose

What small molecule mediates catabolite repression in the lac operon?

cyclic AMP (cAMP)

How is cyclic AMP (cAMP) produced?

from ATP via the enzyme adenylyl cyclase

What protein does cyclic AMP (cAMP) bind to in catabolite repression?

an activator protein called catabolite activator protein (CAP)