Genetics Ch 16 Lac Operon, Transcription Regulation

what cellular processes is gene regulation important for?

1. metabolism
2. response to environmental stress
3. cell division

When can gene expression be regulated?

- before transcription
- during transcription
- after transcription but before translation
- at translation
- after translation

how can gene expression be regulated before transcription

DNA methylation

how can gene expression be regulated during transcription

transcription factors, attenuation

how can gene expression be regulated at translation

translational repressor proteins, riboswitches, antisense RNA can bind to mRNA and prevent translation

how can gene expression be regulated after translation

phosphorylation (covalent modification), feedback inhibition

repressors

proteins that inhibit transcription
Negative Control

operon

group of 2 or more genes under the control of one promoter

- codes for polycistronic mRNA

-> so you only have to upregulate one promoter to control all of the genes

how is the lac operon regulated?

It is turned off by repressors and turned on by the presence of lactose.

catabolite activator protein (CAP)

An activator protein that can bind to the CAP binding site upstream of certain prokaryotic operons, facilitating binding of RNA polymerase and stimulating gene expression.

enzyme adaptation

an enzyme only appears in the cell after it has been exposed to the enzyme's substrate

what regions does the lac operon contain

1. promoter (signals beginning of transcription)
2. CAP site (positive regulation)
3. operator (negative regulation)
4. protein coding genes (lacZ, lacY, LacA)
5. terminator (signals end of transcription)

Operator

negative regulation when bound by repressor protein (the lacL gene product)

lacA

encodes galactosidase transacetylase

--> which covalently modifies lactose by adding a hydrophobic acetyl group which allows it to diffuse outside of the cell

Where is the lac repressor located?

anywhere in the cell because it is a protein that can diffuse

what happens when a repressor protein binds to the operator

it forms a loop --> this structural change in the DNA molecule prevents RNA polymerase from moving

catabolite repression

a second way the lac operon can be transcriptionally regulated

when glucose is depleted _________ is alleviated and the lac operon is _________

catabolite repression; expressed

cyclic AMP (cAMP)

small effector molecule in catabolite repression

how is cAMP formed

from ATP by adenylate cyclase enzyme

when glucose is high, concentration is cAMP in the cell _________

decreases

what happens when CAP is bound to the CAP site

it stimulates the transcription of the lac operon

+lactose, -glucose

high transcription rate of lac operon

-lactose, -glucose

low transcription rate of lac operon

+lactose, +glucose

low transcription rate of lac operon

-lactose, +glucose

very low transcription rate of lac operon

constitutive gene expression

Constant expression of a gene

ex: "housekeeping genes" that are necessary for survival of the organism

housekeeping genes

a gene that is transcribed continually because its product is needed at all times and in all cells

regulated gene expression

the encoded protein is only produced when it is needed

-> expression may be increased or decreased depending on demand

cell signaling

The process of cell-to-cell communication that regulates gene expression

why is gene regulation important for response to environmental stress

living organisms have to adapt to detrimental conditions (ex: low water conditions)

how can gene expression be regulated after transcription but before translation

RNA splicing

activators

proteins that increase transcription
Positive Control

what is the most common way to regulate gene expression in prokaryotes?

at transcription initiation

small effector molecules

molecules that can bind to activators and repressors to influence transcription

what are the two ways inducers can act

1. bind activators and cause them to bind to DNA
2. bind repressors and prevent them from binding to DNA

inducible genes

genes that are normally off but can be turned on in the presence of inducer effector molecules

bacteria have a _______ switch, whereas eukaryotes have a _____ switch

on/off; dimmer

what are the two ways inhibitors can act?

1. co-repressors bind to repressors and cause them to bind to DNA
2. inhibitors bind to activators and prevent them from binding to DNA

repressible genes

genes that are normally transcribed but can be turned off in the presence of inhibitor effector molecules

polycistronic mRNA

molecules of mRNA that code for multiple proteins

what is the lac operon necessary for?

encoding the genes necessary for lactose metabolism

CAP site

Positive regulation-site for catabolite activator protein (CAP)

lacZ

encodes B-galactosidase

-> cleaves lactose
*allows us to measure production or utilization

lacY

encodes lactose permease

*allows lactose to get into the cell (TRANSPORT protein)

stages of breakdown of lactose

1. lactose imported
2. trapped inside cell
3. broken down into galactose and lactose

how is lactose transported into the cell

Active transport where a symporter (lactose permease - encoded by lacY) uses energy from the H+ gradient

why is glucose used before lactose?

it takes much less energy expenditure for the cell

Lac operon is _____ under _____ control

inducible; negative

when the repressor protein binds to the operator region it____

prevents RNA polymerase from transcribing the lac operon

when allolactose (or lactose) binds to the repressor it_____

inactivates the repressor so that it cannot bind to the operattor

the lac operon is _______

inducible (meaning it is only turned on in the presence of lactose)

does the presence of a repressor protein completely inhibit transcription of the lac operon?

no, there are always low levels of basal transcription

-> so that initially the cell can respond to a little bit of lactose

allolactose

inducer molecule that binds to lac repressor; allowing transcription of lacZ, lacY, lacA

diauxic growth

the sequential use of two sugars by a bacterium

what does the laci gene do

encodes the lac repressor

allosteric site

The site on a protein where a small effector molecule binds to regulate the function of the protein.

catabolite repression

lactose metabolizing genes are repressed when the cell can use glucose

System of gene control in some bacterial operons in which glucose is used preferentially and the metabolism of other sugars is repressed in the presence of glucose.

attenuator sequence

A sequence found in certain operons (e.g.,trpoperon) in bacteria that stops transcription soon after it has begun.

osmoregulation

the ability of the cell to regulate how much water it has in it

micF

inhibits the expression of ompF at high osmolarity

ompF

produced at low osmolarity

how does the antisense RNA from the micF gene inhibit translation of the ompF mRNA

the micF RNA physically blocks the ribosome from binding to the ompF mRNA