Gene Regulation in Bacteria

Genetic ____________ bind to the DNA and control the rate of transcription

regulatory proteins

In ___________, transcription terminates soon after it has begun due to the formation of a transcriptional terminator.

attenuation

_________ proteins can bind to the mRNA and prevent translation from starting

Translational repressor

___________ can produce an mRNA confirmation that prevents translation from starting.

Riboswitches

__________ RNA can bind to the mRNA and prevent translation from starting

Antisense

In____________, the product of a metabolic pathway inhibits the first enzyme in the pathway.

feedback inhibition

Covalent modifications to the _________ of a protein can alter its function.

structure

The most common way to regulate gene expression in bacteria is by……

influencing the initiation of transcription

Transcriptional regulation involves the actions of two main types of regulatory proteins

• Repressors → Bind to DNA and inhibit transcription

• Activators → Bind to DNA and increase transcription

These bind to regulatory proteins but not to DNA directly

small effector mlcls

A type of small effector mlcl:

• Bind activators and cause them to bind to DNA

• Bind repressors and prevent them from binding to DNA

inducers

Genes that are regulated by inducers are termed _________

inducible

_________ bind to repressors and cause them to bind to DNA

Corepressors

______ bind to activators and prevent them from binding to DNA

Inhibitors

Genes that are regulated by corepressors and inhibitors are termed _____

repressible

In the absence of the inducer, the repressor protein blocks transcription. The presence of the inducer causes a _________ that inhibits the ability of the repressor protein to bind to the DNA. Transcription proceeds.

conformational change

The activator protein cannot bind to the DNA unless an inducer is present. When the inducer is bound to the activator protein, this enables the activator protein to bind to the DNA and activate transcription. This is a ________gene

inducible

In the absence of a corepressor, this repressor protein will not bind to the DNA. Therefore, transcription can occur. When the corepressor is bound to the repressor protein, a conformational change occurs that allows the repressor to bind to the DNA and inhibit transcription.

This is a _________ gene

repressible

This activator protein will bind to the DNA without the aid of an effector molecule. The presence of an inhibitor causes a conformational change that inhibits the ability of the activator protein to bind to the DNA. This inhibits transcription

This is a _________ gene

repressible

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

This observation was knwn as…..

enzyme adaptation

An operon is a regulatory unit consisting of a few structural genes under the control of _________

one promoter

An operon encodes a ___________ that contains the coding sequence for two or more structural genes

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

polycistronic mRNA

An operon contains several important DNA sequences:

Promoter; terminator; structural genes; operator

The lac operon has two distinct transcriptional units

1. the actual lac operon


1. DNA elements
2. structural genes
2. lac I gene

DNA elements of the lac operon

• Promoter → Binds RNA polymerase

• Operator → Binds the lac repressor protein

• CAP site → Binds the Catabolite Activator Protein (CAP)

lacZ encodes

β-galactosidase

• Enzymatically cleaves lactose and lactose analogues

• Also converts lactose to allolactose (an isomer)

lacY encodes

lactose permease

Membrane protein required for transport of lactose and analogues

lacA encodes

galactoside transacetylase

* Covalently modifies lactose and analogues
* Its functional necessity remains unclear

The lacI gene is not a part of the lac operon because it has its own promoter the ________.

It encodes for _______

i promoter

repressor protein

The lac repressor protein functions as a ______

tetramer

organization of the sequences in the lac operon

i promoter

lacI gene

CAP site

lacP (lac promoter)

operator site (lac O)

lacZ

lacY

lacA

lac terminator

The lac operon can be transcriptionally regulated by

1…..'

The first method is an inducible, negative control mechanism

2….

1. repressor protein
2. activator protein

In the absence of the inducer __________,__ the repressor protein is tightly bound to the ________, thereby inhibiting the ability of RNA polymerase to transcribe the operon

allolactose

operator site

When allolactose is available, it binds to the repressor. This alters the conformation of the repressor protein, which prevents it from binding to the operator site. Therefore, ________can transcribe the operon.

RNA polymerase

The cycle of lac operon induction and repression

1\. When lactose becomes available, a small amount of it is taken up and converted to allolactose by β-galactosidase.

The allolactose binds to the repressor, causing it to fall off the operator site.

2\. lac operon proteins are synthesized.

This promotes the efficient uptake and metabolism of lactose.

3\. The lactose is depleted.

Allolactose levels decrease. Allolactose is released from the repressor, allowing it to bind to the operator site.

4\. Most proteins involved with lactose utilization are degraded.

In the 1950s, Jacob and Monod, and their colleague Arthur Pardee, had identified a few rare mutant strains of bacteria with abnormal lactose adaptation

One type of mutant involved a defect in the lacI gene

* designated lacI -
* It resulted in the _________ of the lac operon even in the absence of lactose
* The lacI– mutations mapped very close to the lac operon

constitutive expression

2 hypothesis that show that lacI- causes constitutive expression:

1. lacI- eliminates function of lac repressor
2. lacI- results in synthesis of internal activator

Experiment 14A

They used bacterial conjugation methods to introduce different portions of the lac operon into different strains

They identified F’ factors (plasmids) that carried portions of the lac operon

Consider an F’ factor that carries the lacI gene, Bacteria that receive this will have two copies of the lacI gene

* One on the chromosome and the other on the F’ factor
* CALLED MEREOZYGOTES OR PARTIAL DIPLOIDS

Merozygotes were instrumental in allowing Jacob, Monod, and Pardee to elucidate the function of the lacI gene

There are two key points

1\.The two lacI genes in a merozygote may be different alleles

2\. Genes on the F’ factor are not physically connected to those on the bacterial chromosome

repressor proteins are an example of

trans-acting elements