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Operon Vocabulary

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• Allosteric Protein

• Promoter

• Gene

• Gene Expression

‘self regulatory biological system in which the output (or response) affects the input; can be positive or negative’

oscillates between two different shapes depending upon what’s bonded to allosteric site

location upstream of gene where RNA polymerase binds to begin transcription

sequence of DNA that codes for proteins

when segment of DNA is transcribed and translated into a functional protein

What is an Operon?A. System within a prokaryotic cell which is involved

in controlling gene expression

B. Operons usually contain clusters of genes which code for proteins (enzymes) that are all involved in one metabolic process

C. Operons are turned ‘on’ or ‘off’ by a DNA switch (segment of DNA) called an operator• If operator is open, then RNA polymerase can bind• If operator is blocked, then RNA polymerase cannot bind

D. Operon = promoter + operator + gene cluster

E. Every prokaryotic gene cluster controlled by operon system; you need to understand function of these two operon systems:

trp operonlac operon

F. There are two main kinds of operon regulatory systems:•repressible operons (ex. trp operon)

Normally, genes are ‘ON’ and presence of aco-repressor TURNS OFF gene expression

•inducible operons (ex. lac operon)Usually genes are “OFF” but the presence of a particular substances (inducer) TURNS ONexpression of one or more genes

Graphic OrganizerCharacteristic

or FeatureRepressible

OperonInducible Operon

Definition

Relationship to metabolism

Repressor protein

produced as

Example

Type of Gene Regulation

Repressible Operons(anabolic metabolic pathway)

A. Utilizes repressor protein which binds to operator and prevents RNA polymerase from attaching to promoter for transcription

B. Repressor protein created by a regulatory gene (regulator) which is independent of the operon with which it represses

C. Repressor protein is allosteric ... has two distinct shapes

D. In repressible operon, repressor protein produced by regulator is in inactive state

E. What the heck does all this mean???

Repressible Operons(anabolic metabolic pathway)

F. Take a look at the trp operon in action! (figure 18.20a, b )

trp operon picture: identify components of operontrp operon (layer 1)trp operon (layer 2)Trp Operon Animation (not from your textbook)

G. Summarize NOW!! {as a flow chart?!}

H. Generally, repressible operons are involved in anabolic pathways The creation of a product (co-repressor) activates the repressor to turn operon OFF (block binding of RNA polymerase at promoter)

Repressible Operons(anabolic metabolic pathway)

Figure 18.3

Repressible Operons(anabolic metabolic pathway)

Figure 18.3

Repressible Operons(anabolic pathway)

Figure 18.3

Inducible Operons(catabolic metabolic pathway)

A. Regulatory gene creates active repressor protein that binds to operator and therefore prevents transcription of gene cluster

B. An inducer is required to ‘inactivate’ repressor and unblock operator...

C. Inducers are usually small molecules that are involved in the catabolic pathway

D. Lactose is inducer of lac operon. Lactose must be present in order to express genes and create enzymes necessary to convert lactose ---> glucose + galactose

Inducible Operons(catabolic metabolic pathway)

E. Check it out: (figure 18.4)Lac Operon (with out lactose)

Lac Operon (with lactose in cell) Lac Operon Animation (not from textbook)Another Lac Operon Animation

F. How is lac operon ‘turned off’? Summarize how inducible operons work in your own words!

G. Generally, inducible operons only transcribe catabolic enzymes when molecule involved in catabolic pathway is present in the environment & bacterial cell!

Inducible Operons(catabolic metabolic pathway: Figure 18.4)

Inducible Operons(catabolic metabolic pathway: Figure 18.4)

The lac Operon - review

E. coli living in an environment withwith lactoselactose:• Inducer (lactose) binds to repressor bound to operator• Shape of repressor protein changes, and repressor

disengages from operator• RNA polymerase is now free to bind to DNA and transcribe

lactose-catabolizing genes!

lactose

Glucose + Galactose

Operon Review

Gene ON

Gene OFF

REPRESSIBLE OPERON

(anabolic pathway)

INDUCIBLE OPERON

(catabolic pathway)

Active repressor bound to operator blocking RNA polymerase

Operator not blocked, RNA polymerase access promoter

How are inducible and repressible operons

SIMILAR?• Explain your answer...

How are inducible and repressible operons

DIFFERENT?• Explain your answer...

Characteristic or Feature

Repressible Operon

Inducible Operon

Definition

Relationship to metabolism

Repressor protein

produced as

Example

Type of Gene Regulation

Genes normally ‘ON’, but can be turned off by co-

repressor

Anabolism – Building molecules

necessary to survival

Inactive

Trp operon

Negative: repressor bound to operator

prevents expression

Genes normally ‘OFF, but can be

turned on by inducer

Catabolism –

Break stg in envir.

Active form

lac operon

Negative: repressor bound to operator

prevents expression

What is the selective advantage to regulating gene expression in prokaryotic cells??

Operons allow bacteria to adapt quickly to changes in their environment.

These single-celled organisms must adapt quickly because they don’t have a lot of ATP to waste making unnecessary proteins

The action of enzymes within the cell alter the raw materials present in the environment in which the bacteria lives

Quick adaptation = survival & >> reproduction!

“Volume control” of lac Operon

E.coli w/ lactose & low [glucose]

• Genes for lactose catabolism are ON

• B/c [glucose] is low, lots of lactose catabolism is required to generate glucose

• Therefore genes are ON and LOUD:– Make lots of mRNA– Do lots of transcription of

genes– Make many catabolic enzy.

E.coli w/ lactose & high [glucose]

Genes for lactose catabolism are ON

B/c [glucose] is high, it is not as important to

metabolize lactose as source for glucose

Therefore genes are ON, but are QUIET

Make less mRNA

Do little transcription

Make few catabolic enz.

How does a bacterium measure the concentration of

glucose in the cell?

How does a bacterium know to increase the ‘volume’ of

transcription based on the amount of glucose in the cell?

Positive Gene Regulation of lac Operon• Recall that genes for lactose catabolism are

only expressed when lactose is present

• What if the cell is in an environment with an adequate supply of glucose andand lactose?

• Which molecule is the first choice of bacteria to use to create ATP: lactose or glucose?

GLUCOSE

Allosteric interaction between a protein and a molecule called

cyclic AMP (cAMP)

•How can the bacterium relate [glucose] to gene expression?

cyclic AMP (cAMP)

ATP --------------> cAMPadenylyl cyclase

•cAMP accumulates when glucose is scarce

•The greater the [cAMP], the more cAMP binds to a protein called cAMP receptor protein (CRP).

•CRP is an activator of transcription!

cAMP and CRP • cAMP binds to allosteric site of CRP and

changes shape of CRP

• The new shape of CRP allows CRP to bind to a location upstream of the lac promoter

• CRP bends DNA and somehow makes it easier for RNA polymerase to bind to DNA and begin transcription

• The binding of CRP to DNA facilitates transcription and is therefore an example of positive gene regulation!

Positive Gene Regulation (figure 18.5)

LACTOSE present & glucose scare so cAMP levels are high; LOTS of lactose-catabolizing enzymes transcribed

(active CRP promotes binding of mRNA at promoter)

Positive Gene Regulation (figure 18.5)

Lactose present AND glucose present so cAMP levels are low; few lactose-catabolizing

enzymes transcribed

Relate operon to recombinant plasmid from transformation lab & biotechnology

Cut arabinose gene with restriction enzyme, but promoter remains intact.

Insert jellyfish gene downstream from arabinose promoter in plasmid...

Arabinose (in agar medium) binds to operator to facilitate binding of RNA polymerase to promoter to transcribe gene.

Now expression of jelly fish gene proceeds and bacteria colonies GLOW GREEN!!

Is the arabinose operon inducible or repressible, based on this information?