UNIT 6

PART 3

*REGULATION USING

OPERONS* Hillis Textbook, CH 11

REVIEW:

Signals that Start and Stop

Transcription and Translation

BUT, HOW DO CELLS CONTROL WHICH

GENES ARE EXPRESSED AND WHEN?

First of all, There is a difference between

regulation in a prokaryote and a eukaryote….

OPERONS - PROKARYOTES Prokaryotes conserve energy by making proteins only when

needed. The most efficient gene regulation is at the level of transcription.

A gene cluster with a single promoter is an operon.

An operator is a short stretch of DNA near the promoter that controls transcription of the structural genes.

1. Inducible operon—turned off unless needed

In inducible systems—a metabolic substrate (inducer) interacts with a regulatory protein (repressor); the repressor cannot bind and allows transcription. Usually control CATABOLIC REACTIONS

2. Repressible operon—turned on unless not needed

In repressible systems—a metabolic product (co-repressor) binds to regulatory protein, which then binds to the operator and blocks transcription. Usually control ANABOLIC REACTIONS.

LAC OPERON - INDUCIBLE A compound that induces protein synthesis is an inducer.

When the enzymes are induced, metabolism will

take place.

E. coli must adapt quickly to supply of food (lactose is a dissacharide example)

Uptake and metabolism of lactose involves three important -galactoside enzymes

-galactoside is a type of glycosidic bond between monosaccharides… if this is present, LACTOSE is present.

If E. coli is grown with glucose but no lactose

present, no enzymes for lactose conversion are produced.

If lactose is predominant and glucose is low, E. coli synthesizes all three enzymes.

If lactose is removed, synthesis stops.

LAC OPERON – INDUCIBLE

LAC OPERON – INDUCIBLE

The lac operon is only transcribed when a -galactoside predominates in the cell:

• A repressor protein is normally bound to the operator, which blocks transcription.

• In the presence of a -galactoside, the repressor detaches and allows RNA polymerase to initiate transcription.

The key to this regulatory system is the repressor protein.

NO lactose, repressor will

not allow transcription =

NO enzymes to metabolize

lactose.

WITH lactose available, the

repressor is removed,

transcription will allow

expression of enzymes to

metabolize lactose

TRP OPERON – REPRESSIBLE A repressible operon is switched off when its repressor is bound to its operator.

However, the repressor only binds in the presence of a co-repressor.

The co-repressor causes the repressor to change shape in order to bind to the promoter and inhibit transcription.

Tryptophan functions as its own co-repressor, binding to the repressor of the trp operon.

NO trp = no repressor and

transcription takes place,

allowing enzymes to be

synthesized for tryptophan.

WITH trp, the repressor

becomes activated and no

transcription takes place.

UNIT 6

PART 3

*REGULATION USING

TRANSCRIPTON

FACTORS* Hillis Textbook, CH 11

TRANSCRIPTION FACTORS -

EUKARYOTES Genes can be regulated at the level of transcription.

Two types of regulatory proteins, called transcription factors, control whether a gene is active.

These proteins bind to specific DNA sequences near the promoter:

1. Negative regulation – prevents transcription

2. Positive regulation – stimulates transcription

THIS IS HOW EUKARYOTIC GENES ARE TURNED “ON” AND “OFF”!

A repressor protein

prevents transcription

An activator

protein binds to

stimulate

transcription

TRANSCRIPTION FACTORS: Transcription factors act at

eukaryotic promoters.

Each promoter contains a core promoter sequence where RNA polymerase binds.

TATA box is a common core promoter sequence—rich in A-T base pairs.

Only after general transcription factors bind to the core promoter, can RNA polymerase II bind and initiate transcription.

REGULATORY PROTEINS: Besides the promoter, other DNA sequences can bind

regulatory proteins that interact with RNA polymerase and

regulate transcription.

Some are positive regulators—activators (DNA sequence is

called an enhancer); others are negative—repressors (DNA

sequence is called a silencer).

ALTERNATIVE SPLICING: Eukaryotic gene expression can be regulated after

the initial gene transcript is made.

Different mRNAs can be made from the same gene by alternative splicing (as introns and exons are spliced out, new proteins are made).

Mechanism for generating proteins with different functions, from a single gene.

TRANSLATIONAL REGULATION: Three ways to regulate

mRNA translation:

• Inhibition of translation with miRNAs

• Modification of the 5′ cap end of mRNA can be modified—if cap is unmodified mRNA is not translated.

• Repressor proteins can block translation directly—translational repressors

Posttranslational aspects of protein synthesis:

Polypeptide emerges from the ribosome and folds into its 3-D shape.

Its conformation allows it to interact with other molecules—it may contain a signal sequence indicating where in the cell it belongs (nucleus, mitochondria, etc.)

In the absence of a signal, the protein will remain where it was produced.

Protein modifications:

Proteolysis—cutting of a long polypeptide chain into final products, by proteases.

Glycosylation—addition of carbohydrates to form glycoproteins

Phosphorylation—addition of phosphate groups catalyzed by protein kinases— charged phosphate groups change the conformation of the protein

LAC OPERON - INDUCIBLE

LAC OPERON - INDUCIBLE

TRP OPERON - REPRESSIBLE

TRP OPERON - REPRESSIBLE

TRANSCRIPTION FACTORS:

TRANSCRIPTION FACTORS:

TRANSCRIPTION

FACTORS

(TATA BOX):

REGULATORY PROTEINS:

ALTERNATIVE SPLICING:

TRANSLATIONAL REGULATION:

PROTEIN MODIFICATIONS: