REGULATION OF GENES

The Operon Discovered in the bacterium, E. coli

Used as a model for gene regulation

An operon is a set of genes and the switches that control the expression of those genes

Jacob and Monod Discovered the operon in the 1940’s

Found two types: inducible (Lac) operon and repressible (tryptophan) operon

The Lac operon is switched off until it is induced to turn on

The tryptophan operon is always in the on position until it is no longer needed

The Lac Operon Lactose is not available to bacteria as an

energy source So, the genes necessary to utilize lactose are

not transcribed

In order for E. coli to utilize lactose, three structural genes must be transcribed in order to produce the enzymes necessary for the breakdown of lactose into glucose and galactose

The Lac Operon The three enzymes necessary are b-

galactosidase, permease and transacetylase These enzymes are coded for by thee

structural genes in the Lac operon

In order for this transcription to occur, RNA polymerase must bind to DNA at the promoter

The Lac Operon If a repressor binds to the operator,

RNA polymerase is prevented from binding to the promoter and transcription of the structural genes is prevented or blocked

The relationship between RNA polymerase and the repressor is an example of noncompetitive inhibition Both substances are competing for two active

sites, one of which blocks the other

The Lac Operon If allolactose, similar to lactose, is present, it acts

as an inducer or allosteric effector

It binds to the repressor, causing the repressor to change shape or conformation

Now, the repressor can no longer bind to the operator and RNA polymerase is free to bind to the promoter

Structural genes are now transcribed and lactose is utilized

Repressed Lac Operon

Tryptophan Operon Repressible; continuously switched on

unless turned off by a corepressor

Consists of five structural genes that code for the enzymes necessary to synthesize the amino acid tryptophan

Tryptophan Operon The repressor molecule encoded by the

regulator gene is initially inactive

RNA polymerase is free to bind to the promoter and transcribe the structural genes, resulting in tryptophan production

Tryptophan Operon When the inactive repressor combines

with a specific corepressor molecule (tryptophan), it changes shape and binds to the operator

This prevents RNA polymerase from binding to the promoter and blocks the further production tryptophan

If tryptophan levels are high, no more is needed, so no more is made

Tryptophan Operon Tryptophan acts as an allosteric

effector

This is an example of a negative feedback mechanism

Prions Prions are not cells and are not viruses

Misfolded versions of a protein normally found in the brain

If prions enter a normal brain, they cause all of the normal versions of the protein to misfold in the same way

Prions Prions are infectious and cause several

brain diseases Scrapie in sheep

Mad cow disease in cattle

Creutzfeldt-Jakob disease in humans

Transposons Transferring genetic elements sometimes

called jumping genes

Discovered by Barbara McClintock

Some transposons jump in a cut-and-paste fashion from one part of the genome to another

Transposons Others make copies of themselves that

move to another region of the genome, leaving the original behind

Two types: insertion sequences and complex transposons

Insertion Sequences Consist of one gene that codes for

transposase, an enzyme that moves the sequence from one place to another

Causes a mutation if it lands within a DNA region that regulates gene expression

Complex Transposons Longer than insertion sequences and

include extra genes Antibiotic resistance or seed color

McClintock hypothesized the existence of transposons when she saw patterns in corn color that made sense only if some genes were mobile