Control Mechanisms
Four Levels of Control of Gene
Expression
Type of Control
Transcriptional
Description
 Regulates which genes are
transcribed.
 Controls rate of transcription.
Posttranscriptional  Modification of mRNA.
 Introns removed & exons
spliced together.
Four Levels of Control of Gene
Expression
Type of Control
Translational
Description
 Controls rate of translation.
 Controls rate of mRNA
activation and destruction.
Posttranslational  Controls rate at which a protein
becomes active and the time it
remains functional.
The lac Operon
E. coli uses β-galactosidase to break down
lactose in order to grow.
 The 3 genes for β-galactosidase are part
of the lac operon.

The lac Operon

When lactose is absent:
 LacI
repressor protein binds to the operator
and partially blocks the promoter.
 Prevents transcription of the lac operon genes.
The lac Operon

With sufficient lactose:
 Binds
to the LacI repressor protein.
 LacI changes its shape, allowing transcription
of the lac operon genes.
WATCH THIS!!!
The trp Operon
E. coli uses tryptophan for the production
of protein.
 The trp operon consists of 5 genes that
code for 3 enzymes needed to synthesize
tryptophan.

The trp Operon

When tryptophan is absent:
 The
shape of the trp repressor protein changes.
 No longer binds to the trp operator.
 RNA polymerase transcribes trp operon genes.
The trp Operon

With sufficient tryptophan:
 Binds
to the trp repressor protein, causing a
change in shape.
 trp repressor-tryptophan complex binds to the
operator.
 Prevents transcription of the trp operon genes.
WATCH THIS!!!
Summary
lac Operon
 Transcription induced when high levels of
lactose present.
 Effector = level of lactose
trp Operon
 Transcription repressed when high levels
of tryptophan present.
 Effector = level of tryptophan