Gene Expression
Gene expression for all genes falls into one of two
categories. (prokaryotes and eukaryotes)
constitutive expression – genes which are always
turned on
1.

2.
known as housekeeping genes
induced expression – genes which are only turned
on as needed
lac Operon
 lac - lactose
 operon – several genes in a sequence all controlled
by a single promoter (mainly in prokaryotes; some
eukaryotes)
promoter
gene 1
operator
gene 2
gene 3
lac Operon
Cells mainly use glucose as a source of energy.
The lac operon is only turned on when glucose is
absent, but lactose is present.
lac Operon Animation
lac Operon Details
Operon codes for 3 enzymes – found in E. coli
1. b-galactosidase (lacZ gene)

enzyme which breaks down lactose
2. permease (lacY gene)

protein transporter which brings lactose into cell
3. transacetylase (lacA gene)

adds acetyl group to galactose
Repressor Protein
repressor protein (lacI gene)
 binds to the operator portion in the presence of
glucose
 prevents RNA polymerase from transcribing genes
when bound to operator
 transcribed by a different gene from the lac operon
When do you want the repressor to bind / not bind to
the operon?
When lactose is absent:
 enzymes are not needed to metabolise lactose
 repressor binds to the operator to inhibit transcription
When lactose is present:
 lactose binds to the repressor protein
 lactose-repressor complex cannot bind to the operator
 transcription can proceed
Effector Molecules
Since lactose is the molecule that determines when
the operon is turned on or off, it is known as an
effector molecule.
 effector molecule – any molecule that can regulate
the activity of a protein
 inducer – effector molecule that binds repressor
protein to cause it to fall off operator
lac Operon Animation
lac Operon Animation