Transcript THE lac OPERON

THE lac OPERON
© 2007 Paul Billiet ODWS
Gene expression

Transcription and Translation of a gene

Each cell contains all your genes but not all are
expressed

Control of gene expression
 ON all the time
 E.g. For respiration
 Only on when needed
 Otherwise it will be wasteful
 Only on in certain organs
Operons

group of genes that
are transcribed at
the same time.

They are only found
in prokaryotes.
Jacob, Monod & Lwoff
© NobelPrize.org
© 2007 Paul Billiet ODWS
E. coli’s respiratory substrates

Glucose
 monosaccharide

Lactose
 disaccharide
 Preferred

So, when glucose is present, lac operon is
switched off
So how is the gene expression of the enzymes involved in the breakdown of
lactose controlled?
The lac Operon in E. Coli

Structural genes

Z: β-galactosidase


Y: Lactose permease




hydrolyses lactose into glucose and galactose
Transports lactose into the cell
Operator region O – the switch
Promoter P – binding site for RNA pol
Regulator gene I – not part of operon; continually
synthesies a repressor protein
1. When lactose is absent

A repressor protein sits on a sequence of DNA just in
front of the lac operon, the Operator site

The repressor protein blocks the Promoter site where
the RNA polymerase settles before it starts transcribing
Repressor
protein
DNA
I
O
Regulator
gene
Operator
site
© 2007 Paul Billiet ODWS
RNA
polymerase
Blocked
z
y
lac operon
a
2. When lactose is present

Lactose fits onto the repressor protein at another active
site

This causes the repressor protein to change its shape (a
conformational change).
DNA
I
© 2007 Paul Billiet ODWS
O
z
y
a
2. When lactose is present

Repressor dissociates from the operator site.

RNA polymerase can now reach its promoter site
DNA
I
© 2007 Paul Billiet ODWS
O
z
y
Promotor site
a
Summary: complete the following table
Lactose
(inducer)
Lactoserepressor
complex
Repressor
RNA
protein
polymeras
e
lac Operon
Choose from the following:
Sits on the promoter site, absent, formed, transcribed, bound to
operator site, not transcribed, lifted off operator site, not formed,
blocked by the repressor, present
© 2007 Paul Billiet ODWS
Summary: complete the following table
Lactose
(inducer)
Lactoserepressor
complex
Repressor
RNA
protein
polymeras
e
lac Operon
absent
Not formed
Bound to
Blocked by
operator site the repressor
Not
transcribed
present
formed
Lifted off
operator site
transcribed
© 2007 Paul Billiet ODWS
Sits on the
promoter
site