Transcript GENE EXPRESSION and the LAC OPERON

GENE EXPRESSION and the
LAC OPERON
We
have about 42 000 genes inside our DNA
that code for proteins. Clearly not all the proteins
are needed at the same time.
Cells
have developed methods to control the
transcription and translation of genes, depending
on their needs.
HOUSEKEEPING
GENES are always needed
and are therefore constantly being transcribed and
translated.
TRANSCRIPTION
FACTORS turn nonhousekeeping genes on when required.

Gene regulation is important for organism
survival and it can occur at four levels:

TRANSCRIPTIONAL: regulates which genes are
transcribed (DNA to mRNA) or regulates the rate
of transcription

POSTTRANSCRIPTIONAL: removing the introns
of the mRNA before it goes off to translation.

TRANSLATIONAL: controls how often and how
fast mRNA transcripts are converted into proteins.

POSTTRANSLATIONAL: controlling the amount
of time before a protein becomes functional.
THE lac OPERON
Lactose is milk sugar and is broken down by E.
coli in the intestines of mammals.
 BETA GALACTOSIDASE is the enzyme
responsible for the breaking down of lactose.

The bacteria have a NEGATIVE CONTROL
SYSTEM that blocks the production of betagalactosidase if lactose is NOT present.
 The system works by inhibiting
TRANSCRIPTION of the gene that codes for
the beta-galactosidase protein.


The beta-galactosidase gene is part of an
OPERON.
An operon is on a prokaryotic DNA molecule.
It contains a PROMOTER, OPERATOR and
some structural genes.
 The promoter and operator do not code for
protein but are important in the REGULATION
of the genes' transcription.


The lac operon consists of a promoter, operator
and three genes (lac Z, lac Y, and lac A) which
code for the proteins and enzymes that break
down lactose.

A REPRESSOR protein, called lacI, binds to the
operator and prevents the genes for beta-galactosidase
from being transcribed (blocks RNA polymerase).

The promoter region and the operator region overlap
(share nucleotides) so when lacI binds to the operator
it is essentially blocking the promoter site where
RNA POLYMERASE wants to bind
In the lac operon system lactose is known as a
signal molecule or an INDUCER.
 When it is present it binds to the lacI protein
which pulls it away from the operator site.


This frees up the promoter site for RNA
polymerase and allows for the proper
TRANSCRIPTION of the genes responsible
for lactose degredation.
THE trp OPERON
Tryptophan is an amino
acid used by E. Coli to
make protein.
 The trp operon is
another example of coordinated regulation.
It contains 5 genes that
make the three enzymes
responsible for
synthesizing tryptophan.

While the lac operon has transcription induced
when lactose is present (in order to make
enzymes to break it down), the trp operon is
repressed when high levels of trp are present
(no need to make any more).
 The EFFECTOR, the cause of the gene being
repressed, is trp.
 Trp is called a CO-REPRESSOR.


When trp levels are high, trp binds to an inactive
repressor protein making it ACTIVE. This
repressor-trp COMPLEX then binds to the
operator site and prevents the gene from being
transcribed.

When levels of trp drop it RELEASES from the
repressor protein and the repressor protein
disengages from the operator so that the trp
genes can be transcribed.