Regulation of Transcription

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Oct 2, 2013 (4 years and 9 days ago)

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Prokaryotic Gene Regulation:

Lecture 5

overview


Generic types of regulation control



Regulation of the “sugar” lactose gene(s) for
the
bactria

e. coli [ referred to as the lac
operon]



Regulation of the expression of the “amino
acid” gene tryptophan in E. Coli. [try operon]



Gene Regulation


All “genes” must have some way of regulating their
expression

in order to
allow them to adopt appropriately to the environment.



In prokaryotic cells the process, owing to the simple nature of the genomic
material, is controlled mainly at the transcription level…



Essentially the molecule “RNA polymerase” must bind to the “exposed “
DNA strand; it must then move, in the 5’ to 3’ direction, transcribing “
all

the DNA of the gene



The transcription is controlled/ regulated at:


RNA polymerase binds to the DNA;


RNA binds DNA transcription begins


RNA is prevented from binding; DNA is not transcribed:
gene not expressed
.




RNA polymerase moves in the 5’ to 3’ direction:


If it is prevented from moving: transcription is stopped :
gene not expressed


Otherwise transcription is completed and the
gene is expressed.





Classes of transcriptional control


Inducible
: gene is expressed only if the
molecule (inducer) is present.


Repressible
: if molecule is present gene
expression is turned off


Negative control
: gene expression occurs
unless it is switched off.


Positive control
: gene is “off” unless it is
switched on.

Regulatory loops: transcription


Feedback loop
: product
of the gene expression
loops back
(directly/indirectly) and
alter the expression of
the same gene (lactose).


The effect can be either :


positive :
increases the
level of transcription [
lac

]


Negative: decreases the
level of transcription [
tryp
]



Gene

Gene product

Sample gene regulatory systems


Lactose “gene” system is “turned on” by its
Inducer: (
lactose
)


Tryptophan “gene” system is “turned off” by
its repressor: (
tryptophan
)


Alternatively they can be described as
Feedback loops under:


Negative control: expression has to be turned
“off”


Positive control: expression must be turned on…




Prokaryotic regulation: lactose


Lactose, a complex sugar (glucose)



In order for
E. Coli

to use (metabolise) the sugar a gene
system referred to as the “
lac

operon
” must be expressed.



In order to ensure efficiency the “
lac

operon
”:


will not be expressed if there is no lactose


will be expressed if there is lactose.



However, Glucose, a more efficient energy source, alters
this function of The
lac

operon
:


Will not be expressed if
glusose

is present


Will not be expressed if no glucose and no lactose


Will be expressed if no glucose but there is lactose


Function of Lac
operon


The term
operon

is the common “gene system” used
in prokaryotic cells and generally a number of genes
are regulated as a one.


In the
E Coli

Lac
operon

there are:



1 repressor gene (
lacI
) and 1 repressor protein


3 structural genes:
LacZ
,
LacY

and
LacA


A
Cis

acting regulatory region (operator)


A promoter (where RNA polymerase binds)


A leader region (not critical to expression)


The
operon

is a positive controlled

Klug chapter 15

Function of Lac
operon


RNA polymerase binds to the promoter
region


The repressor gene produces a product “a
repressor protein”


This binds to the DNA at the operator region
and blocks RNA polymerase moving down
the DNA strand.




If lactose is present it alters the repressor
protein.


The alter repressor protein is unable to bind
to the DNA


RNA polymerase binds to the promoter
region and begins transcribing the 3
structural genes.



When lactose levels drop to zero:
what
happens?

Klug chapter 15

RNA polymerase

Repressor protein

Glucose and the
lac

operon



Lactose is metabolised into
glucose so what happens if
glucose is present.


Catabolite
-
activation
protein (CAP): CAP must be
present to make RNA
polymerase binding
efficiently


In the
pressence

of
glusoce

the CAP is altered and
prevents RNA polymerase
binding to the promoter
region and so prevents
transcription.

Klug chapter 15

The tryptophan
operon


Tryptophan is an essential AA and is
normally made (biosynthesised) by
E
Coli
. If tryptophan is absent in the
growth medium



If tryptophan is present in a growth
medium then the biosynthesis stops
because


The repressor protein is altered by
tryptophan and the modified
repressor protein now binds to the
operator region and blocks RNA
polymerase transcribing the enzymes
required to make tryptophan.



This is an example of a repressor /
repressible operon.



What type of “control” is used by the
tryp

operon

Klug chapter 15

The
tryptophan
operon


In addition in the presence of
tryptophan there is an additional
control mechanism called:


The attenuation regulatory
mechanism
:


In the sequence prior to structural
genes is the attenuator region:


If tryptophan and its gene expression
is repressed they still found that
transcription was initiated… ; there
was “RNA” fragments of leader
[
L
]sequence


Thus altering the repressor protein is
not enough to prevent expression.


It seems that tryptophan also binds
to the attenuator
[
A
]region and
prevents transcription beyond the
leader region.

Attenuator region

Leader region

Klug chapter 15

Exam question


Gene expression can be controlled both
negatively and positively. Explain using suitable
examples how both forms of control are
achieved in prokaryotic cells.


Gene regulatory systems can be controlled via an
inducer or repressor. Discuss the difference
between both methods and illustrate your
answer with suitable example


Distinguish between the complete functionality
of the
tryp

operon and the lac operon [include
glucose/attenuation]