Transcriptional repressors ppt Repressor proteins involved in transcription
Transcription refers to the first step of gene expression where
an RNA polymer is created from a DNA template.
Transcription is performed by enzymes called RNA
polymerases, which link nucleotides to form an RNA strand
(using a DNA strand as a template).
Transcription has three stages: initiation, elongation, and
In eukaryotes, RNA molecules must be processed after
transcription: they are spliced and have a 5' cap and poly-A
tail put on their ends.
Transcription is controlled separately for each gene in your
REGULATION OF TRANSCRIPTION
Transcription in eukaryotic and prokaryotic cells
is controlled by regulatory proteins that bind to
specific regulatory sequences and modulate the activity of
Thus regulatory proteins and regulatory sequences play a vital
role in gene expression.
Any DNA sequence that is responsible for the regulation
of gene expression
Any protein that influences the regions of a DNA molecule
that are transcribed by RNA polymerase during the process
In molecular genetics, a repressor is a DNA- or RNA-binding
protein that inhibits the expression of one or more genes by
binding to the operator or associated silencers.
A DNA-binding repressor blocks the attachment of RNA
polymerase to the promoter, thus preventing transcription of
the genes into messenger RNA.
This blocking of expression is called repression.
Within the eukaryotic genome are regions of DNA known
These DNA sequences bind to repressors to partially or fully
repress the expression of a gene.
Silencers can be located several bases upstream or downstream
from the actual promoter of the gene.
Repressors can also have two binding sites: one for the silencer
region and one for the promoter.
This causes chromosome looping, allowing the promoter region
and the silencer region to come to close proximity.
TYPES OF REPRESSOR PROTEINS
Repressor proteins can be DNA- or RNA-binding:
DNA-binding repressors - block the binding of RNA
polymerase to the promoter. As a result, the gene is prevented
from being transcribed into mRNA
RNA-binding repressors - bind to mRNA, preventing
Examples of repressor proteins are lac repressor that inhibits
the expression of lac operon in E. coli. Another is MetJ,
a methionine repressor of met operon.
PASSIVE REPRESSOR PROTEINS:
Passive repressor proteins do not have intrinsic repressing activity or a
portable repression domain.
Rather, these proteins repress RNA synthesis by competing with
transcriptional activators for DNA binding, by forming inactive
heterodimers with transcriptional activators rendering them incapable of
interaction with DNA, or by binding to co activators required for the
transcriptional activator proteins.
Thus, passive repressor proteins transmit their biological function either
via DNA- or protein–protein interactions.
Inducible cAMP early repressor
bZIP transcription factor family
Sp1-like transcriptional repressors
PASSIVE REPRESSOR PROTEINS
Active mammalian transcriptional repressor proteins have an intrinsic
repression activity that targets the chromatin organization of the
This type of transcriptional repression is activator-independent and
functions over long distances.
Two types of active transcriptional repression can be distinguished:
transcriptional repression via histone deacetylation, and
gene silencing via histone methylation and heterochromatin formation.
Repressor proteins are influenced by the presence of other
molecules, such as co repressors and inducers.
Co repressors are molecules that bind and activate repressors.
Inducers, on the other hand, bind and regulate repressors by
inducing the latter to undergo conformational change thereby
decreasing binding affinity to the operator.