1. THEORY ASSIGNMENT
(Molecular Biology)
Name- Ritika
Course- BSc. Microbiology (H)
Year- Second year, IIIrd sem
College Roll No.- 4549
College id- ritika.mic4540@rla.du.ac.in
Submitted to- Dr. Sunila Hooda
3. OPERON
In genetics, an operon is a functioning unit of genomic DNA
containing a cluster of genes under the control of a single
promoter.
Promoter sequences : DNA sequences that define where
transcription of a gene by RNA polymerase begins.
Operator : a segment of DNA to which a transcription factor binds
to regulate the gene expression by repressing it.
Structural Genes : genes that codes for any RNA or protein product
other than a regulatory factor.
4. HISTORY
Discovered in 1953 by Jacques Monod (French Biochemist) and colleagues,
the trp operon in E. coli was the first repressible operon to be discovered.
While the lac operon can be activated by a chemical (allolactose), the
tryptophan (Trp) operon is inhibited by a chemical (tryptophan).
The trp operon appears to have been an ancient innovation; it was already
present in the common ancestor of Bacteria and Archaea. Although the
operon has been subjected, even in recent times, to dynamic changes in
gene rearrangement, the ancestral gene order can be deduced with
confidence.
5. “What is true for E. coli is also
true for the elephant.”
-Jacques Monod
6. TRP Operon- Introduction
The trp operon is an anabolic operon—a group of genes that is used, or
transcribed, together—that codes for the components for production of
tryptophan. It is a self- regulatory pathway. The trp operon is present in
many bacteria, but was first characterized in Escherichia coli.
It is an example of repressible negative regulation of gene expression. Within
the operon's regulatory sequence, the operator is bound to
the repressor protein in the presence of tryptophan (thereby
preventing transcription) and is liberated in tryptophan's absence (thereby
allowing transcription). Trp operons are repressed when the cell is
saturated with the product of the operon. Repressible system - under
normal conditions, the cell constantly manufactures a product, but is able
to shut down manufacture when the product isn't needed.
7. The trp operon, found in E. coli bacteria, is a group of
genes that encode biosynthetic enzymes for the amino acid
tryptophan. The trp operon is expressed (turned "on") when
tryptophan levels are low and repressed (turned "off")
when they are high. The trp operon is regulated by the trp
repressor.
9. The tryptophan operon is responsible for the production of the amino acid
tryptophan, whose synthesis occurs in five steps, each requiring a particular
enzyme. In E. coli, these enzymes are translated from a single polycistronic mRNA.
Adjacent to the enzyme coding sequences in the DNA are a promoter, an
operator, and two regions called the leader sequence and the attenuator. The
leader and attenuator sequences are transcribed. Trp operon contains five
structural genes: trpE, trpD, (both form antronylase synthase) trpC(Antronylase
synthase), trpB (Tryptophan synthase beta), and trpA (Tryptophan synthase
apha), which encode enzymatic parts of the pathway. Another gene (trpR)
encoding a repressor is located some distance from this gene cluster. trpR has a
promoter where RNA polymerase binds and synthesizes mRNA for a regulatory
protein. The protein that is synthesized by trpR then binds to the operator which
then causes the transcription to be blocked. In the trp operon, tryptophan binds to
the repressor protein effectively blocking gene transcription. In this situation,
repression is that of RNA polymerase transcribing the genes in the operon.
12. Repression-
The operon operates by a negative repressible feedback mechanism. The
repressor for the trp operon is produced upstream by the trpR gene, which
is constitutively expressed at a low level. Synthesized trpR monomers
associate into dimers. This repressor is a co-repressor and is turned on at
later stage. When tryptophan is present, these tryptophan repressor dimers
bind to tryptophan, causing a change in the repressor conformation,
allowing the repressor to bind to the operator. This prevents RNA
polymerase from binding to and transcribing the operon, so tryptophan is
not produced from its precursor. When tryptophan is not present, the
repressor is in its inactive conformation and cannot bind the operator
region, so transcription is not inhibited by the repressor.
13.
14. When the trp level is low:
When the level of tryptophan is low, no tryptophan binds with the
repressor molecule hence there is no change in its shape and
the movement of polymerase chain runs smoothly resulting in
undisturbed transcription and production of tryptophan.
15.
16. Attenuation- translational level
Attenuation is a mechanism for reducing expression of the trp operon when
levels of tryptophan are high. When levels of tryptophan are high,
attenuation causes RNA polymerase to stop prematurely when it's
transcribing the trp operon.
Prokaryotes commonly use attenuation as a mechanism to control gene
expression, but eukaryotes do not. In prokaryotes transcription and
translation are coupled. This makes attenuation possible.
Attenuation is a regulatory mechanism used in bacterial operons to ensure
proper transcription and translation. In bacteria, transcription and
translation are capable of proceeding simultaneously. The process of
attenuation involves the presence of a stop signal that indicates premature
termination.
17. This attenuation process takes place inside leader
sequence which has codons over it. After codon 1,
comes the stop codon the polymerase chain stops at
this codon to receive trp molecules brought by tRNA.
2&3 codon can bind together and form a structure
called as stem loop (anti- terminator) and 3&4 codon
can bind together forming a stem loop (terminator).
Both these structures form under different situation.
18. When the trp levels are low, ribosome polymerase chain slides on
the leader sequence scanning all the codons. Since the concentration
of trp molecules is already low in the cell, tRNA takes time to bring
trp to the polymerase chain, giving it time to wait and stand over
codons 2&3. In this mean time 2&3 form a stem loop called as anti
terminator. The polymerase chain can slide easily over this loop
producing more tryptophan.
When trp level is low (operon turned on)
19.
20. When the trp level is high, Ribosome polymerase cahin slides over
the the leader sequence and scans all the codons present. Since the
trp levels are already high in the cells, t RNA takes no time to bring
tryptophan molecules to the polymerase chain. Now since the chain
has already passes through the site of anti terminator loop (codon
2&3), codon 3&4 form a stem loop now which is called as
terminator. Ribosome polymerase chain is not allowed to pass over
this loop and it falls off, stopping the production of tryptophan
hence regulating the production.
When the levels of trp are high (operon turned off)
21.
22. How is trp operon different from lac operon?
Lac operon is involved with the catabolic process of a sugar, but trp
operon is involved in the anabolic process of an amino acid. Lac operon
gets activated in the presence of lactose, but trp operon gets deactivated
in the presence of tryptophan. the lac repressor binds the DNA sequence
found in the lac operator site, where as the trp repressor recognizes a
different DNA sequence that is found in the trp operator site. By default trp
operon is on but the lac operon is off by default.
