It's about transcriptionalregulation in bacteria by operon.

2. Bacteria employ to control the expression of their genes: by grouping
functionally related genes together so they can be regulated together easily.
Such a group of contiguous, co-ordinately controlled genes is called an operon.
Many operons –eg Lac, Trp, Ara (metabolism)
Lux, Rex (quorum sensing)

3. Control of an operon is a type of gene regulation that enables organisms to regulate
the expression of various genes depending on environmental conditions. Operon
regulation can be either negative or positive by induction or repression.
Negative control involves the binding of a repressor to the operator to prevent
transcription.
In negative inducible operons, a regulatory repressor protein is normally bound to
the operator, which prevents the transcription of the genes on the operon. If
an inducer molecule is present, it binds to the repressor and changes its
conformation so that it is unable to bind to the operator. This allows for expression
of the operon.
Ex- The lac operon is a negatively controlled inducible operon, where the inducer
molecule is allolactose.
In negative repressible operons, transcription of the operon normally takes place.
Repressor proteins are produced by a regulatory gene, but they are unable to bind
to the operator in their normal conformation. However, certain molecules called co-
repressors are bound by the repressor protein, causing a conformational change to
the active site. The activated repressor protein binds to the operator and prevents
transcription.
Ex- The trp operon, involved in the synthesis of tryptophan (which itself acts as the
corepressor), is a negatively controlled repressible operon.

4. Operons can also be positively controlled.
With positive control, an activator protein stimulates transcription by binding to
DNA (usually at a site other than the operator).
In positive inducible operons, activator proteins are normally unable to bind to
the pertinent DNA. When an inducer is bound by the activator protein, it
undergoes a change in conformation so that it can bind to the DNA and activate
transcription.
Eg Ara operon
In positive repressible operons, the activator proteins are normally bound to the
pertinent DNA segment. However, when an inhibitor is bound by the activator, it
is prevented from binding the DNA. This stops activation and transcription of the
system.

5. The lac operon (lactose operon) is an operon required for the transport
and metabolism of lactose in E. coli
Structure:
The lac operon consists of three structural genes, and a promotor,
a terminator, regulator, and an operator.

7. Negative Control of the lac Operon

9. The β-galactosidase reaction: The enzyme breaks the β-
galactosidic bond (gray) between the two sugars, galactose (red)
and glucose (blue), that compose lactose.
Conversion of lactose to allolactose (inducer): A side reaction carried
out by β-galactosidase rearranges lactose to the inducer, allolactose.
What is Inducer?
It is actually an alternative form of
lactose called allolactose. When β-
galactosidase cleaves lactose to
galactose plus glucose, it rearranges
a small fraction of the lactose to
allolactose.
How can lactose be
metabolized to allolactose if
no permease is present to get
it into the cell and no β-
galactosidase exists to
perform the metabolizing
because the lac operon is
repressed?
The answer is that repression
is somewhat leaky, and a low
basal level of the lac operon
products is always present.
This is enough to get the ball
rolling by producing a little
inducer.

10. Catabolic Repression of the lac Operon
Or
Glucose Effect

11. RNA polymerase alone does not bind very well to the lac operon promoter. It might make a
few transcripts, but it won't do much more unless it gets extra help from catabolite activator
protein (CAP). CAP binds to a region of DNA just before the lac operon promoter and helps
RNA polymerase attach to the promoter, driving high levels of transcription.
CAP isn't always active (able to bind DNA). Instead, it's regulated by a small molecule
called cyclic AMP (cAMP). cAMP is a "hunger signal" made by E. coli when glucose levels are
low. cAMP binds to CAP, changing its shape and making it able to bind DNA and promote
transcription. Without cAMP, CAP cannot bind DNA and is inactive.
CAP is only active when glucose levels are low (cAMP levels are high). Thus, the lac operon
can only be transcribed at high levels when glucose is absent. This strategy ensures that
bacteria only turn on the lac operon and start using lactose after they have used up all of the
preferred energy source (glucose).

12. (a) In the absence of lactose to serve as an inducer,
the Lac repressor is able to bind the operator;
regardless of the levels of cAMP and the
presence of CAP, mRNA production is
repressed.
(b) With lactose present to bind the repressor, the
repressor is unable to bind the operator;
however, only small amounts of mRNA are
produced because the presence of glucose
keeps the levels of cAMP low, and thus the
cAMP–CAP complex does not form and bind
the promoter.
(c) With the repressor inactivated by lactose and
with high levels of cAMP present (owing to the
absence of glucose), cAMP binds CAP. The
cAMP–CAP complex is then able to bind the
promoter; the lac operon is thus activated, and
large amounts of mRNA are produced.
(d) When CAP binds the promoter, it creates a
bend greater than 90° in the DNA. Apparently,
RNA polymerase binds more effectively when
the promoter is in this bent configuration.
(e) CAP bound to its DNA recognition site. This part
is derived from the structural analysis of the
CAP–DNA complex

13. So, when does the lac operon really turn on?
The lac operon will be expressed at high levels if two conditions are met:
Glucose must be unavailable: When glucose is unavailable, cAMP binds to
CAP, making CAP able to bind DNA. Bound CAP helps RNA polymerase attach
to the lac operon promoter.
Lactose must be available: If lactose is available, the lac repressor will be
released from the operator (by binding of allolactose). This allows RNA
polymerase to move forward on the DNA and transcribe the operon.

14. Glucose present, lactose absent: No transcription of
the lac operon occurs. That's because the lac repressor
remains bound to the operator and prevents transcription
by RNA polymerase. Also, cAMP levels are low because
glucose levels are high, so CAP is inactive and cannot bind
DNA.
Glucose present, lactose present: Low-level transcription
of the lac operon occurs. The lac repressor is released from
the operator because the inducer (allolactose) is present.
cAMP levels, however, are low because glucose is present.
Thus, CAP remains inactive and cannot bind to DNA, so
transcription only occurs at a low, leaky level.
Glucose absent, lactose absent: No transcription of
the lac operon occurs. cAMP levels are high because
glucose levels are low, so CAP is active and will be bound to
the DNA. However, the lac repressor will also be bound to
the operator (due to the absence of allolactose), acting as a
roadblock to RNA polymerase and preventing transcription.
Glucose absent, lactose present: Strong transcription of
the lac operon occurs. The lac repressor is released from
the operator because the inducer (allolactose) is present.
cAMP levels are high because glucose is absent, so CAP is
active and bound to the DNA. CAP helps RNA polymerase
bind to the promoter, permitting high levels of
transcription.

16. Diauxic Growth Curve
When the bacteria is grown in media containing both
glucose & lactose, a diauxic curve is observed.
Glucose is the preferable source
of carbon over lactose.
A lag phase is observed between
two exponential phases, where
glucose is exhausted & lactose
available in high amount.
During the first phase of
exponential growth, the bacteria
utilize glucose as a source of
energy until all the glucose is
exhausted. Then, after a
secondary lag phase, the lactose
is utilized during a second stage
of exponential growth.

18. Thank you