Agudo, Calumpong, Cogollodo, Erediano, Lendio, Pardillo, Yee
Group 2
March 20, 2020
EXERCISE 8
REGULATION OF GENES AND THEIR PRODUCTS
Objectives:
At the end of this activity, students are expected to be able to explain the overall
process of both the Lac and Tryp operon in the gene regulation. Furthermore, students
are also expected to determine the different molecules and proteins that are significant
in gene regulation.
I. Regulation of Gene Action in Prokaryotes
A. The Lactose Operon
The lac operon is a type of prokaryotic operon and has two kinds of the
transcription control system, which turns the operon "on" or "off" in response to glucose
and lactose levels. These are the negative transcription control system regulated by lac
repressor and the positive transcription control system by catabolite activator protein
(CAP). The structure of lactose operon comprises the following:
(1) the lac I gene, which encodes for the repressor protein responsible for
blocking the transcription
(2) the promoter region acts as a binding of the RNA polymerase
(3) the operator binding site of the lac repressor protein
(4) the three structural genes, which are the lacZ, lacY, and lacA, which code for
the following enzymes beta-galactosidase (for lacZ), lactose permease (for lacY), and
galactose transacetylase (for lacA)
(5) the CAP binding site, which is the regulatory site bounded by catabolite
activator protein.
For the situations when lactose and glucose is either present or absent, the
following happens:
In the absence of lactose and glucose:
Since the lac repressor is bounded to the operator region, the operon is turned
off. This prevents the Rna polymerase from binding to the promoter region and blocking
it from transcribing the operon. Despite the CAP- cAMP complex being bounded to the
binding site; it is still not enough to override the action of the lac repressor protein.
In the presence of lactose but absence of glucose:
The lac repressor protein is inactivated by an allolactose an inducer which
changes the conformation of the repressor allowing it to unbind from the operator
region. Hence, if this happens, the transcription will occur. In addition, with the absence
of glucose in the picture and with the RNA polymerase is not binded well to the DNA, it
will need the help from CAP (Catabolite- Activating Protein). For CAP to bind to the
promoter region, it must bind to cAMP (Cyclic Adenosine Monophosphate). The level of
cAMP is dependent on adenyl cyclase, which is an enzyme that catalyzes the
conversion of ATP to cAMP. In the absence of glucose, the cAMP will bind to CAP,
causing it to change its shape and forming the CAP-cAMP complex, which allows it to
bind to the CAP binding site and promote high levels of transcription.
In the presence of glucose but absence of lactose:
When glucose is present, but lactose is absent, the operon is turned “off". Hence,
there will be no transcription that will take place. This happens since the lac repressor
binds to the operator region blocking the RNA polymerase from binding to the DNA and
with the presence of glucose, it inhibits the adenyl cyclase from producing cAMP
therefore no CAP protein will be activated, and formation of CAP-cAMP complex does
not occur. Hence, CAP cannot bind to its binding site. As a result, no transcription
occurs. Lastly, with the presence of both lactose and glucose, the lac repressor is
inactivated. Therefore, transcription occurs, however, since glucose is also present,
CAP will not be activated preventing it to bind to its binding site. Under this condition,
transcription occurs but only in low levels.
B. The Tryptophan Operon
Tryptophan operon is another type of operon, which is present in prokaryotes.
Tryptophan operon is also a negative repressible transcription control system. The
structure of the tryptophan operon consists of the following:
(1) TrpR which encodes the repressor protein
(2) TryP where the RNA binds
(3) TrpO where the repressor binds
(4) TrpL, which is the regulatory site of attenuation
(5) The five structural genes: trpE, trpD, trpC, trpB, and trpA responsible for
tryptophan biosynthesis.
In a situation when tryptophan is absent or low, the Trp repressor protein is
inactive. Since there will be no tryptophan that will bind to it, the Trp repressor protein
will not bind to the DNA or operator region. Hence, transcription is not blocked since the
RNA polymerase can continue to transcribe the operon.
On the other hand, if tryptophan availability is high, the repressor protein is
activated by the tryptophan, which is a corepressor forming the repressor tryptophan
complex. Therefore, allowing it to bind its binding site in the DNA and blocking or
stopping the transcription process. As a result, there will be no biosynthesis of
tryptophan that will take place.
II. REGULATION OF GENE ACTION IN EUKARYOTES
Eukaryotic gene expression comprises many steps with different genes regulated
at different points. The stages involve in gene expression are the following:
i. Chromatin accessibility
Chromatin may be openly and loosely arranged or tightly compacted. More open
or “relaxed” chromatin makes a gene more available for transcription.
ii. Transcription
Transcription is a key regulatory point for many genes. Sets of transcription factor
proteins bind to specific DNA sequences in or near a gene and promote or repress its
transcription into an RNA.
iii. RNA processing and export
This step involves splicing, capping, and adding a poly-A tail to a primary
transcript or the RNA molecule after which is carried out of the nucleus. Through
alternative splicing, various mRNAs may be made from the same pre-mRNA.
iv. mRNA stability
This step occurs in the cytosol where the mRNA may stay for a long time or are
quickly broken down. This in turn affects the amount of proteins which can be produced
from it. Also, molecules called miRNAs which are small regulatory RNAs bind to target
mRNAs chopping or cutting them of.
v. Translation
In this step, the mRNA is more or less readily translated by the ribosomes to
produce a poly peptide. This step may also be inhibited or increased by certain
regulators.
vi. Protein Activity
Lastly, in here the polypeptide undergoes various modifications through
processing, snipping off of amino acids or proteolytic cleavage, and addition of chemical
groups. This in turn causes affects how the protein behaves.
GUIDE QUESTIONS
1. Given the following genotypes, explain how the mutation (identified by a
superscript will affect the organism grown in the lactose medium).
a. i+ p+ o+ z- y+
lacZ is the gene coding for beta-galactosidase. If a bacterium growing in a
lactose medium with the given phenotype wherein lacZ is a mutant, the lac operon will
be on and transcription will proceed, however, there would be no functional
betagalactosidase enzyme that will be produce. Thus, the lactose will not be broken
down to galactose and glucose which would result with the bacteria not able to utilize
lactose. There will still be functional permease produced.
b. i- p+ o+ z+ y+
The structural gene will not be able to produce a functional repressor protein.
With that, the activity of lac operon will be on whether there is presence of lactose.
There will still be functional beta-galactosidase and permease produced, however the
activity of the lac operon will not be regulated properly due to the mutation in the i gene.
c. i+ p- o+ z- y+
Mutation in the promoter region would cause a problem in the binding of the RNA
polymerase. In the presence of lactose in the environment, no functional
betagalactosidase and permease will be produced because the RNA polymerase
cannot bind properly to the promoter region. There will be no transcription.
2. Give one example of a positive transcriptions control system in bacteria.
Briefly explain how the system controls transcription of the structural genes.
An example of positive control in gene regulation is the glucose effect. In E. Coli,
glucose is the preferred carbon source. The bacterium will consume any glucose that is
available before utilizing another carbon source such as lactose.
The presence of glucose will be involved in diminishing the expression of the lac
operon. This phenomenon is called catabolite repression.
There are two components to consider in order to form a positive form of
regulation, namely: cAMP (Cyclic Adenosine Monophosphate) and CAP (Catabolite-
Activating Protein).
In the presence of glucose, for CAP to bind in the promoter, CAP must bind to
the cAMP. The level of cAMP is dependent on adenyl cyclase, which catalyzes the
conversion of ATP to cAMP. If glucose is present, it will inhibit the adenyl cyclase, which
will cause the level of cAMP in the cell to decline. This condition will inhibit the creation
of cAMP-CAP complex which is important to the positive control of the transcription of
the lac operon.
In the absence of glucose, CAP exerts positive control by binding to the CAP
site, facilitating the binding of RNA polymerase at the promoter region. Hence,
transcription will proceed.
REFERENCES
Khan Academy. Overview of Eukaryotic Gene Regulation. Retrieved on March 19, 2020
from https://www.khanacademy.org/science/biology/gene-regulation/gene-regulation-
ineukaryotes/a/overview-of-eukaryotic-gene-regulation
Losick, R., & Stragier, P. Crisscross regulation of cell-type-specific gene expression
during development in B. subtilis. Nature 355, 601–604 (1992) doi:10.1038/355601a0
Klug, W. S., Cummings, M. R., Spencer, C. A., & Palladino, M. A. (2016). Concepts of
Genetics (Eleventh ed.). Harlow, Essex, United Kingdom: Pearson Education
Limited. Retrieved March 19, 2020
