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Genetics summary chapter 14 VU amsterdam
- Instelling
- Vrije Universiteit Amsterdam (VU)
This summary covers complete chapter 14 of genetics at the Vrije Universiteit Amsterdam.
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Chapter 14 Gene regulation in bacteria- Gene regulation: the phenomenon in which the level of gene expression can vary under different conditions.
In comparison, unregulated genes – constitutive genes - have constant levels of expression in all conditions over time.
Constitutive genes encode for proteins that are needed for the survival of a bacterium. In contrast, the majority of genes
are regulated so that the proteins they encode can be produced at the proper times and in the proper amounts. A key
benefit of gene regulation is that the encoded proteins are produced only when they are needed. Therefore, the cell avoids
wasting valuable energy by making proteins that it does not need. This enables a bacterium to compete as efficiently as
possible for limited resources. Gene regulation is important because bacteria exist in an environment that is frequently
changing with regard to temperature, nutrients and other factors. The following are a few common processes regulated at
genetic level:
1. Metabolism: some proteins function in the metabolism of small molecules. Certain enzymes are needed for a
bacterium to metabolize particular sugars. These enzymes are needed only when the sugars are present in the
bacterium’s environment.
2. Response to environmental stress: certain proteins help a bacterium to survive an environmental stress such as
osmotic shock or heat shock. These proteins are only required when the bacterium is confronted with the stress.
3. Cell division: some proteins are needed for cell division. These are necessary only when the bacterial cell is
getting ready to divide.
The expression of protein-encoding genes, which encode polypeptides, leads to the production of functional proteins. Gene
regulation can occur at any if the steps in the pathway of gene expression (transcription translation). In bacteria, the
most common way to regulate gene expression is by influencing the rate at which the transcription is initiated. We do not
say ‘ the genes are being turned off or on’ but instead, we say ‘ the gene expression is increased/decreases.’
Transcriptional regulation involves often the actions of regulatory proteins that can bind to the DNA and affect the rate of
transcription of one or more nearby genes. Two types of regulatory proteins are common:
1. Repressor: regulatory protein that binds to DNA and inhibits transcription. Transcriptional regulation by a
repressor is termed negative control.
2. Activator: a regulatory protein that binds to DNA and increases the rate of transcription. Transcription regulation
by an activator is termed as positive control.
Small effector molecules also play a role in transcriptional regulation. However, they do not bind directly to the DNA (as in
regulatory proteins) to alter transcription. Rather, an effector molecule exerts its effects by binding to a repressor or
activator (regulatory proteins). The binding of the effector molecule causes a conformational change in the regulatory
protein and thereby influences whether or not the protein can bind to DNA. Genetic regulatory proteins that respond to
small effector molecules typically have two binding sites. One site is where the regulatory protein binds to the DNA ; the
other site is the binding site for the effector molecule.
Regulatory proteins are given names describing how they affect transcription when they are bound to the DNA (repressor
or activator). In contrast, small effector molecules are given names that describe how they affect the transcription when
they are present in the cell at a sufficient concentration to exert their effect.
- Inducer: small effector molecule that causes the rate of the transcription to increase. It can do this in two ways:
1. It can bind to a repressor regulatory protein and prevent it from binding to the DNA
2. It can bind to an activator regulatory protein and causes it to bind to DNA.
In either ways, the transcription rate is increased. Genes that are regulated in this manner are called inducible genes.
Alternatively, the presence of a small effector molecule may inhibit transcription. This occurs in two ways:
1. Corepressor: small effector molecule that binds to a repressor regulatory protein, thereby causing the protein to
bind to DNA.
2. Inhibitor: small effector molecule that binds to activator regulatory protein and prevents it from binding to the
DNA.
In the absence of an inducer, this repressor regulatory protein blocks transcription. The presence of the inducer causes a
conformational change that inhibits the ability of the repressor regulator protein to bind to DNA. Transcription proceeds.
An activator regulatory protein cannot bind to the DNA unless there is an inducer. When the inducer is bound to the
activator regulatory protein, this enables it to bind to the DNA and activate transcription.
In the absence of a corepressor, the repressor regulatory protein will not bind to the DNA. Therefore, transcription can
occur. When the corepressor is bound to a repressor regulatory protein, a conformational change occurs that allows the
repressor to bind to the DNA and inhibit transcription.
, Chapter 14 Gene regulation in bacteria
The activator regulatory protein will bind to the DNA without the aid of a small effector molecule. The presence of an
inhibitor causes a conformational change that inhibits the ability of the activator regulatory protein to bind to the DNA. This
inhibits transcription.
Francois Jacob and Jacques Monad did a research on genes and gene regulation stemmed from an interest in the
phenomenon known as enzyme adaptation – increase in the activity of a specific enzyme brought about the presence of a
specific substance (substrate), occurring without any change in the genotype. Enzymes are composed of proteins. Enzyme
adaptation refers to the observation that a specific enzyme appears within a living cell only after the cell has been exposed
to the substrate of that enzyme. When a bacterium is not exposed to a particular substance, it does not make the enzymes
that are needed to metabolize that substance. To investigate this phenomenon, Jacob and Monad focused their attention
on lactose metabolism in E. coli.
1. The exposure of bacterial cells to lactose increased the levels of lactose-utilizing enzymes by 1000-10000 fold.
2. Antibody and labelling techniques revealed that the increase in the activity of these enzymes was due to the
increases synthesis of the proteins that form these enzymes.
3. The removal of lactose from the environment causes an abrupt termination in the synthesis of the enzymes.
4. The analysis of mutations in the lac operon (operon for the transport and metabolism of lactose within the E.coli)
revealed that each protein involved with lactose utilization is encoded by a separate gene.
These critical observations indicated to Jacob and Monad that each enzyme adaptation is due to the synthesis of specific
proteins in response to lactose in the environment. In bacteria, it is common for a few genes to be arranged together in an
operon – a group of two or more genes that are under the transcriptional control of single promotor. An operon encodes a
polycistronic mRNA, an RNA that contains the sequences of two or more genes.
Why do operons occur in bacteria? One biological advantage of an operon is that is allows the bacteria to coordinately
regulate a group of two or more genes that are involved with a common functional goal: the expression of the genes occurs
as a single unit. To facilitate transcription, an operon is flanked by a promotor that signals the beginning of transcription
and a terminator that signals the end of the transcription. Two or more genes are found between the promotor and
terminator.
Organization of DNA sequences in the lac region of the E. coli chromosome:
1. CAP site (catabolic activator regulatory protein): DNA sequence recognized by an activator regulatory protein
called the catabolic activator protein (CAP).
2. Operator site: sequence of bases that provides a binding site for the repressor protein called the lac repressor.
3. Lacl gene: gene that is not part of the lac operon. The lac operon contains the protein encoding genes called LacZ,
LacY and LacA
a. lacZ: encodes for β-galactosidase
b. lacY: encodes for lactose permease
c. lacA: encodes for galactosidee transacetylase
The lacl gene has its own promotor (i promotor) and is constitutively expressed at fairly low levels. The lacl gene
encodes for the lac repressor, a protein that regulates the lac operon by binding to the operator site and
repressing transcription. This repressor is a homotetramer, a protein that is composed of 4 identical subunits.
Only a small amount of the lac repressor is needed to repress (inhibit) the lac operon.
The CAP site is the binding site for the catabolic activator protein (CAP). The operator site is the binding site for the lac
repressor protein. The promotor (LacP) is responsible for the transcription of the protein-encoding genes LacZ, LacY and
LacA as a single unit, which ends in the Lac terminator. the i promotor is responsible for the transcription of the LacL gene.
Lactose permease allows the uptake of lactose into the bacterial cytoplasm. It cotransports lactose with H +. Because
bacteria maintain H+ gradient across their cytoplasmic membrane, this cotransport permits the active accumulation of
lactose against a gradient. Β-Galactosidase is a cytoplasmic enzyme that cleaves lactose and related compounds into
galactose and glucose. It also converts lactose into allolactose (inducer that inhibits the lac repressor from binding to the lac
operator), which can also be broken down into galactose and glucose.
Lac repressor
The lac operon can be transcriptionally regulated in more than one way. The first mechanism is inducible and under
negative control. This form of regulation involves a lac repressor, which is a protein that binds to the sequence of
nucleotides found in the lac operator site. Once bound, the lac repressor prevents RNA polymerase from transcribing the
LacZ, LacY and LacA genes. The binding of the repressor to the operator site is a reversible process. In the absence of
allolactose, lac repressor is bound to the operator site.
The ability of the lac repressor to bind to the operator site depends on whether allolactose is bound to it or not. Each of the
repressor’s 4 sub units has a single binding site for allolactose, the inducer. When 4 molecules of allolactose bind to the
repressor, a conformational change occurs to prevent lac repressor from binding to the operator site. Under these
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