INTRODUCTION
ransposons which are also called as transposable elements or jumping genes. Transposons
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are segments of DNA that can be transposed from one place to another within the genome
through a cut-and-paste mechanism. Movement of the transposons can occur within a single
chromosome or between various chromosomes. These elements present in all the eukaryotic
and prokaryotic organisms and in some species it occupies high percentage in the genome,
For example, transposable elements comprise approximately 10% of several fish species, 12
% of the C.elegans genome, 37% of the mouse genome, 45% of the human genome and in
some plant species like maize comprises >80%. Accumulation of transposons from bacteria
to humans occurs over time and makes changes in the shape of the genome in the organism
through mobilisation.
ased on the intermediate used for mobilisation of transposable elements they are termed as
B
transposition or retrotransposition.The activity of TEs in the genome can make impact in both
positive and negative ways, for example: TE mobilisation can promote gene inactivation,
modulate gene expression or induce illegitimate recombination, that's why TEs plays a major
role in the genome evolution. Based on the theoretical point of view TE is also called as
selfish DNA or junk DNA
here are two classes of TE, class l and class ll. RNA transposons is the class l which
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functions based on the reverse transcription of an RNA intermediate and it is further
subdivided in two main groups based on the presence of Long Terminal Repeats (LTR)
flanking the retroelement main body.DNA transposons is the class ll which functions based
on the non replicative mechanism. DNA transposons are
made of transposase enzymes flanked by two Terminal Inverted Repeats.
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2874221/
Tn3 TRANSPOSON
n3 transposon is basically a replicative transposon which follows a copy-paste mechanism.
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It is a 4957 base pair mobile genetic element which is present in the almost all the bacterial
phyla includes proteobacteria, firmicutes, and cyanobacteria
Tn3 basically encodes by three proteins they are,
1.β-lactamase (encoded by gene bla).
2.Tn3 transposase (encoded by gene tnpA).
3.Tn3 resolvase (encoded by gene tnpR).
1 .β-lactamase
By hydrolyzing the peptide bond of the distinctive four-membered beta-lactam ring, the
beta-lactamase enzymes render beta-lactam antibiotics inactive. The bacteria gains resistance
as a result of the antibiotic's inactivation.
ole of bacterial transposons in antibiotic resistance:
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Due to the nature of transposable elements, genes can translocate from one DNA molecule to
another, from chromosome to plasmid, and vice versa. This is why many bacterial
transposons also carry genes for antibiotic resistance in addition to the transposase gene. This
causes a genetic flow of genes resistant to antibiotics in the population of bacteria.
The drug resistance gene can spread across the bacterial population when transposons
recombinate with a plasmid vector inside a bacterial cell. This transformation can occur either
horizontally or vertically. This produces dangerous microorganisms that are resistant to
multiple drugs, making diseases harder to treat.
2 .Tn3 transposase
Genome rearrangement results from the reassembly of transposons by transposases, which is
one of the roles of noncoding DNA. Enzymes called transposases are responsible for splicing
and cutting vast amounts of DNA. DNA segments known as transposons have the ability to
travel inside a single cell's genome and take up different locations.
3 .Tn3 Resolvase
DNA recombinase Tn3 resolvase performs a number of functions, such as:
1.DNA recombination: two DNA sequences on the same DNA molecule that are in the right
orientation are cut and rejoined by Tn3 resolvase.
2.Tn3 resolvase aids in the process of Tn3 replication.
3.Genome editing: Recombination systems for genome editing can be designed using the
three-dimensional model of the protein–DNA complex that Tn3 resolvase forms.
I S ELEMENTS
The most basic kind of transposable sequences in bacteria are called insertion sequences, or
IS elements. These sequences can insert through illicit recombination at many locations on
bacterial chromosomes and plasmids.
Usually consisting of a single gene encoding the transposition enzyme, they are brief
sequences.
The first evidence of IS elements was found to be spontaneous insertion in specific lac
operon mutations of Escherichia coli that prevent transcription and translation and render the
gene inactive. It was discovered that the Lac operon gene mutation was unstable, and
molecular investigation shows that additional copies of DNA sequences are present close to
the Lac gene. The additional DNA sequence is lost when the altered E. coli goes through
reverse mutation.
A specific kind of IS element may be found in several copies on a bacterial
chromosome.example, 6 to 10 copies of IS1 are found in the E. coli chromosome
https://www.onlinebiologynotes.com/transposable-elements-in-prokaryotes-bacteria/
n3 transposons are ampicillin-resistance transposon which is archetype, synonymous with
T
“Tn1” or “Tn2.In addition to other mobile genetic elements, family members function as
modular platforms that facilitate the assembly, diversification, and redistribution of an
ever-expanding arsenal of antimicrobial resistance genes. This helps explain why the
emergence of multidrug resistances is posing a challenge to the development of new
treatments. Additionally, they are widely distributed in the horizontal transmission of huge
catabolic operons, which enable bacteria to break down a wide range of chemical families,
including industrial xenobiotic contaminants.
MOLECULAR AND DYNAMIC STRUCTURE OF Tn3 TRANSPOSON
I t is possible to think of mobile genetic components as a juxtaposition of functional modules
that collectively give each element its unique specificities. The Tn3 transposon family
consists of three different types of modules the cointegrate resolution module, which
improves the transposition pathway by lowering the possibility of aberrant replicon fusions
and lessening the transposon's reliance on host recombination functions; various sets of
cargo genes and operons that were assimilated, likely because they proved beneficial to their
host in specific circumstances.
MECHANISM OF TRANSPOSITIONS
1. Identification and Attachment
a . The enzyme transposase:
The transposon encodes the transposase enzyme, which binds to and recognizes the terminal
inverted repeat (IR) sequences that surround the transposon. Because they demarcate the
transposon's boundaries and are essential to its mobility, these repetitions are critical to the
enzyme's activity.
b. Transposase-DNA Complex Formation:
t both ends of the transposon, the transposase attaches to the inverted repeats. The
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transposon's two ends are brought closer together by this interaction, establishing a stable
complex with the DNA.
2. Removal of Transposon:
a . DNA Cleavage:
At the transposon's borders, the transposase enzyme causes double-strand breaks in the DNA.
The transposon is removed from its original site as a result of this cleavage, which takes place
inside the inverted repeats.
b . DNA Intermediate Formation:
The transposon is released as a circular DNA molecule following excision, but the DNA ends
still allow for a covalent bond to form between it and the transposase enzyme. This
intermediary is referred to as a "transposon complex" or "transposon circle" at times.
3.DNA Cleavage and Target Site Recognition
a . Target Site Selection:
Next, the transposase looks for a new place for the transposon to be introduced in the
genome. Though some transposons have less strict criteria, this target location is often a
specific DNA sequence.
b. Double-Strand Break Formation:
he transposase causes a staggered double-strand break in the DNA at the target location.
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Single-stranded overhangs are produced by this break and are required for the transposon's
subsequent insertion.
4. Inserting the Transposon;
a . Transposon Integration:
The target DNA double-strand break is where the transposon is placed. The transposon base
pair terminates with the target DNA's overhanging ends. The target DNA and transposon
DNA are joined by the transposase enzyme.
b . Ligation and Repair of DNA:
Following insertion, cellular DNA repair processes close and fill the spaces left by the
transposon and the target DNA. A common characteristic of many transposons, including
TN3, is the duplication of a few base pairs of the target DNA sequence surrounding the
insertion site as a result of this process.
5. Settlement and Concluding
a . Complexity Resolution:
After the transposon has been effectively inserted into the target location, the transposase
enzyme separates from the DNA. The transposon is now permanently incorporated into the
genome.
b . Target DNA Reconstitution
The procedure is finished when the transposon is smoothly integrated and the target DNA is
returned to its double-stranded state by the cellular repair machinery. As a result, flanking
direct repeats, or duplicated target sequences, are now clearly apparent on both sides of the
transposon, making it a permanent component of the host genome.
3. Effect
1 . Antibiotic Resistance Spread:
The transmission of antibiotic resistance is significantly aided by TN3 transposons. They aid
in the spread of resistance features by incorporating resistance genes into multiple locations
within the bacterial genome or between different bacteria. This may result in the emergence
of bacterial strains that are resistant to many drugs, creating significant difficulties for
infection prevention and management.
b. Evolution and Genetic Variation:
enetic diversity is introduced within bacterial populations by the mobility of TN3
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transposons. Because of this variety, bacteria may quickly adapt to new environments—like
ones containing antibiotics—which can lead to evolutionary changes.
c . Transfer of genes horizontally:
TN3 transposons not only allow transposition within a single genome but also enable
horizontal gene transfer between distinct bacteria. This is especially important since it speeds
up the spread of resistance genes in situations when bacteria are in close proximity to one
another, like in biofilms or the human body.
d . Genetic Engineering Applications:
Genetic engineering and biotechnology have also benefited from our understanding of TN3
transposons. Utilising their capacity to merge with genomes, scientists create instruments for
genetic analysis, mutagenesis, and gene insertion.
