Transcript 3) Direct repeats

Bacterial Transposons
Author
Meenakshi Agarwal, Mehta Gunjan
Mentor
Dr. Santanu Ghosh
Master Layout (Part 1)
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This animation consists of 3 parts:
Part 1 – Definition of transposons and keywords
Part 2 – Types of transposons
Part 3 – Mechanism of transposition
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Part1- Definition of Transposons
Alternate nomenclature:
1) Transposable elements
2) Jumping genes
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3) Mobile DNA
Discovery: First discovered by Barbara McClintock in 1950’s.
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Definition: Capable of independently replicating itself and
inserting the copy into a new position within the same or
another chromosome or plasmid, a process called
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transposition.
Keywords
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1) IS (Insertion sequence) elements: is a small bacterial
transposon that carries only the genes needed for its own
transposition.
2) Inverted terminal repeats: are the short related or identical
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sequences present in reverse orientation at the ends of some
transposons.
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3) Direct repeats: are identical (or closely related) sequences
present in two or more copies in the same orientation in the
same molecule of DNA.
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Keywords
4) Transposase: an enzyme that binds to ends of transposon
and catalyses the movement of the transposon to another part
of the genome by a cut and paste mechanism or a replicative
transposition mechanism.
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5) Resolvase: is the enzyme activity involved in site-specific
recombination between two transposons present as direct
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repeats in a cointegrate structure.
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Part 2-Types of bacterial transposons
There are three types of bacterial tranposons:
1) IS elements
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2) Composite transposon
3) Non-composite transposon
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IS elements

IS
(Insertion
sequence)
elements:
The
simplest
transposons, are autonomous units, each of which codes
only for the proteins needed to sponsor its own
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transposition.

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Inverted repeats + transposase genes
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Step 1:
Structure of IS elements
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Transposase
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protein
Description of
the action/
interactivity
Text to be displayed (if any)
(DT)
Audio Narration
(if any)
IS elements are simplest
1st yellow box will appear
with lable. 2) Arrow from box Transposons and contains
Inverted repeats and
and transposase Protein
tranposase genes,
structure
codes
proteins for
will appear. 3)Two red
boxes(IR) will appear on both transposition protein
ends. 4) sequences with blue
boxes will appear. 5) in last
Insertion sequence, IS1
(text)will appear.
DT1
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Composite transposons

Composite transposons: Composite genetic elements are
larger than IS elements and contain one or more protein-
coding genes in addition to those required for transposition
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e.g. Tn5, Tn9, Tn10.

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Two IS elements + antibiotic resistance gene(s).
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Structure of Composite transposons
Inverted
Repeats
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Structural genes
Inverted
Repeats
antibiotic resistance genes
Inverted IS
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Genes for
Transposition
Description of
the action/
interactivity
1st Black box will appear
with text and lable. 2)
Red and yellow boxes
at both ends With
lables. 3) triangle
shaped arrow and text
inverted IS will appear
Text to be displayed (if any)
(DT)
Audio Narration
(if any)
These transposons
composed of two insertion
sequence, which codes
genes for transposition,
flanking structural genes
which codes for various
proteins and enzymes i.e.
viral or antibiotic resistance
DT1
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Non composite transposons

Non-composite transposons: Non-composite mobile
genetic elements are those which lack IS elements on its
ends e.g. Tn3 and Tn7.
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
Inverted repeats + transposase gene + antibiotic resistance
gene(s).
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Structure of Non-composite transposons
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Description of
the action/
interactivity
1st blue box with label
transposase (tnpA) will
appear. 2) Reolvase
with tnpB will blow up
3) B –lactamase (bla) will
blow up. 4) two yellow
boxes at both ends
with their lable will
appear . 5) arrow with
size 4957bp will
appear
Audio Narration
(if any)
This is the example of NonDT1
composite transposonTn3.These transposons
encode transposition proteins,
have inverted repeats (but no
ISs) at their ends. In addition
to resistance and virulence
genes they may encode
catabolic enzymes
Text to be displayed (if any)
(DT)
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Part 3- Mechanism

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All transposons use a common mechanism in which staggered
nicks are made in target DNA, the transposon is joined to the
protruding ends, and the gaps are filled.
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
The order of events and exact nature of the connections between
transposon and target DNA determine whether transposition is
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replicative or nonreplicative.
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Replicative Transposon
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Replicative transposon is first replicated and then one of the copy
will move to the another location in the genome. Thus, the
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transposon will remain on its original position. “Copy and Paste”
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transposition
involves
two
types
of enzymatic
activity:
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Transposase
transposon;
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that
acts
on
and Resolvase
the
that
ends
acts
on
of
the
the
original
duplicated
copies.
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Replicative
A
group
of transposons
replicative transposition.
related
to
TnA
move
only
by
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Replicative Transposon
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Description of
the action/
interactivity
First donor and recipient DNA
will appear. Dark pink region
from donor molecule will be
replicated and it will be inserted
at the target site in the recipient
molecule.
Audio Narration
(if any)
In replicative transposition, the
transposon is replicated first from the
donor molecule and then it is
inserted at the target site present in
the recipient molecule.
Text to be displayed
(if any)
(DT)
DT1
Replicative Transposon
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Replicative transposition occurs through a cointegrate formation.
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A cointegrate structure is produced by fusion of two replicons, one originally
possessing a transposon, the other lacking it; the cointegrate has copies of
the transposon present at both junctions of the replicons, oriented as direct
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repeats.
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Resolution occurs by a homologous recombination reaction between the two
copies of the transposon in a cointegrate.
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The reaction generates the donor and target replicons, each with a copy of
the transposon.
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Resolvase is the enzyme activity involved in site-specific recombination
between two transposons present as direct repeats in a cointegrate structure.
Replicative Transposon
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The reactions involved in generating a cointegrate have been defined in
detail for phage Mu.
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The process starts with the formation of the strand transfer complex
(sometimes also called a crossover complex).
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The donor and target strands are ligated so that each end of the
transposon sequence is joined to one of the protruding single strands
generated at the target site.
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The crossover structure contains a single-stranded region at each of the
staggered ends. These regions are pseudoreplication forks that provide a
template for DNA synthesis. (Use of the ends as primers for replication
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implies that the strand breakage must occur with a polarity that generates
a 3 ′ –OH terminus at this point.)
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Replicative Transposon
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Description of
the action/
interactivity
As shown in figure, one by one
each step will appear along
with text.
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Audio Narration
(if any)
Replicative transposition occurs
through cointegrate formation.
Nick formation occurs on donor
and recipient molecule.
The single stranded ends of
transposon will be ligated to the
single stranded ends of recipient
molecule.
Text to be displayed
(if any)
(DT)
DT1
Non-replicative Transposon
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another location in the genome. “Cut and Paste”
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Non-Replicative transposon leaves its original place and move to the
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This type of mechanism requires only a transposase.
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The insertion elements and composite transposons like Tn5 and Tn10
use this mechanism.
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Non-replicative transposons leave a break in the donor molecule which is
lethal to the cell unless it is repaired.
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Non-replicative Transposon
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Description of
the action/
interactivity
Donor and recipient molecules
will appear first.
Transposon (Dark Pink) will be
removed from the donor
molecule and will be place at
the target site in the recipient
molecule.
Audio Narration
(if any)
In case of Non-replicative
transposition, transposon removes
from the donor molecule and it
integrates into the recipient
molecule.
The break generated in the donor
molecule can be lethal if it is not
repaired.
Text to be displayed
(if any)
(DT)
DT1
Questionnaire
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1.Transposons are
a.sequences of DNA that move to different positions within the genome of a single cell.
b.sequences of RNA that move to different positions within a cell.
c.sequences of DNA that move from one position to another within two cells.
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2. Tn 10 is a
a.non composite transposon b. a composite transposon c.an mobile genetic element
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3.Transposition is of type
a.replicative only b.non replicative only
c.replicative and non replicative
4. Mobile genetic elements found in bacteria include
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a.TMV,TMV,DHFR b. PCR, DNA,RNA c.Tn 3,Is 10,Tn 10
5.In replicative transposition
a. transposable element is duplicated during the reaction, so that the
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transposing entity is a copy of the original element.
b.Transposition of transposable elements occurs by , "cut-and-paste"
mechanism. c.none of the above.
Answers to the set of questions
1a
2b
3c
4c
5a
Links for further reading
Books

GENES VIII- Benjamin Lewin

Principles of GENETICS – 8th edition

Microbial genetics- David Freifelder
Summary
Transposons are sequences of DNA that can move around to different
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positions within the genome of a single cell.
Three types of bacterial transposons 1) IS elements, 2) Composite and 3)
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Non-composite.
Transposons involves three modes of transposition 1) Replicative, 2) Non-
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replicative and 3) Conservative