Bacterial Transposons

AuthorMeenakshi Agarwal, Mehta Gunjan

MentorDr. Santanu Ghosh

Master Layout (Part 1)

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1 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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1 Part1- Definition of Transposons Alternate nomenclature:

1) Transposable elements

2) Jumping genes

3) Mobile DNA

Discovery: First discovered by Barbara McClintock in 1950’s.

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

transposition.

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1Keywords

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

sequences present in reverse orientation at the ends of some

transposons.

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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1 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.

5) Resolvase: is the enzyme activity involved in site-specific

recombination between two transposons present as direct

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

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

transposition.

Inverted repeats + transposase genes

Step 1: 1

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Structure of IS elements

23 Transposase

protein

1st yellow box will appear with lable. 2) Arrow from box and transposase Protein structure will appear. 3)Two red boxes(IR) will appear on both ends. 4) sequences with blue boxes will appear. 5) in last Insertion sequence, IS1 (text)will appear.

IS elements are simplestTransposons and containsInverted repeats and tranposase genes,codes proteins for transposition protein

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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

e.g. Tn5, Tn9, Tn10.

Two IS elements + antibiotic resistance gene(s).

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Structure of Composite transposons

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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

antibiotic resistance genes

Inverted RepeatsGenes for Transposition

Inverted IS

Structural genesInverted Repeats

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

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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.

Inverted repeats + transposase gene + antibiotic resistance

gene(s).

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Structure of Non-composite transposons

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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

This is the example of Non-composite transposon- Tn3.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

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Part 3- Mechanism

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.

The order of events and exact nature of the connections between

transposon and target DNA determine whether transposition is

replicative or nonreplicative.

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Replicative Transposon• Replicative transposon is first replicated and then one of the copy

will move to the another location in the genome. Thus, the

transposon will remain on its original position. “Copy and Paste”

• Replicative transposition involves two types of enzymatic

activity: • Transposase that acts on the ends of the original

transposon; and Resolvase that acts on the duplicated

copies.

• A group of transposons related to TnA move only by

replicative transposition.

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Replicative Transposon

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interactivityFirst 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.

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.

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Replicative Transposon

• Replicative transposition occurs through a cointegrate formation.

• 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

repeats.

• Resolution occurs by a homologous recombination reaction between the two

copies of the transposon in a cointegrate.

• The reaction generates the donor and target replicons, each with a copy of

the transposon.

• Resolvase is the enzyme activity involved in site-specific recombination

between two transposons present as direct repeats in a cointegrate structure.

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Replicative Transposon• The reactions involved in generating a cointegrate have been defined in

detail for phage Mu.

• The process starts with the formation of the strand transfer complex

(sometimes also called a crossover complex).

• 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.

• 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

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/

interactivityAs shown in figure, one by one each step will appear along with text.

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.The remaining single stranded portion is replicated using the 3’-OH end as a primer.Thus, the cointegrate structure is formed in which two copies of the transposons are present at the junction between donor and recipient molecules.Resolvase will carry out site-specific recombination between the direct repeats present on the transposons in a cointegrate structure.

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Non-replicative Transposon

• Non-Replicative transposon leaves its original place and move to the

another location in the genome. “Cut and Paste”

• This type of mechanism requires only a transposase.

• The insertion elements and composite transposons like Tn5 and Tn10

use this mechanism.

• 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/

interactivityDonor 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.

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.

Questionnaire1

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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.

2. Tn 10 is a

a.non composite transposon b. a composite transposon c.an mobile genetic element

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

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

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 questions1 a2 b3 c4 c5 a

Links for further reading

Books

GENES VIII- Benjamin Lewin

Principles of GENETICS – 8th edition

Microbial genetics- David Freifelder

•Transposons are sequences of DNA that can move around to different

positions within the genome of a single cell.•Three types of bacterial transposons 1) IS elements, 2) Composite and 3)

Non-composite.•Transposons involves three modes of transposition 1) Replicative, 2) Non-

replicative and 3) Conservative

Summary