Bacterial Transposons

1. BACTERIAL TRANSPOSONSBACTERIAL TRANSPOSONS
S.FEROSEKHANS.FEROSEKHAN
FNB-41FNB-41

2. TRANSPOSONS
•“Transposable elements”
• “Jumping genes”
•Mobile DNA
–able to move from one place to another within
a cell’s genome
–sometimes a copy is made and the copy
moves
–insertion requires target DNA sequences

3. Transposon

inverted terminal repeat (ITR)

4. •In the process, they may

- cause mutations.
- increase (or decrease) the amount
of DNA in the genome.
- promote genome rearrangements.
- regulate gene expression.
- induce chromosome
breakage and rearrangement.

5. Discovery of transposons
•Barbara McClintock 1950’s Ac Ds system in
maize influencing kernel color unstable elements
changing map position promote chromosomal
breaks.
•Rediscovery of bacterial insertion sequences
source of polar mutations discrete change in
physical length of DNA inverted repeat ends:
form “lollipops” in EM after denaturation.

6. These mobile segments of DNA are sometimes called
"jumping genes"
There are two distinct types of transposons:

1) DNA transposons
-transposons consisting only of DNA that moves
directly from place to place

2) Retrotransposons
- first transcribe the DNA into RNA and then
- use reverse transcriptase to make a DNA copy
of the RNA to insert in a new location

7. Classification of Transposons into two classes
In both cases ds
DNA intermediate
is integrated into
the target site in
DNA to complete
movement

8. BACTERIAL TRANSPOSONS
● In bacteria, transposons can jump from
chromosomal DNA to plasmid DNA and back.
·

·
● Transposons in bacteria usually carry an
additional gene for function other than
transposition---often for antibiotic resistance.
·

·
● Bacterial transposons of this type belong to
the Tn family. When the transposable elements
lack additional genes, they are known as
insertion sequences.

9. BACTERIAL TRANSPOSONS - TYPES
1. Insertion sequence
2.Composite transposon
3.Tn3-type transposon
4.Transposable phage

10. 1.Insertion sequences
Insertion sequences – IS1 and IS186, present in the 50-
kb segment of the E. coli DNA, are examples of DNA
transposons.

Single E. coli genome may contain 20 of them.

Most of the sequence is taken by one or two genes for
transposase enzyme that catalyses transposition.
IS elements transpose either replicatively or
conservatively.

11. cont….
IS elements

Study of E. coli mutations resulting from insertion of 1-2 kb long
sequence in the middle of certain genes.
Inserted stretches or insertion sequences – could be visualized by EM.
IS - molecular parasites in bacterial cells.
Transposition of IS is very rare – one in 105-107 cells per generation.
Higher rates result in greater mutation rates.

12. Bacterial IS element
Central region encodes for one or two enzymes required for transposition. It is flanked by
inverted repeats of characteristic sequence.
The 5’ and 3’ short direct repeats are generated from the target-site DNA during the insertion of
mobile element.
The length of these repeats is constant for a given IS element, but their sequence depends upon
the site of insertion and is not characteristic for the IS element.
Arrows indicate orientation.

13. Insertion sequences in E.coli
1 or more1426IS4
1 or more1300IS3
51327IS2
8768IS1
No.of.copies/
genome
Size (bp)Elements

15. 2.Composite transposons
Bacteria contain composite mobile genetic elements
that are larger than IS elements and contain one or more
protein-coding genes in addition to those required for
transposition:
Composite transposons - are basically the pair of IS
elements flanking a segment of DNA usually containing
one or more genes, often coding for AB resistance.

They use conservative method of transposition.

16. Cont…
2.Composite transposon
- Antibiotic resistant gene
- Flank by IS element
(inverted or directed repeat)
- Terminal IS can transpose by in self
Ex. Tn5, Tn9, Tn10

18. 3. Tn 3 transposon family
- 5000 bp
- code for Transposase, β-lactamase,
Resolvase
- Function of resolvase
Decrease Transposase production
Catalyse the recombination of
transposon

19. Cont…
•Tn3 – type transposon --- 5kb
•ITR - inverted terminal repeat
•β- lactamase – Resistance gene
ITR
ITR
resolvasetransposase β-lactamase

21. 4.Transposable phage
•Transposable phages – bacterial
viruses which tranpose replicatively as
a part of their normal infectious cycle.
• Integrate into E. coli chromosome at
regulatory element
•Eg. Mu phage

22. Transposable phage
•Transposable phage – 38kb
•ITR - inverted terminal repeats
ITR
ITR
Lysis genes
Integration and
Replication genes
Protein coat
genes

23. Transposable phage - Mu phage

24. Mechanism of transposition
Two distinct mechanisms of transposition:

Replicative transposition – direct interaction
between the donor transposon and the target
site, resulting in copying of the donor element

Conservative transposition – involving
excision of the element and reintegration at a
new site.

25. Mechanism of transposition
1. Replicative transposition

Copy of transposon sequence
Transposase enzyme cut target DNA


Transposition
Duplication of target sequence

26. Replicative transposition

27. 2. Non-replicative (conservative)
transposition
- Cannot copy transposon sequence
- Transposition by cut and paste model
Cut transposon sequence from
donor molecule
attach to target site
Ex. IS10, Tn10

28. Non-replicative (conservative) transposition

29. Mechanism of transposition

30. Evolution of Transposons
•Transposons are found in all major
branches of life.
•It arisen once and then spread to other
kingdoms by horizontal gene transfer.
•Duplications and DNA rearrangements
contributed greatly to the evolution of new
genes.

31. Cont…
•Mobile DNA most likely also influenced the
evolution of genes that contain multiple copies of
similar exons encoding similar protein domains
(e.g., the fibronectin gene).
•The evolution of an enormous variety of
antibiotic resistance transposons and their
spread among bacterial species.
example of genetic adaptation via natural
selection.

32. Transposons causing diseases
•Transposons are mutagens. They can damage the genome of their
host cell in different ways:

1. A transposon or a retroposon that inserts itself into a functional
gene will most likely disable that gene.
2.After a transposon leaves a gene, the resulting gap will probably
not be repaired correctly.
3.Multiple copies of the same sequence, such as Alu sequences
can hinder precise chromosomal pairing during mitosis and meiosis,
resulting in unequal crossovers, one of the main reasons for
chromosome duplication.

33. Cont…
•Diseases caused by transposons include

-hemophilia A and B
-severe combined immunodeficiency
-Porphyria
-Cancer
-Duchenne muscular dystrophy

34. Applications
•The first transposon was discovered in the plant maize
(Zea mays, corn species), and is named dissociator
(Ds).
•Likewise, the first transposon to be molecularly isolated
was from a plant (Snapdragon).
•Transposons have been an especially useful tool in plant
molecular biology.
•Researchers use transposons as a means of
mutagenesis.

35. Cont…
•To identifying the mutant allele.
• To study the chemical mutagenesis methods.
•To study gene expression.
•Transposons are also a widely used tool for
mutagenesis of most experimentally tractable
organisms.

36. QUERIES ?