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1. “TRANSPOSONS”

by:-
Vivek kumar
M.Sc Microbiology
Bangalore University

2. INTRODUCTION

+ Transposons are DNA segments found in the
genomes of living organism .

+ Transposons are capable of moving from
place to another place within a genome.

+ It does not utilize an independent form of
element like phage or plasmid but directly
hops from one site to another.

3. + They carry genes such as resistance to

anti-bacterial, virulence and symbiotic
factors and catabolism of xenobiotics.

« Transposons generate a variety of DNA

e

rearrangements that have profound
consequences for cell survival and
evolution.

+ Transposons are diverse in structure,

highly regulated by a multitude of
mechanism.

4. « They are found to encode a special
protein named transposase which
catalyses the process of transposition.

+ Transposons are particular to different
groups of organism.

+ Transposons have a major impact on
changing the genetic composition of
organism.

5. _DEFINITION OF TRANSPOSONS ____

+ “A DNA sequence that is able to move or
insert itself at a new location in the genome.”

« Transposons are small, mobile DNA
sequences that move around chromosomes
with no regard for homology and insertion of
the transposons may produce deletions,
inversions, chromosomal fusions and even
more complicated rearrangement.

6. VARIOUS NAMES OF TRANSPOSONS ___

Transposons have been variously called —

+ Jumping genes

+ Mobile elements

« Cassettes

+ Insertion sequence

+ Transposable elements

7. DISCOVERY OF JUMPING GENES

+ Transposable elements were discovered by Barbara
Mcclintock in 1965, through an analysis of genetic instability
in maize.

+ The instability involved the breakage of the chromosome
and was found to occur at sites where transposable

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8. CHARACTERISTICS OF TRANSPOSABLE
ELEMENT

+ They are DNA sequence that code for enzymes
which bring about the insertion of an identical
copy of themselves into a new DNA site.

+ Transposition events involve both recombination
and replication processes which frequently
generate two daughter copies of the original
transposable elements. one copy remains at the
parent site while others appears at the target
site.

9. + The insertion of transposable elements invariably
disrupts the integrity of their target genes.

+ Transposable elements carry signals for the
initiation of RNA synthesis, they sometimes
activate previously dormant genes.

+ Atransposon is not a replicon, thus it cannot
replicate apart from the host chromosome the way
that plasmid and phage can.

+ No homology exists between the transposons and
the target site for its insertion.

10. TYPES OF TRANSPOSONS

+ On the basis of their transposition
mechanism, Transposons may be

categorized int-- Cut and paste
« transposons
* replicative
* transposons

¢ retro
+ elements

11. CUT AND PASTE TRANSPOSONS ____

+ They transpose by cutting of the transposable
sequence from one position in the genome and
its insertion to another position within the
genome.

+ The cut and paste transposition involves two
transposase subunits. Each transposase
subunits binds to the specific sequences at the
two ends of transposons. These subunit of
transposase protein then come together and
lead to the excision of transposon.

12. + The excised ‘transposon-transposase complex’
then gets integrated to the target recipient site.
In this manner, the transposons is cut from one
site by a mechanism mediated by transposase
protein.

+ Examples of cut and paste type of transposons
are IS-elements, P-elements, in maize, hobo-
elements in drosophila etc.

13. Donor DNA Tn$ Transposon DNA Donor DNA
U A ES
+ Transposase Binding

Synaptic Complex
Formation

Cleavage

Target DNA
Target capture

Strand transfer

Bei
_®_
wen

14. Repeating Transposon ie
Donor DNA se pi Sequence

Transposase
ransposase Enzym; ‘binds

—_——

Transposition Complex
Formation

y

Excision

y

Target Site Recognized
Target DNA

a]
Target Site
y Transposon is inserted into target DNA

Gee

15. REPLICATIVE TRANSPOSONS __

+ They transpose by a mechanism which involves
replication of transposable sequence and this
copy of DNA, so formed, is inserted into the
target site while the donor site remains
unchanged.

+ In this type of transposition, there is a gain of
one copy of transposon and both the donor and
the recipient DNA molecule are having one-one
transposable sequence each, after
transposition.

+ Example- Tn3-elements.

16. slit

t — —
Cy) Transposition Resolution O)
—— mm
=

(a)

HR
1)

/

Cointegrate

H

Transposition
+ +

Discontinuous ‘cointegrate’

== Donor replicon == Recipient replicon === Transposable element

17. RETRO ELEMENTS

+ Their transposition is accomplished through a
process which involves the synthesis of DNA by
reverse transcription by using elements RNA as
the template.

+ This type of transposition involves an RNA
intermediate, the transposable DNA is
transcribed to produce an RNA molecule.

+ This RNA is then used as a template for
producing a complementary DNA by the activity
of enzyme reverse transcriptase.

18. « The single stranded DNA copy so formed, is
then made double stranded and then
inserted into the target DNA site.

« The transposable elements which requires
reverse transcriptase for their movement are
called retro transposons.

+ The retro elements may be viral or non-viral.
Out of these two, non-viral retro elements are
important and may further be classified as :

19. RUVIRU

+ Retrovirus like elements carry long terminal
repeats(LTR). Examples are copia, gypsy
elements in drosophila.

RETROPOSONS

+ In retroposon LTR are absent. Examples are
LINEs and SINEs in human.

20. RETROTRANSPOSITION

Retrotransposon
TE target site
cr — ——
} transcription
m RNA

} reverse transcription

rn CDNA

hp u
donor site target site

21. TRANSPOSABLE ELEMENTS IN PROKARYOTES _

+ Bacterial transposable elements are of
following types :-

Insertion sequences or IS elements
Prokaryotic transposon element

22. INSERTION SEQUENCE OR IS ELEMENTS

+ They are the transposable sequence which can
insert at different sites in the bacterial
chromosome.

« IS elements contains ITRs(inverted terminal
repeats), these were first observed in E.coli.

+ IS-elements are relatively short usually not
exceeding 2500 bp.

+ The IRTs present at the ends of IS-elements are
an important feature which enables their
mobility.

23. + The IRTs present in the IS-elements of E.coli
usually range between 18-40 bp.

+ The term ‘Inverted Terminal Repeat’(ITR)
implies that the sequence at 5 end of one
strand is identical to the sequence at 5’ end of
the other strand but they run in inverse opposite
direction .

« In E.coli chromosome, a number of copies of
several IS-element like IS1, 152, IS3, IS4 and
IS5 are present.

24. Insertion sequences (IS) are the generic transposable element.
“They are present in large quantities in all bacterial chromosomes
(the number is variable).
«It is a defined sequence of DNA (700-3000 bp long)
> Has flanking inverted repeats and
> Has one or two genes that encodes a transposase
>- a protein involved in movement of this element from one location
to another.

*ORF encodes the transposase
«Inverted repeats are identical and

of variable length
CC «Different IS exist (IS1, IS2,
IR IR 1S50....)

«Function unknown?

A.S. HOZA

25. PROKARYOTIC TRANSPOSON ELEMENT

+ These are also called composite transposons
and are shown by the symbol ‘Tn’.

+ It is made up of two IS elements, one present at
each end of a DNA sequence, which contains
genes whose functions are not related to the
transposition process.

+ These transposons have been found to have
inverted repeats at the ends. the length of these
inverted repeats ranges from a few nucleotides
to about 1500 bp.

26. + It can be said that these are the large
transposons which are formed by capturing of
an immobile DNA sequence within two insertion
sequence thus enabling it to move.

+ Examples of such transposons include the
members of Tn series like, Tn1, Tn5, Tn9,
Tn10, etc.

27. Bacterial composite transposon

Genes for

Inverted transposition

Repeats

Structural genes

Inverted IS

28. EXAMPLES OF TRANSPOSONS

Tn3 transposons of E.coli

Tn3 has 4957 bp and contains three genes such as
tnp A, tnp R and bla,coding respectively for the
following proteins :

Transposase — having 1015 amino acids and

required for transposition.
2) Repressor(resolvase) — containing 185 amino
acids which regulates the transposase.
Beta-lactamase — enzyme which confers
resistance to the antibiotic ampicillin. On both
sides of the Tn3 is an inverted repeat of about
38 bp.

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30. BACTERIOPHAGE MU

The bacteriophage Mu is a temperate
bacteriophage having typical phage properties
and could be regarded as a giant transposon.
- Phage Mu was thought to behave in a unique
manner since it does not multiply in a lytic
way.

- It always starts its life cycle by inserting itself
into the E.coli chromosome at random
locations.

— Tha racııltinn nrnanhana nftan mi itatac tha

31. - Thus, phage Mu was originally known for its
mutagenic properties.

- During later part of phage life cycle, upon
receipt of an appropriate signal, the Mu
prophage somehow generates its
transposition, but also activates the gene
encoding for structural proteins which
packages its DNA.

- Mu phage is unique also in transposing much
often than other transposons, providing nearly
a hundred new copies of Mu during its hour-

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32. lac* gene ö

insertion of Mu

Mu DNA lac

Fig. 20.5. Insertion of phage Mu into a lac gene

renders the gene inactive (lac”).

33. SIGNIFICANCE OF TRANSPOSABLE ELEMENT

>» Transposons may change the structural and
functional characteristics of genome by
changing their position in the genome.

> Transposable element cause mutation by
insertion , deletion, etc.

- It makes positive contribution in evolution as
they have tremendous impact on the alteration
of genetic organisation of organisms.

34. > They are useful as cloning vectors also, in gene
cloning. For example, P-elements are
frequently used as vector for introducing
transgenes into drosophila.

> Transposons may also be used as genetic
marker while mapping the genomes.

» Transposon-mediated gene tagging is done for
searching and isolation of a particular gene.

35. REFERENCE

» PRESCOTT.HARLEY.KLEIN;SIXTH
EDITION INTERNATIONAL EDITION
2005;MICROBIOLOGY;PAGE NO-291-295.
» VERMA P.S.AGARWAL V.K; 2004; CELL
BIOLOGY,GENETICS,MOLECULAR
BIOLOGY,EVOLUTION AND
ECOLOGY;PAGE NO-254-260.

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