Presentation contents: Discovery and Definition of Transposon, Simple transposon and Composite transposons. Here I have explained Barbara McClintock Study of Transposable elements in Corn(Ac and Ds elements). After that Types of Transposable Elements. Then In Simple Transposons or IS elements introduction and how they mediate recombination between two different Plasmids. Introduction of Composite Transposons and their organization (Tn 5 and Tn 10). Introduction of Non-Composite Transposons(Tn 3) and Replicative Transposons.

1. Introduction of Transposable
elements
By Omedul Mondal
MSc Applied Genetics
1st Semester

2. Outline
 Discovery and definition of transposons
Simple Transposons(IS elements)
Composite transposons(Tn10,Tn5)

3. In 1929 Rollins.A Emerson and his maize genetic
group at Cornell University in Ithaca, New York.
Barbara
McClintock

4. Tremendous genetic diversity… due in part to
a remarkable dynamic genome…

5. Why are these kernels spotted?

6. McClintock’s Study of Transposable Elements in
Corn
• In the 1940s and 1950s, Barbara McClintock did a series of elegant
genetic experiments with Zea mays (corn) that led her to hypothesize
the existence of what she called “controlling elements,” which modify
or suppress gene activity in corn and are mobile in the genome.
• McClintock was awarded the 1983 Nobel Prize in Physiology or
Medicine for her “discovery of mobile genetic elements”, i.e
Transposable Elements in corn kernel pigmentation.

7. Corn kernels, some of which show spots of pigment produced by cells in which a transposable element had
transposed out of a pigment-producing gene, thereby allowing the gene’s function to be restored. The cells in the
white areas of the kernel lack pigment because a pigment-producing gene continues to be inactivated by the
presence of a transposable element within that gene

8. Kernel color and transposable element effects in corn. (a) Purple kernels result from the active C gene. (b) Colorless kernels can
result when the Ac transposable element activates Ds transposition and Ds inserts into C, producing a mutation. (c) Spotted kernels
result from reversion of the c mutation during kernel development when Ac activates Ds transposition out of the C gene.

9. • If the corn plant carries a wild-type “C” gene, the kernel is purple.
• If the corn plant carries “c” (colorless) mutations are defective in purple pigment
production, so the kernel is colorless.
• During kernel development, revertants of the mutation occur, leading to a spot of
purple pigment.
• McClintock determined that the original c (colorless) mutation resulted from a
“mobile controlling element” (in modern terms, a transposable element), called Ds
for “dissociation,” being inserted into the C gene
• Another mobile controlling element, an autonomous element called Ac for
“activator,” is required for transposition of Ds into the gene. Ac can also result in Ds
transposing (excising perfectly in this case) out of the c gene, giving a wild-type
revertant with a purple spot.
• The remarkable fact of McClintock’s conclusion was that, at the time, there was no
precedent for the existence of transposable genetic elements. Rather, the genome
was thought to be static with regard to gene locations. Only much more recently
have transposable genetic elements been widely identified and studied, and only in
1983 was direct evidence obtained for the movable genetic elements proposed by
McClintock.

10. McClintock was ahead of her time
1950- TE discovered in Drosophila
1960- TE discovered in bacteria(E.coli)
1980- Electrophoresis
1953- Watson & Crick DNA Double helix Model
1977- Full DNA genome was sequenced in Bacteriophage
1955- Sanger Sequencing
1973- Maxam & Gilbert DNA sequencing
1983- McClintock Won the Nobel Prize

11. Transposons
• Modern research has shown that the stripes and spots on maize kernels are
the result of a genetic phenomenon called transposition.
• Within the maize genome— indeed, within the genomes of most organisms—
geneticists have found DNA sequences that can move from one position to
another.
• These transposable elements—or, more simply, transposons— constitute an
appreciable fraction of the genome.
• In maize, for example, they account for 85 percent of all the DNA. When
transposable elements move from one location to another, they may break
chromosomes or mutate genes.
• Transposons account for 40 percent of the human genome—and they clearly
have roles in shaping the structure of chromosomes and in modulating the
expression of genes.

12. Types Of Transposons
Cut-and-Paste Transposons. Replicative Transposons. Retrotransposons.

13. • Cut and Paste Transposons:- transposition is accomplished by excising an
element from its position in a chromosome and inserting it into another
position. The excision and insertion events are catalyzed by an enzyme called
the transposase, which is usually encoded by the element itself. Geneticists
refer to this mechanism as cut-and-paste transposition because the element
is physically cut out of one site in a chromosome and pasted into a new site,
which may even be on a different chromosome.
• Replicative Transposons:- transposition is accomplished through a process
that involves replication of the transposable element’s DNA. A transposase
encoded by the element mediates an interaction between the element and a
potential insertion site. During this interaction, the element is replicated, and
one copy of it is inserted at the new site; one copy also remains at the
original site. Because there is a net gain of one copy of the element,
geneticists refer to this mechanism as replicative transposition.

14. • Retrotransposons:-transposition is accomplished through a process that
involves the insertion of copies of an element that were synthesized from
the element’s RNA. An enzyme called reverse transcriptase uses the
element’s RNA as a template to synthesize DNA molecules, which are then
inserted into new chromosomal sites. Because this mechanism reverses the
usual direction in which genetic information flows in cells—that is, it flows
from RNA to DNA instead of from DNA to RNA geneticists refer to it as
retrotransposition. Some of the elements that transpose in this way are
related to a special group of viruses that utilize reverse transcriptase—the
retroviruses; consequently, they are called retroviruslike elements. Other
elements that engage in retrotransposition are simply called retroposons.
• Cut-and-paste transposons are found in both prokaryotes and eukaryotes.
Replicative transposons are found only in prokaryotes, and the
Retrotransposons are found only in eukaryotes.

16. Simple Bacterial Transposons(IS elements)
• Insertion sequence (IS), or IS element, is the simplest transposable element
found in bacteria.
• contains only genes required to mobilize the element and insert it into a new
location in the genome. IS elements are normal constituents of bacterial
chromosomes and plasmids.
• IS elements were first identified in E. coli as a result of their effects on the
expression of three genes that control the metabolism of the sugar galactose.
• Compactly organized,consist of fewer than 2500 nucleotide pairs and contain
only genes whose products are involved in promoting or regulating
transposition
• E. coli contains a number of IS elements (e.g., IS1, IS2, and IS10R), each
present in up to 30 copies per genome and each with a characteristic length
and unique nucleotide sequence. IS1 , for instance, is 768 bp long and is
present in 4 to 19 copies on the E. coli chromosome.

17. • All IS elements end with perfect or nearly perfect terminal inverted repeats
(IRs) of 9 to 41 bp. This means that essentially the same sequence is found at
each end of an IS, but in opposite orientations. The inverted repeats of IS1 are
23 bp long.
• IS elements usually encode a protein, the transposase, that is needed for
transposition. The transposase binds at or near the ends of the element and
then cuts both strands of the DNA. This cleavage excises the element from the
chromosome or plasmid, so that it can be inserted at a new position in the
same or a different DNA molecule. IS elements are therefore cut-and-paste
transposons.

19. • When IS elements insert into chromosomes or plasmids, they create a
duplication of part of the DNA sequence at the site of the insertion. One
copy of the duplication is located on each side of the element. These
short (2 to 13 nucleotide pairs), directly repeated sequences, called target
site duplications, arise from staggered cleavage of the double-stranded
DNA molecule.
• IS element inserts into a new location in a chromosome. Insertion takes
place at a target site with which the element has no sequence homology.
First, a staggered cut is made in the target site and the IS element is then
inserted, becoming joined to the single-stranded ends. DNA polymerase
and DNA ligase fill in the gaps, producing an integrated IS element with
two direct repeats of the target-site sequence flanking the IS element. In
this case, direct means that the two sequences are repeated in the same
orientation. The direct repeats are called target-site duplications. Their
size is specific to the IS element, but they tend to be small (4 to 13 bp).

20. IS elements may also mediate recombination
between two different plasmids….

21. • The F plasmid, for example, typically has at least two different IS elements, IS2
and IS3. When a particular IS element resides in two different DNA molecules, it
creates the opportunity for homologous recombination between them. For
instance, an IS element in the F plasmid may pair and recombine with the same
kind of IS element in the E. coli chromosome. Both the E. coli chromosome and
the F plasmid are circular DNA molecules. When an IS element mediates
recombination between these molecules, the smaller plasmid is integrated into
the larger chromosome, creating a single circular molecule.
• Where a plasmid that carries a gene for resistance to the antibiotic
streptomycin (str r ) recombines with a plasmid that can be transferred
between cells during conjugation (a conjugative plasmid).
• Plasmids that transfer genes for antibiotic resistance between cells are called
conjugative R plasmids. These plasmids have two components: the resistance
transfer factor, or RTF, which contains the genes needed for conjugative transfer
between cells, and the R-determinant, which contains the gene or genes for
antibiotic resistance.

22. • Conjugative R plasmids carry several different antibiotic resistance
genes. These plasmids are formed by the successive integration of
resistance genes through IS-mediated recombination events. The
evolution of multiple drug resistance has occurred in several species
pathogenic to humans, including strains of Staphylococcus,
Enterococcus, Neisseria, Shigella, and Salmonella. Today many
bacterial infections causing diseases such as dysentery, tuberculosis,
and gonorrhea are difficult to treat because the pathogen has
acquired resistance to several different antibiotics.

23. Composite Transposons
• Composite transposons are created when two IS elements insert near each other.
• Region between the two IS elements can then be transposed when the elements act
jointly. In effect, the two IS elements “capture” a DNA sequence that is otherwise
immobile and endow it with the ability to move.
• E.g. Tn9,Tn5,Tn10
• Tn10, are complex transposons with a central region containing genes (for example,
genes that confer resistance to antibiotics), flanked on both sides by IS elements (also
called IS modules). Composite transposons may be thousands of base pairs long. The
IS elements are both of the same type and are called ISL (for “left”) and ISR (for
“right”). Depending on the transposon, ISL and ISR may be in the same or inverted
orientation relative to each other.
• In Tn5, the element on the right, called IS50R, is capable of producing a transposase
to stimulate transposition, but the element on the left, called IS50L, is not due to
change in a single nucleotide pair that prevents IS50L from encoding the active
transposase.

24. Genetic organization of composite transposons_ The orientation and length (in nucleotide pairs, np) of the
constituent sequences are indicated. (a) Tn9 consists of two IS1 elements flanking a gene for chloramphenicol
resistance. (b) Tn5 consists of two IS50 elements flanking genes for kanamycin, bleomycin, and streptomycin
resistance. (c) Tn10 consists of two IS10 elements flanking a gene for tetracycline resistance

25. Non Composite Transposons
• Noncomposite transposons , exemplified by Tn3, also contain genes such as
those conferring resistance to antibiotics, but they do not terminate with IS
elements. However, at their ends they have inverted repeated sequences(38 to
40 nucleotide pairs long) that are required for transposition. Enzymes for
transposition are encoded by genes in the central region of noncomposite
transposons.
• Genetic organization of Tn3, has three genes, tnpA, tnpR, and bla, encoding,
respectively, a transposase, a resolvase/repressor, and an enzyme called beta
lactamase. The beta lactamase confers resistance to the antibiotic ampicillin,
and the other two proteins play important roles in transposition.

27. Replicative Transposition

28. References
• Russel J Peter ;I Genetics Molecular Approach; 3rd Edition ; Benjamin
Cummings;
• Snustad , Simmions; Principle of Genetics; Sixth Edition; John Wiley
and Sons