Transcript Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865

‘mobile’ DNA: transposable elements
Transposable elements

Discrete sequences in the genome that have
the ability to translocate or copy itself
across to other parts of the genome without
any requirement for sequence homology by
using a self-encoded recombinase called
transposase
Transposable elements move from
place to place in the genome

1930s Marcus Rhoades and 1950s
Barbara McClintock – transposable
elements in corn

1983 McClintock received Nobel
Prize

Found in all organisms

Most 50 – 10,000 bp

May be present hundreds of times in
a genome
TEs can generate mutations in adjacent genes
TEs in Maize
Fig 15.19 Genes VII by B. Lewin
Classes of transposable elements
Science 12 March 2004: Vol. 303. no. 5664, pp. 1626 - 1632
Common mechanism of transposition


Transposons encode transposases that
catalyse transposition events
Regulation of transposase expression
essential
Fig13.24a: Hartwell
Common mechanism of transposition
Common mechanism of transposition
2 sequential steps
Site specific cleavage of
DNA at the end of TE
Complex of transposasetranspososome
element ends
(transpososome)
brought to DNA target
where strand transfer is
carried out by covalent
joining of 3’end of TE to
target DNA
Common mechanism of transposition


transposase (blue) binds and assembles a paired end complex
(PEC) by dimerization, a process that might involve divalent
metal ions (Me2+).
PEC is then active for the cleavage reactions that remove
flanking donor DNA (thin black lines) and transfer of the
transposon ends into target DNA (black dotted line).
Trends in Microbiology 2005 Vol13(11) pp 543-549
Catalytic domain of transposase involved in a transphosphorylation
reaction that initiates DNA cleavage & strand transfer
Fig 15.14
Fig 15.10
GenesVII Lewin
How transposons move
Transposition can occur via
RNA intermediates
 Class I TEs –
Use a ‘copy & paste’
mechanism

DNA intermediates
Class II TEs
Use a ‘cut and paste’ mechanism
Generally short sequences
See interspersed repeats from the repetitive elements lecture
DNA intermediate
Class II TEs
IS elements and transposons
bounded by terminal inverted repeats (TIR)
Class II TEs
DNA intermediate
Prokaryotic IS elements (e.g. IS10, Ac/Ds, mariner)
encode only transposase sequences
eukaryotic transposons encode additional genes such
as antibiotic resistance genes
Some types of rearrangements mediated by DNA
transposons
Gene (2005)345 pp91-100
Class I TEs encode a reverse transcriptaselike enzyme
Retroposon
retrotransposon
Fig. 13.23 a
Retroposons are
structurally similar
to mRNA
Poly-A tail at 3’ end of
RNA-like DNA
strand
Retrotransposon are
structurally similar
to retroviruses and
are bound by long
terminal repeats
(LTR)
Long terminal repeat
(LTRs) oriented in
same direction on
either end of
element
Class 1 TEs
Retroposons
LTR retrotransposons
Transposons move in different
ways
Classified into 5 families on the basis of their
transposition pathways
1)
2)
3)
4)
5)
DDE-transposases
RT/En transposases
(reverse transcriptase/endonuclease)
Tyrosine (Y) transposases
Serine (S) transposases
Rolling circle (RC) or Y2 transposases
Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)
DDE-transposases
Contains invariant DDE motif
responsible for excision and
integration
DDE motif facilitates catalysis
by divalent metal ions
2 step catalysis occurs on
transpososome
Characterised by target
duplication, the length of
which is specific for each
transposon
Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)
RT/En transposases (reverse transcriptase/endonuclease)
Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)
Tyrosine (Y) transposases
Related to Y recombinases
Transposon is excised out
to generate a circular
intermediate
Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)
Serine (S) transposases
Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)
Rolling circle (RC) or Y2 transposases
Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)
Some transposons can encode
integrons

Integrons are assembly platforms — DNA
elements that acquire open reading frames
embedded in exogenous gene cassettes and convert
them to functional genes by ensuring their correct
expression.
e.g. bacterial Tn7 also encodes an integron — a
DNA segment containing several cassettes of
antibiotic-resistance genes. These cassettes can
undergo rearrangements in hosts that express a
related recombinase, leading to alternative
combinations of antibiotic-resistance genes.
Integrons
Mobile Integrons
Superintegrons
Mazel Nature Reviews Microbiology 4, 608–620 (August 2006)
References
1)
Chapter 9 pp 265-268
HMG 3 by Strachan and Read
2)
3)
Chapter 10: pp 339-348
Genetics from genes to genomes
by Hartwell et al (2/e)
Nature (2001) 409: pp 879-891