Download Transposable Elements

Transposable Elements
(Transposons)
• DNA elements capable of moving ("transposing")
about the genome
• Discovered by Barbara McClintock, largely from
cytogenetic studies in maize, but since found
in most organisms
• She was studying "variegation" or sectoring in
leaves and seeds
• She liked to call them "controlling elements“
because they affected gene expression in
myriad ways
1. Nobelprize.org
(1983 Nobel Prize in
Physiology and Medicine)
2. profiles.nlm.nih.gov/LL/
Corn (maize) varieties
Barbara McClintock 1902-1992
Corn evolution in 7000 yrs of domestication
cob of wild teosinte
cob of Hopi Blue corn
Maize (domesticated corn) kernel
structure
Mutant Kernel Phenotypes
1. Pigmentation mutants
– affect anthocyanin pathway
– elements jump in/out of transcription
factor genes (C or R)
– sectoring phenotype - somatic mutations
– whole kernel effected - germ line
mutation
2.
Starch synthesis mutants
- stain starch with iodine, see sectoring in
endosperm
Some maize phenotypes caused by transposable
elements excising in somatic tissues.
Start with lines that produce kernels defective in starch synthesis
(endosperm phenotypes) or anthocyanin synthesis (aleurone and
pericarp phenotypes) because of an inserted element, and the
element excises during development.
Somatic Excision of Ds from C
Wild type
Fig. 23.9
Mutant
Sectoring
Other Characteristics of McClintock's
Elements
• Unstable mutations that revert frequently but often
partially, giving new phenotypes.
• Some elements (e.g., Ds) correlated with
chromosome breaks.
• Elements often move during meiosis and
mitosis.
• Element movement accelerated by genome
damage.
Molecular Analysis of Transposons
• Transposons isolated by first cloning a gene that
they invaded. A number have been cloned this way,
via "Transposon trapping“.
• Some common molecular features:
– Exist as multiple copies in the genome
– Insertion site of element does not have extensive
homology to the transposon
– Termini are an inverted repeat
– Encode “transposases” that promote movement
– A short, direct repeat of genomic DNA often
flanks the transposon : “Footprint”
Ac and Ds
•
•
•
•
Ds is derived from Ac by internal deletions
Ds is not autonomous, requires Ac to move
Element termini are an imperfect IR
Ac encodes a protein that promotes
movement - Transposase
• Transposase excises element at IR, and also
cuts the target
Structure of Ac and Ds deletion
derivatives
Ds is not autonomous, requires Ac to move!
Fig. 23.10
How duplications
in the target site
probably occur.
Duplication
remains when
element excises,
thus the
Footprint.
Fig. 23.2
Mu/MuDR (Mutator)
• Discovered in maize; differs significantly
from Ac and En/Spm families
• Autonomous and non-autonomous
versions; many copies per cell
• Contain a long TIR (~200 bp)
• Transpose via a gain/loss (somatic
cells) or a replicative (germline cells)
mechanism.
Structure of MuDR
(autonomous Mu)
and its promoters.
• MuDrA and B
expressed at high
levels in dividing cells
and pollen, because of
transcriptional
enhancers.
• MURA is transposase
& has NLS.
• MURB needed for
insertion in somatic
cells.
RetroTransposons
Can reach
high numbers
in the
genome
because of
replicative
movement.
Fig. 7.34 in Buchanan et al.
Control of Transposons
• Autoregulation: Some transposases are
transcriptional repressors of their
own promoter(s)
• e.g., TpnA of the Spm element
• Transcriptional silencing: mechanism
not well understood but correlates
with methylation of the promoter
(also methylation of the IRs)
Biological Significance of
Transposons
• They provide a means for genomic change
and variation, particularly in response to
stress (McClintock’s "stress" hypothesis)
(1983 Nobel lecture, Science 226:792)
• or just "selfish DNA"?
• No known examples of an element playing a
normal role in development.