Transposable Elements

Manifestations of transposable elements in the garden QuickTimeª and a TIFF (Uncompressed) decompressor are needed to see this picture.

•Variegated phenotypes first characterized in maize by Barbara McClintock in the 1940’s as “unstable genetic loci” •Her findings proved not only to be true, but also a

Genetics meets genomics

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• Transposons are found in all eukaryotes and prokaryotes studied to date • Genome-sequencing projects reveal that transposons contribute significantly to nucleotide content of many eukaryotic genomes • 45% of human genome • 50-80% of some large plant genomes (e.g. maize and barley)

• Mutable nature of transposons recognized as a driving force of evolution

Types of transposons

•Class I Elements: Retrotransposons Integrated RNA POL II RNA retroelement intermediate

RT

cDNAINTEGRASE Integration to copy a new site

ass II Elements: “cut and paste” DNA-type transposon TRANSPOSASE

Integrated DNA element

Excision from donor site

TRANSPOSASE

Integration to a new site

Basic structure of transposons • Transposons are mobile genetic elements capable of jumping around the genome • Terminal repeats (TR, TIR) • Subterminal regions

• Autonomous

• Encode functional transposase which catalyzes excision and reinsertion activity

• Non-autonomous

• Internally deleted elements • Activated in trans by autonomous element transposase

Transposase

Trse

DNA-type transposons move via a “cut and paste” mechanism Transposase

Donor site Transposon

Transposase binds to subtermin regions of transposon

Transposase dimerization brings transposon ends together

Transposase clips transposon from donor locus

Transpososome attacks DNA at new target site

New insertion results in duplication of target site sequence

Transposon regulation • Because of mutagenic nature, survival of the host depends on keeping transposition under control • Many elements are reversibly inactivated • Silenced by various host mechanisms including methylation, chromatin modification, and RNAi

• Active elements are strictly regulated by the host • Various strategies to minimize damage to host • Host-directed alternative splicing of Drosophila P-element restricts transposition to germline