Chapter 11-b Transposon

Transposon• Transposable Elements: An Overview

• Transposable Elements in Bacteria

• Cut-and-paste Transposonin Eukaryotes

• Retroviruses and Retrotransposons

• Transposable Elements in Humans

Transposons

A transposable element moves from one DNA address to another

Originally discovered in maize, transposons have been found in all kinds of organismsBacteriaPlantsHumans (40%)

Transposition

Categorization of transposon

Although each kind of transposable elements has its own special characteristics, most of them can be classified into one of three categories based on how they transpose.

1. Cut-and-paster transposon

2. Replicative transposon

3. Retrotransposons

Cut-and-paster transposons Transposon is accomplished( 熟练的 ) 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( 转座酶 ),which is encoded by the elements itself.

Geneticists refer to this mechanism as cut-and-paster transposons ( 切粘转座子 ) because the elements is physically cut out of one site in a chromosome and paster into a new site, which may even be on a different chromosome.

Cut-and-paster transposons

Replicative Transposons( 复制转座子 )

Transposition is accomplished through a process that involves replication of the transposable element’s DNA.

A transposase encoded by the element mediated 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

Retrotransposons ( 反转录转座子 )

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.

It flows from RNA to DNA

Key Points -1

A cut-and-paster transposonis excised from one genomic position and inserted into another by an enzyme, the transposase, which is usually encoded by the transposon itself.

A replicative transposon is copied during the process of transposition.

A retrotransposon produced RNA molecules that are reverse-transcribed into DNA molecules; these DNA molecules are subsequently inserted into new genomic position.

Key Points -2

The cut-and-paster transposonsare found in both prokaryotes and eukaryotes( 切粘转座子真核和原核都有 ).

The replicative transposons are found only in prokaryotes( 复制转座子仅存在于原核 ).

Retrotransposonare found only in eukaryotes (反转座子仅存在于真核 ).

Transposition Mechanisms

Transposons are sometimes called “jumping genes”, DNA doesn’t always leave one place for another

When it does, nonreplicative transposition “Cut and paste” Both strands of original DNA move together

from 1 place to another without replicating Transposition frequently involves DNA replication

1 copy remains at original site New copy inserts at the new site Replicative transposition “Copy and paste”

Transposable Elements in Bacteria

Bacterial transposons move within and between the bacterial chromosome and various types of plasmids.

细菌转座子在细菌染色体内部或在细菌染色体和不同类型的质粒之间移动。

Main types of transposable elements

The insertion sequences( 插入序列 ), or IS elements, are simplest, containing only genes that encode proteins involved in transposition.

The composite transposons(Tn) 复合转座子 and Tn3 elementsare more complex, containing some genes that encode products unrelated to the transposition process.

IS Elements ( 插入序列元件 ) ISs are the simplest type of bacterial transposon. They contain only the elements necessary for their

own transpositon.Short inverted repeats at their endsAt least 2 genes coding for an enzyme, transposase

They consist of fewer than 2500 bp. The smallest, IS1, is 768 bp long.

Because these terminal sequences are always in inverted orientation with respect to each other, they called inverted terminal repeats( 反向末端重复 ).

IS Transposition

Structure of an inserted IS50 element showing its terminal inverted repeats and target site duplication.

The terminal inverted repeats are imperfect because the fourth nucleotide pair from each end is different.

Transposase

At least some IS elements encode a protein that is needed for transposition.

This protein, called transposase, seems to bind at or near the end of the element, where is cuts both strands of the DNA.

IS elements are therefore cut-and-paster transposons.

Production of target site duplications by the insertion of an IS element.

Composite Transposons

Composite transposons consists of two IS elements flanking a region that contains one or more genes for antibiotic resistance. 2.5 kb -20 kb

In effect, two IS elements “capture” a DNA sequence that is otherwise immobile and endow it with the ability to move ( 实际上是两个 IS 元件捕获了一个不能移动的 DNA 序列，并赋予了它移动的能力 ).

Composite Transposons -Tn5

Tn5 consists of two IS50 elements flanking genes for kanamycin( 卡那霉素 ), bleomycin( 博来霉素 ), and streptomycin resistance.

The flanking IS element is inverted

Composite Transposons - Tn9

•Tn9 consists of two IS1 elements flanking a gene for chloramphenicol( 氯霉素 ) resistance.

•In Tn9, the flanking IS elements are in the same orientation with respect to each other

Composite Transposons - Tn10

Tn10 consists of two IS10 elements flanking a gene for tetracycline resistance. The flanking IS element is inverted

Tn3 Elements

The elements in this group of transposons are larger than IS elements and usually contain genes that are not necessary for transposition—a feature that is also characteristic of composite transposons( 与复合转座子的相同点 ).

However, unlike the composite transposons, the Tn3 elements do not have IS elements at each of their ends.

Instead, the Tn3 elements have simple inverted repeats 30 to 40bp long at their termini.

The Tn3 elements also produce target site duplication when they insert into DNA ( 与复合转座子的相同点 ).

Genetic organization of Tn3 There are three genes, TrpA, TrpB, and bla, encoding,

respectively, a transposase, a resovase( 解离酶 )/repressor, and an enzyme called beta lactamase( 内酰胺酶 ) . The beta lactamase confers resistance to the antibiotic ampicillin, and the other two proteins play important roles in transposition.

Cut-and-paster Transposons in Eukaryotes

Transposable elements were discovered by analyzing genetic instabilities in maize; genetic analyses have also revealed transposable elements in Drosophilia.

There are many different types of transposons in eukaryotes. These elements vary in size, structure and behavior. Some are abundant in the genome, others rare.

a) 不依赖供体序列与靶位点间序列的同源性

b) 转座不是简单的转移，涉及转座子的复制

Hotspots ( 热点 )

Regional preference ( 在 3kb 区域内的随机插入 )

d) 某些转座因子（ Tn3 ）对同类转座因子的插入具有排他性

（免疫性）

e) 靶序列在转座因子两侧会形成正向重复

f) 转座因子的切除与转座将产生复杂的遗传学效应

The feature of Transposon-based DNA recombination

c) 转座插入的靶位点并非完全随机（插入专一型）

Types of transposons in eukaryotes

Ac and Ds Elements in Maize

P Elements and Hybrid Dysgenesis in Drosophila (P 元件和果蝇的杂交不育 )

Mariner, an Ancient and widespread Transposon ( 海员，古老而广泛的转座子 )

Eukaryotic TransposonsTransposons have powerful selective forces on

their sideTransposons carry genes that are an advantage

to their hostsTheir host can multiply at the expense of completing

organismsCan multiply the transposons along with rest of their

DNAIf transposons do not have host advantage, can

replicate themselves within their hosts

Ds and Ac of MaizeDs (dissociator) cannot transpose on its ownMust have help from an autonomous transposon,

Ac (for activator)Ac supplies transposaseDs is an Ac element with most of its middle removedDs needs

A pair of inverted terminal repeatsAdjacent short sequences that Ac transposase can

recognize

Structures of Ac and Ds

P Elements

The P-M system of hybrid dysgenesis in Drosophila is caused by conjunction of 2 factors: Transposable element (P) contributed by the male M cytoplasm contributed by the female allows transposition of

the P elementHybrid offspring of P males and M females suffer

multiple transpositions of P element

23.4 Retrotransposons

Retrotransposons replicate through an RNA intermediate

Retrotransposons resemble retrovirusesRetroviruses can cause tumors in vertebratesSome retroviruses cause diseases such as AIDS

Before studying retrotransposons, look at replication of the retroviruses

DNA RNA DNA

Retroviruses

Class of virus is named for its ability to make a DNA copy of its RNA genome

This reaction is the reverse of the transcription reaction – reverse transcription

Virus particles contain an enzyme that catalyzes reverse transcription reaction

Retrovirus Replication Viral genome is RNA, with

long terminal repeats at each end

Reverse transcriptase makes linear, ds-DNA copy of RNA

ds-DNA copy integrates back into host DNA = provirus

Host RNA polymerase II transcribes the provirus to genomic RNA

Viral RNA packaged into a virus particle

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