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1. Acquisition of new sequences- horizontal transfer of genetic material between genomes by extrachromosomal elements
2. Rearrangements of existing sequences- transfer within the genome
21.1 Introduction
Evolution of genomes via:
3
1. Acquisition of new sequences- horizontal transfer of genetic material between genomes by extra-chromosomal elements
a) Bacteria- plasmids move by conjugation (F plasmid, Hfr)
b) Phages – spread by infection (transduction)
c) Both can transfer host genome with its own replicon
4
2. Rearrangements of existing sequences- transfer within the genome
a) Unequal recombination- mispairing in homologous recombination
b) Nonreciprocal recombination- results in duplication of loci; one copy- original function, the other- evolves
c) Transposable elements (transposons)
5Figure 21.1
6
Transposons (transposable elements) definition
“discrete sequences in the genome that are mobile- able to transport themselves to other locations within the genome.”
7
Basic concept for Transposons
1. Move directly from one site in the genome to another (do not need other vectors)
2. not rely on any relationship between sequences at the donor and recipient sites- Non-homologous recombination.
3. Internal counterpart to vectors that move sequences between genomes (phages & plasmids). May provide a major source of mutations in the genome.
4. Two classes: a) DNA transposon; b) Retroviruses & retroposons
5. Transposons found in both prokaryotes and eukaryotes.
8
Bacterial vs. Eukaryotic Tranaposons
Bacterial transposons carry genes that transpose themselves.
Eukaryotes• Although many are defective (lost ability to
transpose independently) and rely on the enzymes from a few functional transposons.
• Many number and variety of transposons included.
9
Transposable elements can promote rearrangement of the genome directly or indirectly:
• directly (Tn itself) 1.cause deletions or inversions or 2.lead to movement of host sequences to new
locations.
• indirectly 1.serve as substrates for cellular recombination
systems as “portable regions of homology” 2.two copies-sometimes on different
chromosomes in eukaryotes- provide sites for reciprocal recombination.
3.These types of exchanges lead to deletions, insertions, inversions, or translocations.
10
Natural selection view for Transposons
Neither advantage nor disadvantage on the phenotype.
But the selfish DNA concerned only with its own propagation (parasite to the genome; yet-selective advantage)
Is an independent entity that residues in the genome.
11
Good comment for Transposons
• Any transposition event conferring a selective advantage, e.g., genetic rearrangement.
• It will lead to preferential survival
12
Other features of Transposon
1. Transposons are DNA elements that transpose to different places on the DNA.
2. Transposition is the movement of the transposon.
3. It requires special protein factors particularly to cut and ligate the DNA
transposase
4. NO homology is required between the transposon and the target sequence.
5. First identified in bacterial operon-spontaneous silencing.
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21.2 Insertion Sequences Are Simple Transposition Modules
• An insertion sequence is a transposon that codes for the enzyme(s) needed for transposition flanked by short inverted terminal repeats (IR, ITR).
• is closely related rather than identical
• at the ends of transposon
• are autonomous unit- sponsors its own transposition
• Recognition of the ends (IR)- critical in transposition; point mutations abort it
KEY CONCEPT
14
• IR (inverted terminal repeat)
• The target site is random, hotspots, or
preferred (bent DNA; consensus sequence; inactive region).
duplicated during insertion forms DR (direct repeat).
• The length of the DR is:– characteristic for any particular
transposonFigure 21.2
IRDR
KEY CONCEPT
KEY CONCEPT
15
Rate comparison
1. Rate of transposition:
~10-3-10-4 per element per generation
2. Rate of spontaneous mutation:
~10-5-10-7 per generation
3. Rate of reversion (by precise excision of the IS
element)
~10-6-10-10 per generation
~103 times less frequent than insertion
16
21.3 Composite Transposons Have IS Modules
• Composite transposons have a central region flanked by an IS element at each end.
• Central region carry other genes (antibiotic resistance or other markers).
• IS elements only carry enzymes needed for transposition (transposase, resolvase).
IS-L IS-R
KEY CONCEPT
17
• Either one or both of the IS elements of a composite transposon may transposition.
• A functional IS module can transpose either itself (Fig.21.2) or the entire transposon
• An active IS element at either end may also transpose independently.
Figure 21.3
DR
IR
NOT identical but close related
KEY CONCEPT
19Figure 21.04: IS elements can mobilize other sequences.
Inside-out
With marker high freq. under selection > IS10
Transposition frequency declines with distance between IS10.
length-depend factor determines the size of common composite Tn.
Mobility frequency: IS10 > Tn10
20
Summary on Transposons
1. The smallest transposons are called insertion sequences (IS elements).
2. IS elements have inverted repeats at either end and a transposase gene in between; they do NOT have any resistance or other markers.
3. Transpoase is responsible for: (1) creating a target site (random, or consensus seq); (2) recognize the ends (IS) of the transposon.
4. Two IS elements flanking a marker gene(s) form a composite transposon.
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21.4 Transposition Occurs by Both Replicative and Nonreplicative Mechanisms
Figure 21.5
ATGCATAC
GT
corrected
All transposons use a common mechanism
1
2
3
The stagger between the cuts determines the length of the direct repeats.
The target repeat is characteristic of each transposon; reflects the geometry of the cutting enzyme
KEY CONCEPT
22
• The order of events and exact nature of the connections between transposon and target DNA determine whether transposition is:– Replicative & Nonreplicative (cut-and-paste)
The use of staggered ends is common to all transpositions: three types• Replicative [R] TnA
• Nonreplicative [N] IS, Tn10 and Tn5
• Conservative [C] (nonreplicative)
• Some Tn use only one type• Others multiple types: R and N/C IS1, IS903, R or N/C phage Mu
KEY CONCEPT
23
Animation
24
Figure 21.6Replicative
Transposon is duplicated; a copy of the original element is made at a recipient site (TnA); donor keeps original copy
Transposition- an increase in the number of Tn copies
ENZs: transposase (acts on the ends of original Tn) and resolvase (acts on the duplicated copies)
25
Figure 21.7 Nonreplicative transpositionNonreplicative
a) Transposon moves from one site to another and is conserved; breaks in donor required host repair system.
b) IS, Tn10 and Tn5 use this mechanism; no Tn copy increase
c) ENZs: only transposase
26
Figure 21.08: Movement conserves bonds.
Conservative (nonreplicative)
a) Tn excised from donor and inserted in target – • every nucleotide bond is conserved like in lambda
integration (Site-Specific Recombination) • large elements (episomes?)
b) ENZs: transposase (related to integrase family)
P.851Epiosome: is a plasmid able to integrate into bacterial DNA
27
21.5 Transposons Cause Rearrangement of DNA
• Homologous recombination between multiple copies of a transposon causes rearrangement of host DNA. [deletion, inversion]
• Homologous recombination between the repeats of a transposon may lead to precise or imprecise excision.
KEY CONCEPT
28Figure 21.09: Direct repeats recombine to excise material.
Recombination using cellular enzymes
Reciprocal recombination between two copies of the transposon
Lost from the cell
Single copy on chromosome
IS1L IS1R
•recombination between two elements in same orientation (DR)
Tn inserts a copy at a second site near its original location
Step 1
Step 2
29
Figure 21.10: Inverted repeat recombination inverts material.
(A) (B)
(A)
(B)
(B) (A)
Excisions not supported by Tn’s:
Precise excision - removes transposon & one copy of duplicated sequence; rare Tn10= ~10-9; recombination between 5-9 bp duplicated target sequences
Imprecise excision - leaves a remnant of the transposon; Tn10= ~10-6.
sufficient to prevent target gene reactivation. recombination between 24 bp IS-modules of a composite Transposon
Recombination using cellular enzymes
not relies on Tn-coded function
P854
P859
•recombination between two elements in opposite orientation (IR)
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21.6 Common Intermediates for Transposition
• Transposition starts by forming a strand transfer complex.– The transposon is connected to the target site
through one strand at each end. (1 x 2)
KEY CONCEPT
Both replicative and non-replicative transposition use a common mechanism:
IS elements, prokaryotic & eukaryotic transposons, and bacteriophage Mu, retroviral DNA and the first stages of immunoglobulin recombination use the similar mechanism.
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2. Transposon nicked at both ends; target nicked at both strands
3. Nicked ends joined crosswise; covalent connection between the transposon and the target
1. Synapsis stage- two ends of transposon are brought together [shown after cleavage but actually occurs previously]
Joining transposon to its target- common pathway
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• The Mu transposase forms the complex by:
1. synapsing the ends of Mu DNA
2. followed by nicking
3. then a strand transfer RX
Figure 21.12
KEY CONCEPT
1
2
3
Replicative transposition follows if the complex is replicated.
Nonreplicative transposition follows if it is repaired.
Animation show the detail
Next step differs and determines the type of transposition:
MuAtetramer
33
Figure 21.12
1
2
A Mu transposon passes through 3 stable stages:
- MuA binds to ends as tetramer forming a synapsis.
- MuA subunits act in trans to cut next to R1 and L1 (coordinately; two active sites to manipulate DNA).
- MuA acts in trans to cut the target site DNA and mediate in trans strand transfer
• Mu integrates by nonreplicative transposition;
• during lytic cycle- number of copies amplified by replicative transposition
MuA also binds to internal site- needed for complex formation but not strand cleavage
3cuts in trans
transfers in trans
34
Figure 21.12
1
2
3
In strand transfer complex transposon is connected to the target site through one strand at each end
cuts in trans
transfers in trans
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Figure 21.12
1
2
3
The MuB protein chooses targets;
Mu Tn moves >10-15 kb away from original site (target immunity).
MuB binds to the MuA-Mu DNA complex; MuA causes MuB to hydrolize ATP; MuB released from the donor DNA.
MuB binds nonspecifically to the target DNA and stimulates the recombination activity of MuA in transposition complex.
MuA clears MuB from the donor - gives preference for transposition to the target.
36
21.7 Replicative Transposition Proceeds through a Cointegrate
• Replication of a strand transfer complex generates a cointegrate:
– A fusion of the donor and target replicons.
• The cointegrate has two copies of the transposon.
– They lie between the original replicons.
Figure 21.13
by resolvase
(homolgous recombination)
KEY CONCEPT
37
• Recombination between the transposon copies regenerates the original replicons, but the recipient has gained a copy of the transposon.
• The recombination reaction is catalyzed by a resolvase coded by the transposon.
KEY CONCEPT
38Figure 21.14: Mu transposition uses a crossover intermediate.
39
21.8 Nonreplicative Transposition Proceeds by Breakage and Reunion
• Nonreplicative transposition results if:– a crossover structure is nicked on the unbroken pair of
donor strands and– the target strands on either side of the transposon are
ligated
Figure 21.15
KEY CONCEPT
donor target
40
• Two pathways for nonreplicative transposition differ according to whether:
– the first pair of transposon strands are joined to the target before the second pair are cut (Tn5), or
– whether all four strands are cut before joining to the target (Tn10)
KEY CONCEPT
41
Figure 21.16: Transposition can use cleavage and ligation.
• Two pathways for nonreplicative transposition differ according to :
– whether all four strands are cut before joining to the target (Tn10) or
– whether the first pair of transposon strands are joined to the target
before the second pair are cut (Tn5),
KEY CONCEPT
42Figure 21.17: Tn5 is cleaved from flanking DNA.
• Two pathways for nonreplicative transposition differ according to :
– whether all four strands are cut before joining to the target (Tn10) or
– whether the first pair of transposon strands are joined to the target before the second pair are cut (Tn5),
1. nicking
2.interstrand Rx
3
4. cleavage
43
Figure 21.18: Transposon ends are joined.
dimer Tn 5 and Tn 10 transposases both function as dimers.
44
21.9 TnA Transposition Requires Transposase and Resolvase
• Replicative transposition of TnA requires:– a transposase to form the cointegrate structure– a resolvase to release the two replicons
• The action of the resolvase resembles lambda Int protein.
• It belongs to the general family of topoisomerase-like site-specific recombination (SSR) reactions.– They pass through an intermediate in which the protein is
covalently bound to the DNA.
KEY CONCEPT
45
Figure 21.19: TnA transposon organization is conserved.
Transposase
• Resolvase• Repressor (mutant Tn freq)↗
Internal res site
feedback
(38bp)
•Replicative Tn, •Non-IS-dependent, •DR (~5bp) generated at target sites.
Control regions
Limiting factorin transposition
Dual role
Res1. Can be replaced by RecA-mediated general recombination, but less efficient.2. 15-20 bp of res site are identical to att.3. But protein mechanism is different: res- intramolecular resolution. Att- intermolecular resolution
TnA features
46
21.10 Transposition of Tn10 Has Multiple Controls
Figure 21.20: Tn10 has two promoters.
IS10Linactive
Promoter strong
weak
RNA stable
OUT RNA function as an antisense RNA.One copy no effect5 copies significant
• Multicopy inhibition reduces the rate of transposition of any one copy of a transposon when other copies of the same transposon are introduced into the genome
KEY CONCEPT
47
Figure 21.21
Tn must maintain min freq to survive, but too great freq may damage the host cells.
• Multiple mechanisms affect the rate of transposition.
KEY CONCEPT
48
21.11 Controlling Elements in Maize Cause Breakage and
Rearrangements
• Transposition in maize was discovered because of the effects of chromosome breaks.– The breaks were generated
by transposition of “controlling elements (Transposon)- Ds element.”
KEY CONCEPT
Figure 21.22: Transpositions are clonally inherited.
49
• The break generates one chromosome that has:– a centromere– a broken end – one acentric fragment
• The acentric fragment is lost during mitosis; – detected by the
disappearance of dominant alleles in a heterozygote.
Figure 21.23. A break at controlling element causes loss of an acentric fragment
KEY CONCEPT
(site for chr. breakage)
dominant
recessive
50
• Fusion between the broken ends of the chromosome generates dicentric chromosomes.– These undergo further cycles of
breakage and fusion.
• The breakage-fusion-bridge cycle is responsible for the occurrence of somatic variegation.
Figure 21.24 Ds provides a site to initiate the chromatid breakage-fusion-bridge cycle.
KEY CONCEPT
51
Figure 21.25
21.12 Controlling Elements Form Families of Transposons
The numbers, types, and locations of the control elements are characteristic for each maize strain.
KEY CONCEPT
Each family of controlling elements in maize has two classes: autonomous and nonautonomous
Note! Ds = Dt
Common feature
Specific feature
52
Mutator transposon is one of the simplest elements
MuDR (autonomous element) code for genes: • mudrA codes for MURA transposase• mudrB codes for nonessential accessory protein
In MuDR, de-methylation of the terminal repeats increases transposase experssion.
53
Figure 21.26: The Ac element gas five exons that code for a transposase; Ds element have internal deletions.
Ac element
Ds elements have internal deletions.
•Ds alone could not induce the breakage. (need Ac)•Transposition of Ac/Ds occurs by a non-replicative mechanism.•Phage using Ac/Ds results in DNA methylation change.
May code for a repressor of transposition
transposase
That inactivate the trans-acting transposase
Ds is an incomplete version of Ac itself
54
• Autonomous controlling elements code for proteins that enable them to transpose.
• Nonautonomous controlling elements have mutations that eliminate their capacity to catalyze transposition (internal sequences).– They can transpose when an autonomous element provides
the necessary proteins via trans-acting transpoase.
• Autonomous controlling elements have changes of phase (reversible), when their properties alter as a result of changes in the state of methylation.
KEY CONCEPT
[note! Ds is coded for deleted form of transpoase; Ac is complete functional; that is why always written in Ac/Ds system]
55
Transposable elements in eukaryotes:
Barbara McClintock (1902-1992)Cold Spring Harbor Laboratory, NY
Nobel Prize in Physiology and Medicine 1983
“for her discovery of mobil genetic elements”
• Studied transposable elements in corn (Zea mays) 1940s-1950s(formerly identified as mutator genes by Marcus Rhoades 1930s)
Nonautonomous DNA tn (Ds) require the activator (Ac) to be in the same cells.
56
21.13 Spm Elements Influence Gene Expression
• Spm elements affect gene expression at their sites of insertion, when the TnpA protein binds to its target sites at the ends of the transposon.
• Spm elements are inactivated by methylation.
KEY CONCEPT
57
Figure 21.27: Spm/En has two genes. tnpA a spliced 2500-bp mRNA (Exon1-11); tnpB 6000-bp mRNA (containing ORF1+ORF2)
defective
dSpm
58
21.14 The Role of Transposable Elements in Hybrid Dysgenesis
• P elements are transposons that are carried in P strains of Drosophila melanogaster (fly), but not in M strains.
• When a P male is crossed with an M female, transposition is activated.
[Note! M male x P female, transposition is inactivated]
hybrid dysgenesis
KEY CONCEPT
p853
59
• The insertion of P elements at new sites in these crosses:– inactivates many genes– makes the cross infertile
Figure 21.28 Hybrid dysgenesis is asymmetrical
= infertile
• Dysgenesis is principally a phenomenon of the germ cells.
• P-specific sequences can induce dysgenesis by insertional inactivation.
• P-specific seq are many (30-50 copies) and locate in different chr., but not in M strain.
60
21.15 P Elements Are Activated in the Germline
• P elements are activated in the germline of P male x M female crosses.
• This is because a tissue-specific splicing event removes one intron (somatic expression).– This generates the coding
sequence for the transposase.
Figure 21.29 Pelement has four exons; the first three are spliced together in somatic expression; all four are spliced together in germline expression
KEY CONCEPT31bp
IRGenerate DR of target DNA (~8bp)
61
• The P element also produces a repressor of transposition.– It is inherited
maternally in the cytoplasm.
Figure 21.30
•The presence of the repressor explains why M male x P female crosses remain fertile.
KEY CONCEPT
No repressor
Retroviruses and Retroposons
Chapter 22
高雄醫學大學生物醫學暨環境生物學系張學偉 助理教授Email: [email protected] 分機 : 2691
Retro-transposons
63
22.1 Introduction
Figure 22.1 Reproductive cycles (continuous) of retroviruses and retroposons.
Retroposons are confined to an intracellular cycle.
Single strand• Retrovirus -Transposition involved RNA intermediate is unique to eukaryotes.RNA DNA RNA
• Retroposon -Transposition through RNA intermediate. (similar)Itself no transposition activity but with active element sequence [with help from retrovirus]
Like a lysogenic bacteriphage
64
22.2 The Retrovirus Life Cycle Involves Transposition-Like Events
• A retrovirus has two copies of its genome of single-stranded RNA.
• An integrated provirus is a double-stranded DNA sequence.
KEY CONCEPT
65
• A retrovirus generates a provirus by reverse transcription of the retroviral genome.
Figure 22.2
integrase
Long terminal repeat
66
22.3 Retroviral Genes Codes for Polyproteins
Figure 22.3
A typical retrovirus has three genes
5-cap polyA
gap-pol
Env proteins
gag & pol in different frame: gap >> gag-pol
Continued
R seqment
67
• Gag and Pol proteins are translated from a full-length transcript of the genome.
• Translation of Pol requires a frameshift by the ribosome.
• Env is translated from a separate mRNA that is generated by splicing.
• Each of the three protein products is processed by proteases to give multiple proteins.
KEY CONCEPT
68
Figure 22.04: HIV buds from the membrane.
Photo courtesy of Matthew A. Gonda, Ph.D., Chief Executive Officer, International Medical Innovations, Inc.
Viron- physical virus particles p.866
Viroid- small infected nucleic acids without protein coats.
比較
A process that is reversed during infection
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22.4 Viral DNA Is Generated by Reverse Transcription
• Retroviruses are called Plus (+) strand viruses because the viral RNA itself codes for the protein products.
• Complementary DNA of Virus RNA called minus (-) strand DNA.
another strand in duplex DNA called plus (+) strand DNA.
• RNase H degrade the RNA part of RNA-DNA hybrid.
70
22.4 Viral DNA Is Generated by Reverse Transcription
A short sequence (R) is direct repeated at each end of the viral RNA.– The 5’ and 3’
ends are R-U5 and U3-R, respectively.
Figure 22.5 Retrovirial RNA ends in direct repeat (R), the free linear DNA ends in LTR and the provirus ends in LTRs that are shortened by two bases each.
R segmentKEY CONCEPT
71
Figure 22.6-a
• Reverse transcriptase starts synthesis when a tRNA primer binds to a site 100 to 200 bases from the 5’ end.
KEY CONCEPT
(+)
(-)
(-)
72
Figure 22.6-b
•When the enzyme reaches the end, the 5’-terminal bases of RNA are degraded.
–This exposes the 3’ end of the DNA product.
KEY CONCEPT
•The exposed 3’ end base pairs with the 3’ terminus of another RNA genome.
• Synthesis continues, generating a product in which the 5’ and 3’ regions are repeated.
–This gives each end the structure U3-R-U5.
(-)
(-)
(-)
73
• Similar strand switching events occur when reverse transcriptase uses the DNA product to generate a complementary strand.
Figure 22.7 Synthesis of plus-strand DNA requires a second jump.
KEY CONCEPTDNA (-)
RNA
DNA (-)DNA (+)
74
Strand switching is an example of the copy choice mechanism of recombination.
Figure 22.8
KEY CONCEPT
DNA (-)
RNA (+)
copy choiceA type of recombination used by RNA virus, in which the RNA polymerase switches from one template to another during synthesis
p869
75
Linear DNA is inserted directly into the host chromosome by the retroviral integrase enzyme.
Figure 22.9
KEY CONCEPT
Two base pairs of DNA are lost from each end of the retroviral sequence during the integration reaction.
The organization of proviral DNA in a chromosome is the same as a transposon.– The provirus is flanked by short
direct repeats (DR) of a sequence at the target site.
Cp. Fig22.5
22.5 Viral DNA Integrates into the Chromosome at radom sites.
76 Part of Fig. 22.5
Left LTRResponse for initiating transcription of provirus
Right LTRSometimes (rarely) sponsor transcription of host seq near integration site.
LTR also carries an enhancer that acts on cellular and viral seq.
U3 carries promoter
77
22.6 Retroviruses May Transduce Cellular Sequences
Figure 22.10: Replicative-defective transforming viruses have a cellular seq substituted for parial viral seq.
From spliced RNA copies of cellular seq. c-onc
Onc = Oncogenesis, transform ability
c-onc usually interrupted by intronsv-onc is un-interrupted
78
Figure 22.11
Transforming retroviruses are generated by a recombination event: A cellular RNA sequence replaces part of the retroviral RNA.
Helper virus
79
22.7 Yeast Ty Elements Resemble Retroviruses
Figure 22.12: Ty elements have two genes.
Ty = Transposon yeast
• interspersed repeat DNA
• Same transpose mechanism to retrovirus
• Freq < bacterial Tn
• Two major classes:
Ty1 (30 copies)
Ty917 (6 copies)
•ps: , 100 copies, considerable heterogeneity
80
• Ty elements are classic retroposons, with a reverse transcriptase activity.– They transpose via an RNA
intermediate.
Figure 22.13
KEY CONCEPT
Ty elements does not give rise to infectious particles, but virus-like particles (VLPs) accumulate within the cells.
81
Endogenous retroviruses
• Ty transposons have a similar organization to endogenous retroviruses.
• endogenous retroviruses are retroviruses derived from ancient infections of germ cells in humans, mammals and other vertebrates; as such their proviruses are passed on to the next generation and now remain in the genome.
• Most retroviruses (such as HIV-1) infect somatic cells, but some can also infect germline cells (cells that make eggs and sperm)
http://en.wikipedia.org/wiki/Endogenous_retrovirus
KEY CONCEPT
82
• copia is a retroposon that is abundant in D. melanogaster.
Figure 22.15
22.8 Many Transpable Elements Reside in D. melanogaster.
83
22.9 Retroposons Fall into Three Classes
Retroposons of the viral superfamily are transposons that mobilize via an RNA that does not form an infectious particle.
Figure 22.16
KEY CONCEPT
1 2 3
LTR
84
• Some retroposons directly resemble retroviruses in their use of LTRs. - Others do not have LTRs.
• Other elements can be found that were generated by an RNA-mediated transposition event;– But they do not themselves code for enzymes that can catalyze
transposition.– [Just need help]
Figure 22.17
KEY CONCEPT
Plant
•contain another type of small mobile element, called MITE (miniature inverted-repeat transposable element)
•No relationship to SINE, LINE
85
• Transposons and retroposons constitute almost half of the human genome.
Figure 22.18
KEY CONCEPT
Only one SINE have been active in the human lineage: the common Alu element
86
87
22.10 The Alu Family Has Many Widely Dispersed Members
• A major part of repetitive DNA in mammalian genomes consists of repeats of a single family:– organized like transposons – derived from RNA polymerase III transcripts
KEY CONCEPT
•Individual members of the Alu family are related rather than identical.
•Alu sequence is related to 7SL RNA, a compartment of the signal recognition particle.
88
22.11 Processed Pseudogenes Originated as Substrates for Transposition
• A processed pseudogene is derived from an mRNA sequence by reverse transcription.
Figure 22.19
KEY CONCEPT
No intron
DRDR
RNA polmerase II
89
Evidence:
1. many of the poly-A retroposons that have been detected by large-scale genomic sequencing are truncated elelments. most of these are missing region from 5’end. lost the ability to transpose.
2. Processed pseudogenes not expressed by cell due to lack of promoter, intron or truncate near 5’end. (many cellular gene had been truncated at 5’end) these pseudogenes are often flanked by short repeat this is structure of LINE-promoted transposition of cellular mRNA.
90
processed pseudogenes • do not carry any information used to transposition.• do not carry out reverse transcription of RNA.• A dead ends of evolution.
Active LINE element• provides most of the RTase activity• Acts for transposition on: (1) its own (2) SINE• For generating processed pseudogenes.
Summary
91
22.12 LINES Use an Endonuclease to Generate a Priming End
• LINES do not have LTRs.
• They require the retroposon to code for an endonuclease that generates a nick to prime reverse transcription.
Figure 22.20
DNA-binding protein
RT-ase endonuclease
KEY CONCEPT
5’5’ 3’
92
Note
• Although transposition of cellular RNA can occur, it is a rare event.
• LINE-encoded protein (ORF1&2) bind immediately to their own RNA during translation
show highly preference to its own RNA rather than the cellular RNA.
93
Figure 22.21: LINES proteins are cis-acting.
•Reverse transcription often does not proceed fully to the end, so the copy is inactive.
•Original from RNA pol II lacks of promoter are necessary inactive.
94
Figure 22.22: Autonomous act on nonautonomous elements.
For transposition to survive, they must occur in the germline.
