Download Composite Transposons

TRANSFERIMIENTO LATERAL DE GENES
F+
F-
Unlike the shorter attachment pili (fimbriae),
this long type of pilus is used for transfer of
genes in conjugation and is often called a sex
pilus.
Fig. 8.7 transmission electron micrograph of F pilli of E. coli
F' cells
Formation of an F' cell from
an Hfr cell, and transfer of a
bacterial chromosome
segment to a recipient cell.
F+
F-
An F+ x F- mating- The F+ cell transfers one strand of DNA from its plasmid to the F- cell via
the conjugation bridge. As this occurs, the complementary strands of F plasmid DNA are
synthesized.
Fig. 8.8 An F+ x F- mating.
F', formed by the excision of the F
factor plasmid from the Hfr strain
contains some host genes in addition to
the gene for F plasmid.
The F' transfers its genetic material
like an F+ strain
Fig. 8.10 The formation and transfer of F’ plasmids
Viral reproduction: the lytic cycle
Generalized schematic for
viral reproduction in a host
bacterium, through the lytic
cycle.
In the lytic cycle, the virus
(phage) multiplies in the host
cell and the progeny viruses
are released by lysis of cell.
Viral reproduction: the lysogenic cycle
Generalized schematic for
viral reproduction in a host
bacterium, through the
lysogenic cycle.
In the lysogenic cycle, viral
DNA is integrated into the
host genome and replicates
as the chromosome
replicates, producing
lysogenic progeny cells
Generalized transduction: Lytic phage
Specialized transduction: Lysogenic phage
Transposable elements in
prokaryotes

Insertion sequence (IS) elements

Transposons (Tn)

Bacteriophage Mu
Insertion elements and transposons
Insertion sequences (IS) are short DNA sequences, about
700 to 5000 bp which can move from one location in a
DNA sequence to another. They have short 16-41 bp
inverted repeats on their ends. They encode a
transposase which catalyses site-specific recombination.
Simple transposons are mobile genetic elements in which
a one or more genes are flanked by two insertion
sequences.
Composite transposons
Structures of some bacterial transposable elements.
(A) A composite transposon contains antibiotic genes flanked by two
insertion sequences as direct or inverted repeats Shown here is the
Tn5 transposon, with inverted repeats.
(B) The Tn3 transposon.
Insertion sequence (IS)
elements
•
Simplest type of transposable element found in bacterial
chromosomes and plasmids
•
Encode only genes for mobilization and insertion
•
Range in size from 768 bp to 5 kb
•
IS1 first identified in E. coli’s glactose operon is 768 bp
long and is present with 4-19 copies in the E. coli
chromosome
•
Ends of all known IS elements show inverted terminal
repeats (ITRs)
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Integration of IS element in
chromosomal DNA
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Three different mechanisms
for transposition
• Conservative transposition
• Replicative transposition
• Retrotransposition
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Three different mechanisms
for transposition
• Conservative transposition: The element itself
moves from the donor site into the target site
• Replicative transposition: The element moves a
copy of itself to a new site via a DNA intermediate
• Retrotransposition: The element makes an RNA
copy of itself which is reversed-transcribed into a
DNA copy which is then inserted (cDNA)
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Conservative transposition
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Replicative transposition
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Retrotransposition
common feature of mobile elements
• Generation of short direct repeats flanking the newly inserted element
• This results for a staggered cut being made in the DNA strands at the site of
insertion
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Transposons (Tn)
• Similar to IS elements but are more complex
structurally and carry additional genes
• 2 types of transposons:
• Composite transposons
• Noncomposite transposons
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Composite
transposons
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IS10R is an autonomous element, while IS10L is non-autonomous
Composite Transposons
• Tetracycline resistance is carried by a transposable element
• The transposon is a composite transposon, composed of ISelements flanking an included sequence, in this case containing
an antibiotic resistance gene
• IS10R is an autonomous element
• while IS10L is non-autonomous
• Composite transposons probably evolved from IS elements by
the chance location of a pair in close proximity to one another.
Inactivation of one element by mutation would not harm ability
to transpose and would assure continued transposition of the
entire transposon
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Noncomposite
transposons
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Noncomposite transposons
(Tn)
• Carry genes (e.g., a gene for antibiotic resistance)
• Ends are non-IS element repeated sequences
• Tn3 is 5 kb with 38-bp ITRs and includes 3 genes; bla
(-lactamase), tnpA (transposase), and tnpB (resolvase,
which functions in recombination)
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Examples of DNA-intermediate
mobile elements
• Insertion Sequences (IS) elements in bacteria
• P elements in Drosophila
• AC/DS (dissociation) elements in maize
• AC is a full-length autonomous copy
• DS is a truncated copy of AC that is non-autonomous, requiring AC in
order to transpose
• At least seven major classes of DNA transposons in the human
genome (3% of total genome)
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Structure and transposition of a transposable
element
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