8. • When resistance plasmids were first discovered there was much speculation
as to how a single element could have evolved to carry a number of different
antibiotic-resistance genes, and in particular how apparently related plasmids
could have different combinations of such genes (or, conversely, how
otherwise dissimilar plasmids could carry related resistance genes).
• It was assumed that a basic plasmid, having the ability to
replicate independently but not carrying any other information,
had somehow picked up a resistance gene from the chromosome
of a resistant host strain.
9. • Transfer of this plasmid to an otherwise sensitive strain then produces a
selective advantage for that strain, and therefore indirectly a selective
advantage for this ‘new’ plasmid.
• As the plasmid moves from one organism to another it has the opportunity to
acquire additional resistance genes, thus giving rise to a family of plasmids
containing different combinations of resistance genes.
10. • Since this model implies that unrelated plasmids could pick up the same gene
independently, this would explain the widespread distribution of certain resistance
genes, notably a type of β-lactamase (the enzyme that destroys penicillin and hence
confers resistance to penicillins).
• This particular enzyme, the TEM β-lactamase, is the commonest type among
plasmids in the Enterobacteriaceae, and is also present in many members of other
genera.
11. • The reason for the ubiquity of the TEM β-lactamase became apparent from the
discovery that this gene could move (transpose) from one plasmid to another.
• This is exemplified by the conjugation experiment shown
diagrammatically in Figure.
• A strain of E. coli containing two different plasmids, one with an ampicillin-
resistance gene and one conferring resistance to kanamycin, is used as a donor.
• The recipient strain is sensitive to both
drugs (but resistant to nalidixic acid).
• Plating the mixed culture on a medium containing nalidixic acid, ampicillin and
kanamycin will therefore select for recipient cells that have received both
resistance genes from the donor.
• It was found that resistance to both antibiotics was transferred at a rate much
higher than would be predicted from the rate of independent transfer of the two
plasmids.
• It was also found that the recipients that were resistant to both drugs contained a
single plasmid carrying both resistance genes.
12.
13. • This effect was not due to ordinary recombination between the
two plasmids, since it occurred equally well in recombination-
deficient (recA) strains.
• From additional evidence, it was deduced that the ampicillin-
resistance gene had moved (transposed) from one plasmid to
the other.
• The term transposon was coined to signify an element that
was capable of such behaviour,
i.e. A mobile genetic element containing additional genes
unrelated to transposition.
14. • This movement of resistance genes can occur not only
between two plasmids, but also from plasmid to
chromosome and vice versa.
• It therefore provides part of the explanation for the rapid
evolution of resistance plasmids, and also of plasmids
that carry genes other than antibiotic resistance.
• Although this discussion has focussed on antibiotic
resistance, other plasmid borne genes are also known
to be transposable on occasions.
• As with bacterial plasmids, there are two reasons why
the literature is dominated by antibiotic resistance: the
widespread use of antibiotics has provided an
unusually strong selective pressure for their
development and dissemination, and antibiotic
resistance is a very convenient genetic marker, thus
making it much easier to study than other types of
genes.
18. TYPES OF TRANSPOSONS
• 1) CLASS I ELEMENTS
• IT IS SHOW COPY AND PASTE.
• THAT IS FOUND IN EUKARYOTES.
• IT IS ALSO NON AS REPLICATIVE
TRANSPOSONS.
• 2) CLASS II ELEMEMT
• IT IS SHOW CUT AND PASTE
• THAT IS FOUND IN PROKARYOTES.
• IT IS ALSO KNOWN AS
CONSERVATIVE TRANSPOSONS.
19. STRUCTURE OF TRANSPOSONS
1)VIRAL
TRANSPOSONS
•IT HAVING LTR
ELEMENT.
•FOUND IN-
-RETROVIRAL
-DROSOPHILA –COPIA
-YEAST
2)NON VIRAL
TRANSPOSONS
NON-LTR
•TWO TYPES-
1) LINES
2) SINES
3) BACTERIAL
TRANSPOSONS
•IT HAVING IS ELEMENT.
•FOUND IN –
-BACTERIA
-DROSOPHILA
-CORN
21. •The structure of a simple transposon, Tn3, is shown in Figure 7.4; it
consists of
about 5000 bp and has a short (38 bp) inverted-repeat sequence at each
end.
• It is therefore analogous to an insertion sequence, the distinction being
that a
transposon carries an identifiable genetic marker, in this case the
ampicillin resistance gene (bla, β-lactamase).
• Tn3 codes for two other proteins as well: a
transposase (TnpA), and TnpR, a bifunctional protein that acts as a
repressor
and also is responsible for one stage of transposition known as
resolution
STRUCTURE OF TRANSPOSONS
22.
23. • Some transposable elements have a more complex structure than
Tn3.
• These composite transposons consist of two copies of an insertion
sequence
on either side of a set of resistance genes.
• For example, the tetracycline resistance transposon Tn10, which is
about 9300 bp in length, consists of a
central region carrying the resistance determinants flanked by two
copies of the IS10 insertion sequence in opposite orientations (Figure
7.5;)
• IS10 itself is about 1300 bp long with 23 bp inverted-repeat ends and
contains a transposase gene
24.
25. • Composite transposons may have their flanking IS regions in inverted
orientation or as direct repeats.
• For example, Tn10 and Tn5 (Figure 7.6)
Both have inverted repeats of an insertion sequence (IS10 and IS50
respectively) at
their ends, while Tn9 has direct repeats of IS1.
• The transposition behaviour of such composite elements can be quite
complex; the insertion sequences themselves may transpose independently
or transposition of the entire region may occur.
• Furthermore, recombination between the IS elements can occur,leading
to deletion or inversion of the region separating them
26.
27. • Even more complex arrangements can occur. For example, Tn4
appears to be related to Tn21 but contains a complete copy of Tn3
within it.
• The ampicillin-resistance gene of Tn4 can thus be transposed as part of
the complete Tn4 transposon, or by transposition of the Tn3 element.
• Thus several layers of transposons can occur, nested within one
another.
28. • Composite transposons such as Tn10 are also known as class I transposons,
• whereas transposons such as Tn3, flanked by inverted repeats rather than IS
elements, are referred to as class II transposons, or non-composite
transposons.
• Note that this description is used merely to distinguish them from the
composite transposons.
• It does not imply that their structure is in any sense
simpler; indeed, as we shall see in the next section, transposons such as Tn21
are composed of bits and pieces from many sources.
29. • Extremely complex and large transposons can be built up by insertion
of additional genes within an existing transposon.
• We have already seen (Figure 7.6) that this can happen by insertion of one
transposon within
another one. But this is not the whole story.
• Many large transposons have been identified that are related to Tn21, but contain
different (more or fewer)resistance genes (Figure 7.7). There is a reason for this.
INTEGRONS
30.
31. • Many transposons carry a region known as an integron, which is an
assembly platform for the integration of genes originating from other
genetic elements.
• This region contains an integrase gene and a specific attachment site (attI)
where additional genes are inserted (Figure 7.8).
• A typical integron also has a conserved sequence (3
-CS) at the 3 end, containing genes for resistance
to quaternary ammonium disinfectants (qacE) and sulphonamides (sul).
• The gene to be inserted is excised from elsewhere as a circular molecule
containingjust a single gene with an imperfect inverted repeat (the attC
site) at its 3end. This is known as a cassette
32.
33. • The integrase carries out site-specific recombination between the attI and
attC sites, which results in integration of the cassette adjacent to attI.
• The attI site remains available for the insertion of further genes, enabling
the build-up of an array of several cassettes within the integron.
• A further twist to the story is that the gene cassettes do not normally contain
a promoter.
• However, there is a promoter region within the integron itself, upstream
from the insertion site, so each gene cassette is transcribed from the
integron promoter
34. • Integrons are not only found in the Tn21 family of transposons.
• Several classes of recognizably distinct integrons have been found in other
transposons, as well as in plasmids that do not carry functional transposons.
• They therefore represent a significant additional mechanism for the evolution
of
bacterial plasmids and the spread of antibiotic resistance.
35. • Although much of our knowledge of integrons has been derived from their
role in the evolution of transposons, and hence of plasmids, they occur in
other contexts as well.
• The chromosomes of many bacteria, notably Vibrio cholerae, contain one
or more regions with a similar structure, that is they have an integrase, a
promoter, and an attachment site, followed by gene cassettes.
• But in this case, instead of a handful of cassettes as is typical of a
transposon,they have a very large number of cassettes (over 200 in some
species of Vibrio,representing about 3% of the genome). These are known
as superintegrons.
• The function of superintegrons is unclear, and many of the genes they
carry are of unknown function, but they provide the cell with a mechanism
for recruiting additional genes from other sources and therefore contribute
to the genetic versatility of the bacterium.
36. ISCR ELEMENTS
• We are building up a picture of transposons as complex structures.
• They may contain other transposons within them, or insertion sequences,
either on their own or as two copies flanking one or more resistance
genes (thus mobilizing those genes by formation of a composite
transposon).
• Or they may contain an integron into which additional gene cassettes can
be inserted.
• There is a further complication. Many transposons are found to carry an
additional conserved region, known as a common region (CR), usually
close to the 3-CS region of an integron,followed by one or more
resistance genes.
• These lack the usual properties associated with gene cassettes, and have
been acquired by another mechanism.
• The CR codes for a protein, usually known as Orf513, which is related to
37. • We encountered IS91 earlier in this chapter as an example of an
insertion
sequence that lacked inverted-repeat ends and transposed by a
rolling-circle
• mechanism. One characteristic of this mechanism is that the
replication may
not terminate accurately at the end of the insertion sequence but
may extend
to copying adjacent genes.
• These genes will therefore be incorporated into
the transposed element, thus mobilizing the genes concerned
(Figure 7.9).
38. • This is in effect another form of gene mobilization by an insertion
sequence,and hence these elements are sometime referred to as
ISCR elements.
39.
40. What are transposons used for?
As genetic tools, DNA transposons can
be used to introduce a piece of foreign DNA into
a genome. Indeed, they have been used
for transgenesis and insertional mutagenesis in
different organisms, since these elements are
not generally dependent on host factors to
mediate their mobility.
41. Are transposons good or bad?
As with most transposons, LINE-1
migrations are generally harmless. In fact,
LINE-1 has inserted itself around our
genomes so many times over the course of
human evolution that it alone makes up as
much as 18% of our genome! ... LINE-1
insertions have been linked to different
kinds of cancer, including colon cancer
42. Do humans have transposons?
Transposable elements (TEs) are mobile repetitive
sequences that make up large fractions of
mammalian genomes, including at least 45% of
the human genome (Lander et al. ... Information
on human DNA transposons is currently very
scarce. This type of element makes up 3% of our
genome
43. Can transposons cause mutations?
Transposons are mutagens. They can cause
mutations in several ways: If a transposon inserts
itself into a functional gene, it will probably damage it.
Insertion into exons, introns, and even into DNA
flanking the genes (which may contain promoters and
enhancers) can destroy or alter the gene's activity.
