MBG 2040 Lecture Notes - Lecture 11: Multiple Drug Resistance, Transposable Element, Transposase
2 May 2018
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Transposable Genetic Elements
Overview:
• Introduction to transposable elements
• Transposable elements in bacteria
• Cut-and-paste transposons in eukaryotes
• Retro-transposons, retro-posons, and infectious retroviruses
• Transposable elements in humans
• Evolutionary implications and applications
Barbara McClintock: discovery of jumping genes (mobile genetic elements)
• Geneticist with passion for maize genetics
• Key person in development of cytogenetics
• Proposed concept of 'jumping genes' in 1948
o Nobel Prize in 1983
Transposable Elements
• Transposable elements (or transposons) are found in genomes of all kinds of organisms, from
bacteria to plants to humans
• Are structurally, mechanistically, and functionally diverse
• Three basic categories based on distinct mode of transposition:
o Cut-and-paste transposition: element is literally cut out of one site in a chromosome and
pasted into a new site on the same or a different chromosome
o Replicative transposition (copy-and-paste): element is first replicated, and one copy is
inserted at a new site, while the other copy remains at the original site
o Retro-transposition: DNA element is first transcribed into RNA, which is reverse transcribed
back into DNA, followed by insertion of the DNA into a new site on the same or different
chromosome
• *see categorization of transposable elements by transposition mechanism
Summary:
• Cut-and-paste transposon is excised from one genomic position and inserted into another by a
special enzyme (transposase) which is usually encoded by the transposon itself
• Replicative transposon is copied during the process of transposition, followed by insertion into a
different spot in the genome DNA
• Retro-transposon first produces RNA molecule that is subsequently reverse-transcribed into a
cDNA molecule
o These DNA molecules are then inserted into new genomic locations
Transposable elements in bacteria:
• Bacterial transposons move within and between chromosomes and plasmids
• Two types:
o Cut-and-paste transposon
• Insertion Sequences (IS elements)
▪ Simplest transposons detected in bacteria
▪ First detected in certain lac- mutants of E. coli which often revert back to
wildtype
▪ IS elements are compactively organized and contain only genes which encode
enzymes involved in transposition
▪ Inverted terminal repeats are found at both ends of the IS elements
• Single nucleotide difference between ends
find more resources at oneclass.com
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• Target site duplications are direct repeat
▪ Not all IS elements encode the enzyme transposase
▪ Insertion of an IS element causes direct duplication at target site:
• Two strands of target DNA are cleaved by transposase
• The IS element is inserted into the gap created by staggered cleavage of the
target DNA
• DNA synthesis fills in the gaps on each side of the IS element, producing a
direct duplication of the target site
▪ Formation of conjugative R plasmids from recombination of IS elements:
• Conjugative R plasmids are responsible for the spread of multiple drug
resistance genes among species of bacteria (medical implications)
• Have two components:
• TRF: the resistance transfer factor (contains genes required for
conjugative transfer between bacterial cells)
• R-determinant contains genes for antibiotic resistance
• Composite transposons
▪ Created when two IS elements insert near each other on the bacterial
chromosome
▪ Denoted by the symbol Tn
• Tn9 = Camr: resistance to chloramphenical
▪ *see slide
o Replicative transposons: Tn3 elements
• Steps:
▪ Transposase encoded by Tn3 catalyzes the formation of a co-integrate between
the donor and recipient plasmids
• During this process, Tn3 is replicated so there is a copy of the element at
each junction in the co-integrate
▪ Resolvase produced by tnpR gene resolves the co-integrate by mediating
recombination between the two Tn3 elements
▪ Donor and recipient plasmids separate, each with a copy of Tn3
Summary:
• IS elements are cut-and-paste transposons that reside in both the chromosome and the plasmids of
bacterial cells
• IS elements can mediate recombination between different DNA molecules
• Conjugative plasmids can move transposons among the same or different bacterial species through
conjugation, hence spreading antibiotic resistance genes carried by them
• Composite transposons consist of two IS elements flanking a region of DNA that contains one or
more antibiotic resistance genes
Cut-and-paste Transposons in Eukaryotes:
• Transposable elements were first discovered through analysis of genetic instabilities in maize
o Genetic analyses also revealed transposable elements in Drosophila
• Ac and Ds controller elements in maize
o First discovered by Barbara McClintock in 1930s-1940s
o Aleurone colour is affected by the CI (colour inhibition) allele, encoding a dominant inhibitor
of pigmentation
o Mosaics with pigmented patches were caused by the loss of the CI allele due to transposition
of nearby element (Ds)
o Ac/Dc System:
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Introduction to transposable elements: transposable elements in bacteria, cut-and-paste transposons in eukaryotes, retro-transposons, retro-posons, and infectious retroviruses, transposable elements in humans, evolutionary implications and applications. Barbara mcclintock: discovery of jumping genes (mobile genetic elements: geneticist with passion for maize genetics, key person in development of cytogenetics, proposed concept of "jumping genes" in 1948, nobel prize in 1983. Transposable elements in bacteria: bacterial transposons move within and between chromosomes and plasmids, two types, cut-and-paste transposon. Insertion sequences (is elements: simplest transposons detected in bacteria, first detected in certain lac- mutants of e. coli which often revert back to wildtype. Is elements are compactively organized and contain only genes which encode enzymes involved in transposition. Inverted terminal repeats are found at both ends of the is elements: single nucleotide difference between ends, target site duplications are direct repeat, not all is elements encode the enzyme transposase.
