13 - Transposons, ICEs and Pathogenicity Islands Flashcards
Transposition
- Mechanism by which a gene moves from one location to another
- Can be found on chromosomal or plasmid DNA
Three types of transposable element
- Bacteriophages
- Insertion sequences
- Transposons
Common aspects of transposable elements
- Flanked by direct repeats
- Have inverted repeats/direct repeats and encode a transposase
- Has no machinery to replicate itself when it is not inserted into plasmid or chromosome
Transposase
Mediates the transposition with high sequence specificity or little sequence specificity
Mechanism of movement of transposable elements
- Simple/conservative (cut and paste)
- Replicative (copy and paste)
Types of transposable elements
- Insertion elements
- Composite transposon/nonreplicative (cut and paste mechanism)
- Composite transposon/replicative (copy and paste mechanism)
Insertion elements
Inverted repeats flanking transposase
Composite transposon/ non-replicative (cut and paste mechanism)
Inverted repeats flanking transposase AND accessory genes such as antibiotic resistance or toxin
Composite transposon/ replicative (copy and paste mechanism)
Inverted repeats flanking transposase AND resolvase AND accessory genes such as antibiotic resistance or toxin
Non replicative (cut and paste) transposition
- Transposon protein (TnpA) is expressed from the Tn gene
- TnpA binds to the IR
- Catalyses cleavage of the dsDNA backbone to release the transposon DNA
- TnpA remains bound to the
ends of the fragment and creates a circle - TnpA recognises an attachment site in the target DNA and catalyses insertion of the Tn into this site
Conservative transposition (e.g Tn5)
- RecA independent
- TnpA recognizes the IR and the target sequence
- TnpA introduces single strand cuts into the target site and each side of the transposon
- Single strand overhangs
are created - TnpA religates dsDNA phosphodiester backbone flanking the transposon
- Results in duplicated target site flanking the transposon
Frequency of transposition
1 in 10^4 to 10^7 cells
Replicative transposition (e.g Tn3)
- TnpA introduces single strand cuts at the ends of the IR and the target site
- Original and target sites fuse through single stranded copies of the transposon (forming a co-integrate)
- DNA polymerase fills in the
second strand forming a cointegrate - TnpR (resolvase) catalyses
ssDNA exchange between copies of Tn at the resolution (res) site within the transposon - Co-integrates resolve into two replicons each with
copy of the Tn
Results of inaccurate excision of transposon
- Can move genes to another location in the same genome
- Can move genes onto a resident conjugative plasmid and then move into another
recipient cell which could be a different strain or a different species - A compound transposon can acquire all types of functions including it’s own transfer functions so it does not need a plasmid
Integrative and Conjugative element (ICE)
Inverted repeats flanking integrase AND excisionase AND accessory genes
such as antibiotic resistance AND transfer genes
Genomic islands
Plasmids, bacteriophages and transposons captured in the genome (2/500kb)
Pathogenicity islands (PAIs)
- A region of chromosomally located DNA encoding virulence determinants or antibiotic resistance with a different GC% and codon usage to the rest of the chromosome,
- Flanked by direct repeats
- Inserted into specific site (tRNAs most common location)
- Lost ability to move
Why have PAIs lost ability to move
Mutations in direct repeats or in intergrase
Pathogenicity
Ability to cause disease
Virulence
Degree or intensity of pathogenicity
Virulence determinant
gene encoding a molecule contributing to pathogenesis
Pathogenic potential
Degree of damage caused
to the host (morbidity and mortality)
Infectivity
Ability to establish a focal point of infection in the host
Invasiveness
Ability of the organism to
spread to adjacent sites in
the body
What is evolution of pathogenic bacteria from commensal organisms driven by
- Random mutation
- Acquisition of MGE
- Environmental selection
Examples of MGEs
- Bacteriophage
- Plasmids
- Transposons/IS element/integron
- ICE
- Genomic islands (e.g. PAIs)
EPEC
- Enteropathogenic E. coli (infant diarrhoea)
- T3SS found on PAI
- EAF plasmid carrying pilus for adherence
EHEC
- Hemorrhagic colitis (HC) and Hemolytic Uremic Syndrome (HUS).
- Plasmid that secretes hemolysin
- Bacteriophage that secretes toxins
T3SS
- Basal structure in bacteria
- Needle inserts into eukaryotic cell
- Effector proteins are pumped from the bacterium into the eukaryotic cell
Uses of T3SS
- Invasion/multiplication within host cell by intracellular pathogens
- Attachment to host cells by extracellular pathogens
