Exam 3 Flashcards
What is another name for Transposable elements? Why?
Jumping Genes - because they can be seen in one location, then unexpectedly move to another.
How are transposable elements labeled?
Tn prefix, then a number in italics (eg. Tn10)
What is transposition?
It is a form of illegitimate or non-homologous recombination that is catalyzed by transposases (often encoded by the Tn)
What two ways can transposable elements move between hosts?
Via transducing particles and conjugative plasmids
Describe replicative vs. non replicative transposition
In non-replicative transposition, the transposon is excised and moves to a new location, leaving nothing behind in the original location.
In replicative transposition the transposon replicates and the new sequence inserts at a new site, leaving the original sequence behind at the original location.
Why is transposition a rare event?
It prevents the genome from being scrambled which makes it a highly regulated process.
Describe the structure of a transposon
It encodes a transposase and potentially other genes (eg. AMR), which are flanked on either side by inverted repeates which are critical in the transposition process
What are direct repeats
These are short sections of host DNA that flank teh IR’s they arise from repair of the target site insertion by DNA Pol I. The length of DRs is characteristic of each transposon.
What are the three types of transposable elements?
Insertion sequences, Composite Transposons, Non-composite transposons.
Describe insertion sequences
Smallest type of DNA transposons (~1kb). Mostly harbor only single transposase genes. E. coli K12 harbors 4 IS elements.
IR–Transposase–IR
Describe compositie transposons
These are formed when two of the same IS elements bracket an additional gene. Transposition of the entire transposon involves utilization of the outside ends of the transposon. The IS elements that make up the composite can be capable of independent transposition. Many R plasmids acquired AMR from composite transposons.
IR–Transposase–IR–gene–IR–Transposase–IR
Describe non-composite transposons.
The limits of the transposon are flanked by short IR’s. In addition to the transposase between the IR’s is another gene (often AMR). This gene is part of the minimum transposable unit.
IR–transposase–gene(AMR)–IR
What are integrons? How do they relate to transposons.
Composite transposons often harbor integrons.
These are cassettes of genes encoding resistance to multiple antibiotics .
The transposon will often encode an integrase, which catalyzes integration of the resistance genes into the transposon by site-specific recombination.
What is target site specificity in relation to transposons?
Most transposons exhibit a tropism for certain DNA sequences. Some avoid highly transcribe gens, while others are attracted to DNA that is actively being replicated.
How do transposons impact genes adjacent to the insertion site? How is this different between monocystronic and polycystronic genes?
Monocystronic - Insertion –> KO
Polycystronic - if IS contains termination sequence –> no txn of downstream genes.
What are outward facing promoters? What do they do?
They are promoters on transposons that may lead to enhanced gene expression near the insertion site.
What are three mechanisms of regulation of transposition?
Transposon-encoded repressor proteins
Anti-sense RNA’s
Expression of a defective transposase.
What is a dominant negative mutation and how does this impact transposition?
The expression of a defective transposoase that titrates concentration of active transposase oligomers is a dominant negative mutation. In Tn5, active truncated transposase is part of oligomer that forms transposase, it is not active.
