1. Microbial Genetics (Micr340)
Lecture 12
Transposition

2. Nonhomologous recombination
No requirement for two DNAs being of the same or similar nucleotide sequences
Needs enzymes that recognize specific regions in DNA
Mechanisms include:
transposition
phage integration and excision
resolution of cointegrates

3. Transposition
Transposons – DNA elements that can hop (transpose) from one place in DNA to another
Movement by a transposon is called transposition, catalyzed by enzymes called transposases
Transposons usually encode their own transposases
Transposons are known to exist in all organisms on earth

4. History of Transposons
Transposons (jumping genes) were first discovered in Maize (corn) by Barbara McClintock.
Transposons were later found in bacteria, flies and humans.
This discovery led to the awarding of Nobel Prize to McClintock in 1983.

5. Transposition
Many transposons are essentially cut out of one DNA and inserted into another
Other transposons are copied and then inserted elsewhere
Donor DNA and target DNA

6. http://highered. mcgraw-hill

8. Structure of Bacterial Transposons
All contain repeats at their ends, usually inverted repeats (IR)
Presence of short direct repeats in the target DNA that bracket the transposon
The sites of insertion are different among target DNAs

9. Types of Bacterial Transposons
Smallest bacterial transposons are called insertion sequence elements (IS elements); they only encode transposase enzymes
Composite transposons – formed by two IS elements of the same type, bracketing other genes
Noncomposite transposons are bracketed by two short inverted repeats, with multiple resistance markers.

10. Structure of a Bacterial transposon

11. IS Elements and Composite Transposons

12. Composite transposons

13. Noncomposite transposons

14. Assay of transposition

15. Transposon Mutagenesis
A gene that has been marked with a transposon is relatively easy to map
Genes marked by transposons are very easy to be cloned
Transposon mutagenesis to identify essential genes (related to research of my lab).

16. Properties of transposons for mutagenesis
Transpose at a fairly high frequency
Not very selective in target sequence
Carry selectable marker (antibiotic resistance genes)
Broad host range for transposition

17. Tn5 mutagenesis

18. Cloning genes mutated by transposition

19. Isolating random gene fusions