Mobile Genetic Elements: IS Elements and Transposons

• Discovery of IS elements in bacteria

• polar mutations
• heteroduplex analysis


 
 

• types of IS

• Structure of a generic IS element

• inverted repeats
• transposase gene

• Structure of transposons

• simple (Type II) transposons
• composite (Type I) transposons

• Mechanisms of transposition

• conservative transposition involves the movement of the element from donor to recipient, such that donor loses the transposon
• in replicative transposition, the transposon jumps to recipient, but also remains in donor DNA
• this mechanism involves a co-integrate form
• called replicative because replication precedes transposition
• target sites and the consequence of transposition
• target sites are variable short DNA regions of recipient (2 to 12 bp)
• almost always are duplicated when the transposon jumps in
• thus, even conservative transposition can be mutagenic

• Conjugative transposons

• Capable of jumping from one DNA molecule to another (transposition) AND jumping from one cell to another (conjugation or transfer).
• These tend to be very large and complex
• First one discovered, Tn916, carries a tetracycline resistance gene that is of a particular type (called tetM).  This gene is all over the bacterial world, and it probably was spread by this "promiscuous" transposon.