Further information
Mick Chandler, Laboratoire de Microbiologie et Génétique Moléculaire, CNRS, Toulouse France, presents this plant and microbial sciences research seminar on: ” A fork at the end of the road: transposition of a novel and widespread family of insertion sequences.”
Abstract: DNA transposons are ubiquitous and play a major role in shaping the genomes of both prokaryotes and eukaryotes. Their movement is catalysed by transposon-specific enzymes known as transposases. In spite of the number and diversity of transposable elements, there are only a limited number of chemical mechanisms by which they can move. The best characterised of these is that catalysed by the large group of so-called DDE transposases. These use a hydroxyl group as a nucleophile to attack the appropriate phosphodiester DNA bonds at each transposon end and in the target DNA. Transposons with this chemistry are the most numerous identified to date and transpose using double strand DNA intermediates.
Recently we have identified a second major transposon group and have demonstrated that they use an entirely different type of mechanism involving an obligatory single strand DNA intermediate. These transposons use a transposase (Y1 transposases) related to relaxases of conjugative plasmids and rep proteins of certain viruses and plasmids with rolling circle replication in which the nucleophile used is an active-site tyrosine.
I will describe our present model of transposition of members of this extensive family. This is based on understanding the entire transposition cycle of one example of this family (IS608 from Helicobacter pylori) in vitro and on the unique transposase structure resolved at 2.8A with and without the DNA substrates.
I will also present the results of genetic and biochemical experiments and of in silico genome mining which demonstrate that transposition of these elements occurs preferentially at the replication fork and during repair of damaged DNA.