Mu transposition is mechanistically similar to HIV integration and to Immunoglobulin rearrangements

Mu is a bacterial virus which propagates itself by repeated transposition/integration into the genome of its host. Cancer-causing retroviruses such as HIV use a similar mechanism to integrate into their host genomes. In the figure below, note the similarities between the Mu and HIV systems: Both genomes are flanked by the terminal dinucleotide CA which is cleaved to yield 3' OH groups. The strand transfer or joining step is identical in both systems and leads to integration of the viral genome into the host (target) genome. The chemistry of this class of phosphoryl transfer reactions is common to immunoglobulin rearrangements and RNA splicing.

An available in vitro system for Mu transposition allows us to ask questions about the structure and function of transposition proteins, nucleoprotein complexes and reaction intermediates at a level of detail not currently possible with retroviral or immunoglobulin systems. Some of the most interesting aspects of Mu transposition, such as the requirement for a cis-acting enhancer (see below) to facilitate assembly of the catalytically competent transposase complex, are also of fundamental significance to other processes such as transcription, replication and recombination.

The Mu transposase is by far the most efficient transposase known. The Mu transpososome is assembled by interactions of transposase subunits with the left and right ends of Mu and an enhancer located in between. We have now determined the path of the DNA within the three-site LER synapse using a new method called 'difference topology'. As shown below, the three sites (enhancer is red and the two Mu ends are blue)

interwrap around each other five times. This is the most complex DNA arrangement seen to date within a recombination synapse. This new methodology allows us to ask questions about the order with which the sites come together, the roles of individual subsites in the assembly process, as well as the role of accessory factors.

We have also recently developed a higly efficient plasmid-based in vivo transposition assay. Hand-in-hand with the in vitro assay, we are in a unique position to delve even deeper into mechanism of transposition. In the future, will be focusing on in vitro and in vivo pathways of transpososome assembly and transposition, including target site selection and target immunity.

 

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