Mu has a served as a paradigm for transposition for the last three decades. While much has been learned about the chemistry of phosphoryl transfer, we are still in the dark about important aspects of nucleoprotein assembly. We know little of the manner in which the enhancer (E) interacts with the Mu ends (L and R) to initiate the process that eventually leads to formation of an LER complex and finally the active transposase (MuA) tetrainer. We know even less about how Mu ends are held together in the tetramer or how capture of target DNA occurs. The similarities between the mechanism of transposition of Mu and the mechanism of integration of HJV DNA heightens the significance of our research. The specific aims of this proposal are to 1) explore the architechture of the LER complex using 'difference topology', 2) determine MuA - Mu end and MuA - MuB contacts using in vivo suppression analysis as well as in vitro experiments with a view to understanding how target DNA is delivered to the ends, 3) investigate a recently discovered 'disintegration' reaction catalyzed by MuA and use it to understand the mechanism of 'conservative' Mu integration, and 4) explore similarities between the Mu and HIV systems using hybrid transposase-integrase proteins.