Structure-function relationships in the transposition protein B of bacteriophage Mu.

The B-protein of phage Mu, which is required for high frequency intermolecular transposition in vivo, shows ATPase activity in vitro, binds nonspecifically to DNA, and stimulates intermolecular strand transfer. To elucidate the structural bases for B-protein function, it was subjected to limited proteolysis with two different proteases, trypsin and chymotrypsin. The resulting fragments were mapped by amino acid sequencing. These data show that the B-protein is organized in two domains: an amino-terminal domain of 25 kDa and a carboxyl-terminal domain of 8-kDa. A fragment analogous to the amino-terminal domain, produced by deleting the 3′ end of a cloned B gene, proved to be insoluble and had to be renatured after elution from a sodium dodecyl sulfate gel. The renatured protein retains ATP-binding activity and to a lesser extent the DNA-binding activity of the MuB protein, but is unable to hydrolyze ATP or function in transposition. We also show in this study that efficient DNA-strand transfer by the B-protein occurs even in the absence of a detectable ATPase activity or in the presence of adenosine 5′-O-(thio)triphosphate (ATP gamma S).