One of the transposition proteins of Tn7, TnsB, was overexpressed, and purified substantially. The protein has a subunit molecular weight of around 85 kDA, which is consistent with prediction from the DNA sequence, and appears to exist as a monomer in solution. The identity of the polypeptide was confirmed by N-terminal sequencing, which also demonstrated that the site of translational initiation is the first AUG in the reading frame, and that there is no N-terminal processing of the protein.