Tn3 transposition immunity is conferred by the transposase-binding domain in the terminal inverted-repeat sequence of Tn3.

A series of mutant terminal inverted repeats (IRs), having 2 bp substitutions at various sites within the 38-bp IR sequence of the ampicillin-resistance transposon Tn3, were tested for transposition immunity to Tn3. Mutations within region 1-10 in the IR did not affect transposition immunity, while mutations within region 13-38 inactivated the immunity function. These two regions corresponded to domain A which was not bound specifically by Tn3 transposase and to domain B which was bound by the transposase, respectively. This indicates that specific binding of transposase to domain B within the IR sequence is responsible for transposition immunity.