Separate structural and functional domains of Tn4430 transposase contribute to target immunity

Like other transposons of the Tn3 family, Tn4430 exhibits target immunity, a process that prevents multiple insertions of the transposon into the same DNA molecule. Immunity is conferred by the terminal inverted repeats of the transposon and is specific to each element of the family, indicating that the transposase TnpA is directly involved in the process.However, the molecular mechanism whereby this protein promotes efficient transposition into permissive targets while preventing transposition into immune targets remains unknown. Here, we demonstrate that both functions of TnpA can be uncoupled from each other by isolating and characterizing mutants that are proficient in transposition (T+) but impaired in immunity (I-). The identified T+/I- mutations are clustered into separate structural and functional domains of TnpA, indicating that different activities of the protein contribute to immunity.Combination of separate mutations had synergistic effects on target immunity but contrasting effects on transposition. One class of mutations was found to stimulate transposition, whereas other mutations appeared to reduce TnpA activity. The data are discussed with respect to alternative models in which TnpA acts as a specific determinant to both establish and respond to immunity.