Identification of a transposon Tn3 sequence required for transposition immunity

A plasmid containing transposon Tn3 is immune to further insertions of Tn3. This phenomenon works in cis and is referred to as transposition immunity. We have used the ability of Tn3 to form cointegrates between two plasmids to develop a quantitative assay to detect transposition immunity. Presence of Tn3 on both the plasmids reduces the cointegration frequency to less than 1/100 of parental. Using this assay, we have determined that (i) tnpR is not required for immunity, (ii) only the terminal 38 base pairs of Tn3 need be present to confer immunity, and (iii) other parts of Tn3 appear not to confer immunity.