Absence of cis-acting transposition immunity with Tn7.

It is possible in two steps to insert into the plasmid RP4 two copies of the transposon Tn7. This was demonstrated using a wild-type Tn7 in the first step, and a Tn7 derivative (carrying an additional marker), in the second step. The two successive transpositions occurred with the same polarity and frequency. The genetic structures of the resulting plasmids, predicted from the phenotypes of the bacterial host, were confirmed by direct analysis of the plasmid DNAs. Thus, the phenomenon of cis-acting transposition immunity, described with Tn1 or Tn3, does not take place in the case of Tn7.