The stable carriage of two TnA units on a single replicon

Bacterial plasmids which contain a copy on TnA are refactory to the uptake of a second by transposition. However plasmids containing two such copies can be constructed by in vitro recombination techniques. Some plasmids containing two copies of TnA have been obtained by conventional transposition, but in all cases they arose by the virtually simultaneous insertion of both units into a replicon that carried no TnA. All stable plamids containing two copies of TnA carried the transposons in opposite orientation.