Inversions and deletions generated by a mini-gamma delta (Tn1000) transposon

Intramolecular transposition by an engineered derivative of the transposon gamma delta (Tn1000) is described. This 1-kb element contains inverted repeats of the 40 bp of the delta end of gamma delta, bracketing a kan gene, but it contains no resolution site. Transposition was analyzed in two plasmids; one contained two contraselectable (conditional lethal) genes (thyA and sacB) adjacent to the mini-gamma delta element in a 13.0-kb pBR322/pUC-based two-component plasmid (a heterodimer), and the other contained a different contraselectable gene (strA [rpsL]) in a 13.2-kb three-component plasmid (a heterotrimer). Selection for loss of function of a single contraselectable gene yielded inversions and deletions. Each inversion plasmid was 1 kb larger than the parent plasmid: it had a second copy of mini-gamma delta inserted in the contraselected gene, with that copy plus the intervening segment inverted, and the 5-bp target site duplicated. Each deletion plasmid was smaller than the parent plasmid and had a deletion that extended from one transposon end into or through the contraselected gene for distances of up to 9.4 kb. The frequencies of deletions versus inversions ending in a single target gene were similar, although overall, deletions outnumbered inversions because deletions, but not inversions, into sites beyond the contraselected gene inactivate it. This work also demonstrates that thyA (which encodes thymidylate synthetase) is a useful contraselectable marker.