Abstract
The large (14 kb; kb = 10
3 bases) bacterial transposon, Tn7 (encoding resistance to trimethoprim and streptomycin/spectinomycin), has unusual properties. Like other elements, Tn7 transposes with low efficiency and low target-site specificity, but Tn7 also transposes, with high frequency in a unique orientation, to a preferred “attachment” site, called
attTn7, in the
Escherichia coli chromosome and similarly into plasmids containing
attTn7. We developed a novel bacteriophage M13-based assay system to measure the transposition frequency of Tn7 to M13mp phage vectors containing
attTn7 on a cloned 1 kb fragment of chromosomal DNA. Phage harvested from a Tn7 donor strain were used to infect recipient bacteria with selection for trimethoprim resistance. Transposition frequency, expressed as the number of trimethoprim-resistant colonies per plaque-forming unit, was found to be ~10
−4 to M13mp: :
attTn7, in contrast to 10
−10 to M13mp recombinants with ~1 kb insertions of other, “generic brand”, DNA. By deletion analysis of M13mp: :
attTn7, we show that
attTn7 is contained within a 64 base-pair region; sequences adjacent to the actual insertion site and encoding the carboxy terminus of the
glmS gene are required. This assay also provided evidence for transposition immunity conferred by the right end of Tn7.