Evidence that coupling sequences play a frequency-determining role in conjugative transposition of Tn916 in Enterococcus faecalis

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Abstract
The conjugative transposon Tn916 (encodes resistance to tetracycline), originally identified in Enterococcus faecalis, moves by an excision-insertion process in which the rate-limiting step is believed to be excision. Individual transposon-containing strains exhibit characteristic mating frequencies which range over several orders of magnitude; the basis of this phenomenon is addressed in the present study. We were able to generate independent single-copy insertions in identical target locations and with similar orientations within a plasmid hemolysin determinant (cylA); however, transposition from this site occurred at very different frequencies (10(-8) to 10(-4) per donor) depending on the individual isolate. DNA sequencing analyses showed that the coupling (junction) sequences differed between isolates and thus appeared to be responsible for differences in excision frequencies. Other experiments showed that inducible transcription into either end of the transposon had no significant effect on transfer.