Tn7 transposition creates a hotspot for homologous recombination at the transposon donor site

Abstract

Homologous recombination at the bacterial transposon Tn7 donor site is stimulated 10-fold when Tn7 is activated to transpose at high frequency in RecD- Escherichia coli, where recombination is focused near the ends of double-chain breaks. This is observed as an increase in recombination between two lacZ heteroalleles when one copy of lacZ carries within it a Tn7 that is transposing at high frequency. This stimulation of recombination is dependent upon the presence of homology with the donor site, is independent of SOS induction, and is not due to a global stimulation of recombination. When stimulated by Tn7 transposition, the conversion events giving rise to Lac+ recombinants occur preferentially at the site of Tn7, suggesting that transposition is stimulating gene conversion at the donor site. These results support the model that Tn7 transposition occurs by a 'cut and paste' mechanism, leaving a double-chain break at the donor site that is repaired by the host homologous recombination machinery; normally, repair would use homology in a sister chromosome to regenerate a copy of the transposon. This proposed series of events allows transposition that is nonreplicative, per se, to be effectively replicative.