Transcription-dependent recombination induced by triple-helix formation.

The homologous recombination between direct repeat sequences separated by either 200 or 1000 bp was induced by active transcription of the downstream gene when poly(dG)-poly(dC) sequences exist between the two direct repeats. This dG tract-mediated and transcription-induced recombination was RecA independent, and the frequency of recombination was dependent on both the length and the orientation of the poly(dG)-poly(dC) sequences relative to the gene. An intramolecular dG.dG.dC triplex formation was detected in Escherichia coli cells in a length-dependent manner when the transcription of the downstream gene was activated. We suggest that the negative superhelical strain generated by active transcription of the downstream gene induces poly(dG)-poly(dC) sequences to adopt a triple-helix structure in vivo and that this structure brings two remote sequences together to stimulate homologous recombination.