Instability of plasmid DNA maintenance caused by transcription of poly(dT)-containing sequences in Escherichia coli.

A series of pUC19-derived plasmids which contain a (dA)34 x (dT)34 tract was constructed to examine the effect of this sequence on plasmid maintenance in Escherichia coli. When this sequence was placed downstream from the lacZ promoter (lacZp), plasmids transcribing the (dT)34 strand were rapidly lost from the cells irrespective of the orientation relative to the replication origin, while plasmids transcribing the (dA)34 strand were stably maintained. A plasmid with a deletion within the lacZp region, preventing transcription from the (dT)34 strand, exhibited no such instability. The apparent transcription-dependent plasmid instability was not observed for the TcR (tetracycline-resistance gene) promoter which has weaker activity than lacZp. Similar strand-specific instability was observed for another microsatellite sequence, (dG)34 x (dC)34. These findings suggest that accumulation of transcription products of the poly(dT) sequences could block DNA replication.