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).(dT)(34) t ract was constructed to examine the effect of this sequence on plasmid maintenance in Escherichia coli. When this sequence was placed downst ream from the lacZ promoter (lacZp), plasmids transcribing the (dT)(34 ) strand were rapidly lost from the cells irrespective of the orientat ion relative to the replication origin, while plasmids transcribing th e (dA)(34) strand were stably maintained. A plasmid with a deletion wi thin the lacZp region, preventing transcription from the (dT)(34) stra nd, exhibited no such instability. The apparent transcription-dependen t plasmid instability was not observed for the TCR (tetracycline-resis tance gene) promoter which has weaker activity than lacZp. Similar str and-specific instability was observed for another microsatellite seque nce, (dG)(34).(dC)(34). These findings suggest that accumulation of tr anscription products of the poly(dT) sequences could block DNA replica tion.