TRANSCRIPTION-INDUCED DELETIONS IN ESCHERICHIA-COLI PLASMIDS

Characterization of functions that render DNA susceptible to rearrange ment is important for a better understanding of genome instability. In a previous work, we showed that sequences located downstream of a str ong promoter are particularly prone to deletion. in this paper, the pa rameters that influence transcription-induced deletions were studied. pBR322 derivatives carrying the M13 (+) replication origin and a P-Tac -dependent transcription region were used. Deletion formation was anal ysed in the presence of the replication protein of M13, which introduc es a nick at the phage replication origin, and in a rep(-) strain to a void M13-driven replication. Our study showed that: (i) 4 h after indu ction of transcription, a few per cent of the plasmids have experience d a deletion; (ii) these deletions result in joining of the M13 replic ation origin to a nucleotide located in or downstream of the transcrib ed region; (iii) deletion formation strongly depends on the orientatio n of transcription, on promoter strength and transcript length, but is independent of translation; (iv) formation of transcription-induced s upercoiling domains does not induce deletion formation. We propose tha t deletions in the transcribed region result from collisions between c onverging replication and transcription machineries.