Replication-transcription conflicts trigger extensive DNA degradation in Escherichia coli cells lacking RecBCD

and proceed in opposite directions with high speed and processivity until they fuse and terminate 19 in a specialised area opposite to oriC. Proceeding forks are often blocked by tightly-bound 20 protein-DNA complexes, topological strain or various DNA lesions. In Escherichia coli the 21 RecBCD protein complex is a key player in the processing of double-stranded DNA (dsDNA) ends. 22 It has important roles in the repair of dsDNA breaks and the restart of forks stalled at sites of 23 replication-transcription conflicts. In addition, ΔrecB cells show substantial amounts of DNA 24 degradation in the termination area. In this study we show that head-on encounters of replication 25 and transcription at a highly-transcribed rrn operon expose fork structures to degradation by 26 nucleases such as SbcCD. SbcCD is also mostly responsible for the degradation in the termination 27 area of ΔrecB cells. However, additional processes exacerbate degradation specifically in this 28 location. Replication profiles from ΔrecB cells in which the chromosome is linearized at two 29 different locations highlight that the location of replication termination can have some impact on 30 the degradation observed. Our data improve our understanding of the role of RecBCD at sites of 31 replication-transcription conflicts as well as the final stages of chromosome duplication. 32 However, they also highlight that current models are insufficient and cannot explain all the 33 molecular details in cells lacking RecBCD