Controlling Genome Topology with Sequences that Trigger Post-replication Gap Formation During Replisome Passage:  The E. coli RRS Elements
We report that the Escherichia coli chromosome includes novel GC-rich genomic structural elements that trigger formation of post-replication gaps upon replisome passage. The two nearly perfect 222 bp repeats, designated Replication Risk Sequences or RRS, are each 650 kbp from the terminus sequence dif and flank the Ter macrodomain. RRS sequence and positioning is highly conserved in enterobacteria. At least one RRS appears to be essential unless a 200 kbp region encompassing one of them is amplified. The RRS produce lagging strand ssDNA gaps, about 2000 bp long, upon replisome passage, regardless of genome placement. Deletion of both RRS elements has substantial effects on global genome structure and topology. We hypothesize that RRS elements serve as topological relief valves during chromosome replication and segregation. There have been no screens for genomic sequences that trigger transient gap formation. Functional analogs of RRS could be widespread, possibly including some enigmatic G-quadruplexes.