Direct visualisation of post-replication gap formation at the bacterial RRS

Replication risk sequences (RRS) are recently discovered genomic structural elements that trigger post-replication gap formation during replisome passage. In E. coli, the two RRS elements are nearly perfect 222-bp G-quadruplex-containing repeats that flank the terminal domain and are highly conserved in both sequence and genomic position across enterobacteria. We report here the first direct visualisation of RRS function in vitro using single-molecule methods. When the G4 strand of the RRS element is positioned on the lagging-strand template, gaps are formed essentially every time a replisome encounters it. An increase in ssDNA in the synthesised DNA is readily seen using ssGAP-seq methods. When the G-quadruplex strand of the RRS is positioned on the leading-strand template, gaps are formed, albeit at lower frequency. However, the continued DNA synthesis in a rolling-circle assay indicates that the gaps are still formed on the lagging strand, indicating that the RRS complementary strand has a significant but reduced capacity to form a structure that triggers lesion skipping. The results document the potency of the RRS as a trigger for gap formation, suggesting a possible function for at least some eukaryotic G-quadruplexes.