Characterization of a pathway of genomic instability induced by R-loops and its regulation by topoisomerases in E. coli.

The prototype enzymes of the ubiquitous type IA topoisomerases (topos) family are Escherichia coli topo I (topA) and topo III (topB). Topo I shows preference for relaxation of negative supercoiling and topo III for decatenation. However, as they could act as backups for each other or even share functions, strains lacking both enzymes must be used to reveal the roles of type IA enzymes in genome maintenance. Recently, marker frequency analysis (MFA) of genomic DNA from topA topB null mutants revealed a major RNase HI-sensitive DNA peak bordered by Ter/Tus barriers, sites of replication fork fusion and termination in the chromosome terminus region (Ter). Here, flow cytometry for R-loop-dependent replication (RLDR), MFA, R-loop detection with S9.6 antibodies, and microscopy were used to further characterize the mechanism and consequences of over-replication in Ter. It is shown that the Ter peak is not due to the presence of a strong origin for RLDR in Ter region; instead RLDR, which is partly inhibited by the backtracking-resistant rpoB*35 mutation, appears to contribute indirectly to Ter over-replication. The data suggest that RLDR from multiple sites on the chromosome increases the number of replication forks trapped at Ter/Tus barriers which leads to RecA-dependent DNA amplification in Ter and to a chromosome segregation defect. Overproducing topo IV, the main cellular decatenase, does not inhibit RLDR or Ter over-replication but corrects the chromosome segregation defect. Furthermore, our data suggest that the inhibition of RLDR by topo I does not require its C-terminal-mediated interaction with RNA polymerase. Overall, our data reveal a pathway of genomic instability triggered by R-loops and its regulation by various topos activities at different steps.