Transcriptional R loops are anomalous RNA:DNA hybrids that have been detected in 
organisms from bacteria to humans. These structures have been shown in 
eukaryotes to result in DNA damage and rearrangements; however, the mechanisms 
underlying these effects have remained largely unknown. To investigate this, we 
first show that R-loop formation induces chromosomal DNA rearrangements and 
recombination in Escherichia coli, just as it does in eukaryotes. More 
importantly, we then show that R-loop formation causes DNA replication fork 
stalling, and that this in fact underlies the effects of R loops on genomic 
stability. Strikingly, we found that attenuation of replication strongly 
suppresses R-loop-mediated DNA rearrangements in both E. coli and HeLa cells. 
Our findings thus provide a direct demonstration that R-loop formation impairs 
DNA replication and that this is responsible for the deleterious effects of R 
loops on genome stability from bacteria to humans.