Effects of RNA polymerase modifications on transcription-induced negative supercoiling and associated R-loop formation

Transcription in the absence of topoisomerase I, but in the presence of DNA gyrase, can result in the formation of hypernegatively supercoiled DNA and associated R-loops. In this paper, we have used several strategies to study the effects of elongation/termination properties of RNA polymerase on such transcription-induced supercoiling. Effects on R-loop formation were exacerbated when cells were exposed to translation inhibitors, a condition that stimulated the accumulation of R-loop-dependent hypernegative supercoiling. Translation inhibitors were not acting by decreasing (p)ppGpp levels as the absence of (p)ppGpp in spoT relA mutant strains had little effect on hypernegative supercoiling. However, an rpoB mutation leading to the accumulation of truncated RNAs considerably reduced R-loop-dependent hypernegative supercoiling. Transcription of an rrnB fragment preceded by a mutated and inactive boxA sequence to abolish the rrnB antitermination system also considerably reduced R-loop-dependent supercoiling. Taken together, our results indicate that RNA polymerase elongation/termination properties can have a major impact on R-loop-dependent supercoiling. We discuss different possibilities by which RNA polymerase directly or indirectly participates in R-loop formation in Escherichia coli. Finally, our results also indicate that what determines the steady-state level of hypernegatively supercoiled DNA in topA null mutants is likely to be complex and involves a multitude of factors, including the status of RNA polymerase, transcription-translation coupling, the cellular level of RNase HI, the status of DNA gyrase and the rate of relaxation of supercoiled DNA.