Mitomycin-C-induced changes in the nucleoid of Escherichia coli K12

The SMC-like RecN protein is at the crossroads of several genotoxic stress responses in Escherichia coli

Introduction

DNA damage repair (DDR) is an essential process for living organisms and contributes to genome maintenance and evolution. DDR involves different pathways including Homologous recombination (HR), Nucleotide Excision Repair (NER) and Base excision repair (BER) for example. The activity of each pathway is revealed with particular drug inducing lesions, but the repair of most DNA lesions depends on concomitant or subsequent action of the multiple pathways.

Methods

In the present study, we used two genotoxic antibiotics, mitomycin C (MMC) and Bleomycin (BLM), to decipher the interplays between these different pathways in E. coli. We combined genomic methods (TIS and Hi-SC2) and imaging assays with genetic dissections.

Results

We demonstrate that only a small set of DDR proteins are common to the repair of the lesions induced by these two drugs. Among them, RecN, an SMC-like protein, plays an important role by controlling sister chromatids dynamics and genome morphology at different steps of the repair processes. We further demonstrate that RecN influence on sister chromatids dynamics is not equivalent during the processing of the lesions induced by the two drugs. We observed that RecN activity and stability requires a pre-processing of the MMC-induced lesions by the NER but not for BLM-induced lesions.

Discussion

Those results show that RecN plays a major role in rescuing toxic intermediates generated by the BER pathway in addition to its well-known importance to the repair of double strand breaks by HR.

DNA damage-dependent recruitment of nucleotide excision repair and transcription proteins to Escherichia coli inner membranes

The entire process of nucleotide excision repair (NER) in Escherichia coli has been reconstituted in vitro from purified proteins and defined DNA substrates. However, how this system is organized in vivo in unclear. We report here the isolation and characterization of macromolecular assemblies containing NER and transcription proteins from E. coli. This ensemble consists of at least 17 proteins. They are recruited, as a consequence of DNA damage induced by UV irradiation, to the inner membrane. The UV-induced 6-4 photoproducts are also relocated to the inner membrane following UV-irradiation of the cells. This recruitment process is dependent on the uvrA, uvrC and recA gene products. These results suggest that at least part of the repair process may associate with the inner membrane and also provide insights into understanding the cellular organization of repair processes.

Effects of quinolones on nucleoid segregation in Escherichia coli

The effects of quinolone antibiotics on nucleoid segregation in growing Escherichia coli were examined by using fleroxacin (Ro 23-6240, AM 833) as a prototype compound. At levels that were close to its MIC and induced growth arrest and filamentation, fleroxacin caused large nucleoids to appear in midcell, suggesting inhibition of nucleoid segregation. With increasing fleroxacin concentrations, nucleoids became progressively smaller, suggesting inhibition of DNA replication. Removal of fleroxacin restored normal cell and nucleoid morphology in filaments with large nucleoids but not in filaments with small nucleoids. The results are consistent with inhibition of chromosome decatenation at low quinolone concentrations (bacteriostatic effect) and DNA supercoiling at high concentrations (bactericidal effect).