RdgB acts to avoid chromosome fragmentation in Escherichia coli

Summary Bacterial RecA protein is required for repair of two‐strand DNA lesions that disable whole chromosomes. recA mutants are viable, suggesting a considerable cellular capacity to avoid these chromosome‐disabling lesions. recA‐dependent mutants reveal chromosomal lesion avoidance pathways. Here we characterize one such mutant, rdgB/yggV, deficient in a putative inosine/xanthosine triphosphatase, conserved throughout kingdoms of life. The rdgB recA lethality is suppressed by inactivation of endonuclease V (gpnfi) specific for DNA‐hypoxanthines/xanthines, suggesting that RdgB either intercepts improper DNA precursors dITP/dXTP or works downstream of EndoV in excision repair of incorporated hypoxathines/xanthines. We find that DNA isolated from rdgB mutants contains EndoV‐recognizable modifications, whereas DNA from nfi mutants does not, substantiating the dITP/dXTP interception by RdgB. rdgB recBC cells are inviable, whereas rdgB recF cells are healthy, suggesting that chromosomes in rdgB mutants suffer double‐strand breaks. Chromosomal fragmentation is indeed observed in rdgB recBC mutants and is suppressed in rdgB recBC nfi mutants. Thus, one way to avoid chromosomal lesions is to prevent hypoxanthine/xanthine incorporation into DNA via interception of dITP/dXTP.