The inhibition of the SOS response by the mazEF pathway required the participation of EDF.

We determined the SOS response by measuring the fluorescence of the reporter plasmid pL(lexO)-gfp (A, B, C), and by LexA degradation (D, E, F). We compared E. coli strain MC4100relA⁺ (A and D) with strains MC4100relA⁺ΔclpX (A and D), MG1655 (B and E), or BW25113 (C and F); the strains in A, B, and C harbored plasmid pL(lexO)-gfp. We grew the cells in M9 media supplemented with ampicillin (100 µg/ml), with shaking. When the culture reached O.D.₆₀₀ 0.5–0.6, we added (or not) EDF (10 ng/ml) or iEDF (100 ng/ml). These cultures were incubated without shaking at 37°C for 30 min, after which we added NA (10 µg/ml) to each sample. Immediately after adding NA, we measured fluorescence (FU) by fluorometer or LexA degradation (as described in Materials and Methods) over a period of 4 hours. The values shown are relative to those of cells that had not been treated with NA. All data are representative of three independent experiments. The colors surrounding the blots in D, E, and F correspond to the colors representing the samples in A, B, and C.