The inhibition of the SOS response by the <i>mazEF</i> pathway required the participation of EDF.

<p>We determined the SOS response by measuring the fluorescence of the reporter plasmid pL(<i>lexO</i>)-<i>gfp</i> (<b>A, B, C</b>), and by LexA degradation (<b>D, E, F</b>). We compared <i>E. coli</i> strain MC4100<i>relA</i>⁺ (A and <b>D</b>) with strains MC4100<i>relA</i>⁺Δ<i>clpX</i> (<b>A</b> and <b>D</b>), MG1655 (<b>B</b> and <b>E</b>), or BW25113 (<b>C</b> and <b>F</b>); the strains in A, B, and C harbored plasmid pL(<i>lexO</i>)-<i>gfp</i>. 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 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0114380#s4" target="_blank">Materials and Methods</a>) 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.</p

The SOS Response is Permitted in <i>Escherichia coli</i> Strains Deficient in the Expression of the <i>mazEF</i> Pathway

<div><p>The <i>Escherichia coli</i> (<i>E. coli</i>) SOS response is the largest, most complex, and best characterized bacterial network induced by DNA damage. It is controlled by a complex network involving the RecA and LexA proteins. We have previously shown that the SOS response to DNA damage is inhibited by various elements involved in the expression of the <i>E. coli</i> toxin-antitoxin <i>mazEF</i> pathway. Since the <i>mazEF</i> module is present on the chromosomes of most <i>E. coli</i> strains, here we asked: Why is the SOS response found in so many <i>E. coli</i> strains? Is the <i>mazEF</i> module present but inactive in those strains? We examined three <i>E. coli</i> strains used for studies of the SOS response, strains AB1932, BW25113, and MG1655. We found that each of these strains is either missing or inhibiting one of several elements involved in the expression of the <i>mazEF</i>-mediated death pathway. Thus, the SOS response only takes place in <i>E. coli</i> cells in which one or more elements of the <i>E. coli</i> toxin-antitoxin module <i>mazEF</i> or its downstream pathway is not functioning.</p></div