Limited concentration of RecA delays DNA double-strand break repair in R1

Jolivet, E.*; Lecointe, F.*; Coste, G.*; Sato, Katsuya*; Narumi, Issei; Bailone, A.*; Sommer, S.*

To evaluate the importance of RecA in DNA double strand break (DSB) repair, we examined the effect of low and high RecA concentration such as 2,500 and 100,000 molecules per cell from the inducible P promoter in in absence or in presence of IPTG, respectively. We showed that at low concentration, RecA has a negligible effect on cell survival after -irradiation when bacteria were immediately plated on TGY agar whereas it significantly decreased the survival to -irradiation of cells while overexpression of RecA can partially compensate the loss of DdrA protein. In contrast, when cells expressing limited concentration of RecA were allowed to recover in TGY liquid medium, they showed a delay in mending DSB, failed to reinitiate DNA replication and were committed to die during incubation in liquid medium. A deletion of resulted in sensitivity to -irradiation and mitomycin C treatment. Interestingly, constitutive high expression of RecA compensates partially the sensitization to mitomycin C. The cells with low RecA content also failed to cleave LexA after DNA damage. However, neither a deletion of the gene nor the expression of a non cleavable LexA(Ind) mutant protein, had an effect on survival or kinetics of DNA DSB repair compared to their counterparts in as well as in bacteria expressing limiting concentration of RecA, suggesting an absence of relationship between the loss of viability, the delay in the kinetic of DSB repair, and the absence of LexA cleavage. Thus, LexA protein seems to play no major role in the recovery processes after -irradiation in .