Figure 6.

Double-stranded DNA breaks and DNA repair stress in PNA-treated cells. Strain SMR14350 was grown exponentially in AB medium, diluted to OD₄₅₀ = 0.05 and treated with NrdA-PNA (8 μM), 2-base mismatch-PNA (8 μM) or ciprofloxacin (0.0125 mg/L) or left non-treated for 4 h. (a) In vivo visualization of double-stranded DNA breaks by Gam-GFP foci. Scale bar is 5 μm. (b) Gam-GFP foci/cell was determined for three independent experiments [non-treated, n = 3687; NrdA-PNA, n = 3860; 2-base mismatch-PNA, n = 2811; ciprofloxacin (CIP), n = 2553]. Data are given as percentage of cells with Gam-GFP foci. Shown is the mean±SD based on three determinations. ns, not significant; ***, P<0.01. (c) Susceptibility of cells deficient in major DNA damage repair pathways towards NrdA-PNA. All the cells were derivatives of MG1655 and were grown exponentially in MHB-I medium, diluted to about 1–5 × 10⁵ cfu/mL and treated with 4 μM NrdA-PNA for 5 h. WT (crosses), ΔrecA (filled circles), ΔrecBCD (open circles), ΔruvABC (filled squares) and ΔrnhB (open squares). Cell viability is displayed as cfu/mL. For details, see the Materials and methods section. Shown is the mean±SD based on three determinations.