FIG 6.

Effect of polyploidy level on UV resistance of the cells. (A) Schematic diagram of the assay. A mid-log-phase DnaA^WT or DnaA^R328H culture was inoculated into fresh inorganic medium and then cultured for 6 h at the indicated temperature under 70-µE m⁻² s⁻¹ illumination. Cell density was adjusted to an OD₇₅₀ of 0.2, and a serial dilution series (10⁻¹ to 10⁻⁵) was prepared using fresh medium. The diluted cells were spotted (for panel B) and plated (for panels C and D) on agar medium and then irradiated under 0- to 600-J m⁻² UV-C (254 nm), respectively. The plates were further incubated at 30°C under illumination (70 µE m⁻² s⁻¹) for 7 days to allow surviving cells to form colonies. (B and C) The cell survival rate was calculated by counting the colony formation units. Results of DnaA^WT and DnaA^R328H cells cultured at 20°C, 30°C, or 40°C under 300-J m⁻² UV-C irradiation are shown. Error bars represent the standard deviation (n = 3 biological replicates). Two-tailed t tests between the indicated strains or conditions were performed. *, P < 0.05; **, P < 0.01. (D) Flow cytometry analysis showing the DNA level at hour 6 in DnaA^WT and DnaA^R328H cells cultured at 20°C, 30°C, or 40°C. Black line, DnaA^WT; blue line, DnaA^R328H; A.U., arbitrary unit. (E) The survival rate of the wild-type cells was determined after irradiation with 0- to 600-J m⁻² UV-C at 20, 30, or 40°C. Error bars represent the standard deviation (n = 3 biological replicates).