a, Differential interference contrast (DIC) cell images of strains TB28 (wt), CH34/pBL145 (∆ftsN / cIts PλR::ftsN1–90), PM17 (ftsWE289G), PM18 (ftsWE289G ∆ftsN), and PM6 (ftsIR167S). Cells were grown overnight in LB medium at 37 °C (CH34/pBL145) or 30 °C (all other sttrains), diluted to OD600 = 0.02 in LB, and grown to OD600 = 0.6–0.7 at 30 oC (causing depletion of FtsN in the CH34/pBL145 cells shown). Scale bars: 4 µm. b, DIC cell images of strains TB77 (ftsNslm117), and PM4 (ftsNslm117 ftsIR167S). Overnight cultures were diluted to OD600 = 0.05 in M9-glucose medium and grown to OD600 = 0.5–0.6 at 30 oC. Note that the ftsNslm117 allele corresponds to an EZTnKan-2 transposon insertion in codon 119 of ftsN, leading to a pronounced, but non-lethal, cell constriction defect. Also note that the ftsIR167S allele in strain PM4 largely overcomes this defect. Scale bars: 8 µm. Representative micrographs were from three independent experiments. For the number of cells and characterization of cell morphology, see Supplementary Table 7.