Ags1 is responsible for the secondary septum formation and the correct primary septum structure. (A) WT cell morphology and septum formation details. (B–D) Absence of Ags1 generates (B) twisted primary septa, (C) septa with no secondary septum, and (D) a remedial internal cell wall and secondary septum layer. Cells were grown in EMM+T for 3 (−S) or 6–9 h (+S) and analyzed by TEM. ICW, internal cell wall; Mtd, materiel triangulaire dense; Mtd?, diffuse Mtd; Ps, primary septum; RSs, remedial secondary septum; Ss, secondary septum; arrow, primary septum end anchoring (WT) or defective anchoring (Ags1 absence) into the cell wall. Bars: (WT, top) 1 µm; (rest of panels) 0.5 µm.