Fig 3. Models of division site positioning during vegetative growth and sporulation in B. subtilis.

(A) During vegetative growth, upon initiation of DNA replication, the future division site is marked by a putative, but not yet identified, mid-cell defining factor. Upon segregation of the chromosomes (shown in grey inside of the cells), the nucleoid occlusion factors (blue lines above the nucleoids) clear the mid-cell site, while the Min system (shown in red) blocks Z-ring formation at the cell poles. The septum is formed with high precision at the mid-cell site. (B) During sporulation, the Z-ring forms approximately ¹/₆ of a cell length from one of the cell poles with high precision. In this cell cycle stage, the replicated chromosomes are in the form of axial filaments, and the Min system proteins are localized at the cell poles. It is not known how Z-ring formation prevails over the negative regulation of nucleoid occlusion and the Min system. (C) SpoIIE has a crucial role in precisely localizing the sporulation septum; in its absence, the asymmetric septum forms in an average of ¹/₄ a cell length from the nearest cell pole. This septum is thicker than the wild type sporulation septum and resembles the vegetative like septum. ΔSpoIIE cells cannot form spores. (D) RefZ has a moderate influence on the localization of the sporulation septum; in its absence the septation position moves farther from the cell pole, to an average position of ¹/₅ of the cell length. (E) MinCD influences the localization of the sporulation septum, and in cells lacking these proteins the septum position moves much closer to the cell pole, to an average position of ¹/₈ a cell length. In addition, cells lacking MinCD can also position the sporulation septum close to the mid-cell site, with lower frequency and precision. (F) This model shows how the Z- and E-ring can recognize the asymmetric site of septation within a narrow window formed by the negative cell division regulators RefZ and MinCD. However, the possible existence of an additional positive regulator, which helps to form the Z-ring specifically at this site cannot be ruled out.