Model for the role of Noc and Min in ensuring efficient utilization of the division site in <i>B. subtilis</i>.

<p>(A) Early in the cell cycle, possibly upon completion of the initiation phase of DNA replication (see Moriya <i>et al</i>, 2010) a midcell-defining factor marks the position of the future division site at which the Z ring will assemble. At this stage, midcell becomes competent for Z ring assembly. However the early utilization of this division site by the Z ring is blocked by nucleoid occlusion (demonstrated by the red arrow). (Bi) In wild-type cells, the midcell site becomes unmasked (in green) upon segregation of the bulk of the replicated chromosomes (shown in grey). This causes Noc and other nucleoid occlusion factors to clear the central region of the cell. (Bii) As the cells elongate, other division sites competent for Z ring assembly become available at the cell quarters. These sites are masked (in red) by the combined activity of Noc (and possibly other nucleoid occlusion factors), and Min that acts at a distance from the pole, allowing FtsZ to concentrate to the midcell division site only. The red elongated triangles below each cell correspond to the position and concentration of Min (higher at the poles). (Biii) The quarter (potential) division sites only become available after Z ring constriction is initiated at the first (midcell) site, allowing separated daughter cells to initiate a new division cycle. (Ci) In the absence of Noc and MinCD, despite midcell being competent for Z ring assembly, utilization of this site is delayed due to the titration of FtsZ to the cell poles, and the inability of FtsZ to reach a high enough threshold concentration at midcell to form a ring. (Cii) As the cells grow, FtsZ can eventually start to accumulate at midcell, but also at other division sites that would normally be blocked by Noc and MinCD (in yellow). (Ciii) Z ring assembly at the medial division site (midcell) occurs very late, well after wild-type cells would have already completed a round of division, causing a severe delay in cell division. The dashed green lines along the length of the cell indicate the constant dynamic movement of FtsZ throughout the cell.