Abstract
Probably one of the most controversial questions about the cell division of Bacillus subtilis, a rod-shaped bacterium, concerns the mechanism that ensures correct division septum placement–at mid-cell during vegetative growth but closer to one end during sporulation. In general, bacteria multiply by binary fission, in which the division septum forms almost exactly at the cell centre. How the division machinery achieves such accuracy is a question of continuing interest. We understand in some detail how this is achieved during vegetative growth in Escherichia coli and B. subtilis, where two main negative regulators, nucleoid occlusion and the Min system, help to determine the division site, but we still do not know exactly how the asymmetric septation site is determined during sporulation in B. subtilis. Clearly, the inhibitory effects of the nucleoid occlusion and Min system on polar division have to be overcome. We evaluated the positioning of the asymmetric septum and its accuracy by statistical analysis of the site of septation. We also clarified the role of SpoIIE, RefZ and MinCD on the accuracy of this process. We determined that the sporulation septum forms approximately 1/6 of a cell length from one of the cell poles with high precision and that SpoIIE, RefZ and MinCD have a crucial role in precisely localizing the sporulation septum. Our results strongly support the idea that asymmetric septum formation is a very precise and highly controlled process regulated by a still unknown mechanism.
Highlights
The cell division processes in E. coli and B. subtilis have been studied intensively for decades
The first clear morphological feature of sporulation in B. subtilis is the polar cell division that starts with migration of the Z-ring from mid-cell to the two cell poles on a spiral trajectory, in a process that depends on the sporulation-specific overexpression of ftsAZ and the presence of SpoIIE [21]
SpoIIE localization serves as a good marker for determining the septation site, so we used the localization of a SpoIIE-Ypet fusion to determine the position of the sporulation division site
Summary
The cell division processes in E. coli and B. subtilis have been studied intensively for decades. The earliest visible event in cell division is the formation of a Z-ring by FtsZ, a tubulin like protein, at the future septum site. At least two distinct mechanisms contribute to the precise placement of the division machinery: nucleoid occlusion and the Min system [1]. In B. subtilis, the main player of nucleoid occlusion is Noc; a DNA associated protein that blocks division from taking place over the chromosome [2]. The second mechanism, Min system, includes four proteins: MinC, MinD, DivIVA, and MinJ [3]. MinC is the actual cell division inhibitor that directly binds to FtsZ and is activated by MinD, a membrane-associated ATPase [4].
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