Nuclear segregation in Bacillus subtilis

Abstract

JACOB, BRENNER AND CUZIN¹ suggested that prokaryote nuclear segregation was achieved by cell surface extension between surface sites to which the chromosome is permanently attached (Fig. 1a). Their model (model A) showed nuclear segregation occurring in newly divided cells, with all surface extension in one cycle occurring between the nuclei. Thus at cell division, nuclei would be arranged symmetrically in the half cell but asymmetrically during the interdivisional period. If nuclear segregation occurs during mid-cycle and cell extension is continuous throughout the cycle, the nucleus cannot reach a symmetrical position (that is at 25% of the cell length) by the end of the cycle relying solely on length extension. To overcome these difficulties, Clark² proposed that the nucleus is always located at the junction of old and new membranes with growth occurring on one side of the nucleus (model B, Fig. 1b). At nuclear segregation, the growing point divides allowing growth of the cell envelope between the nuclei as shown in Fig. 1b. Nuclei remain attached at the site of envelope extension, giving a newborn cell with an asymmetrically arranged nucleus. Donachie and Begg³ have presented evidence for terminal growth regions in slow growing cells as predicted by Clark's model. A third possibility is that the nucleus is located centrally in new born cells and moves to a position at the centre of a half cell at the time of segregation (model C, Fig. 1c). These, and other possibilities, can be explored by determining the position of nuclei in relation to cell length in exponential phase cells.