SummaryWe have identified the Bacillus subtilis homologue of the essential cell division gene, ftsL, of Escherichia coli. Repression of ftsL in a strain engineered to carry a conditional promoter results in cell filamentation, with a near immediate arrest of cell division. The filaments show no sign of invagination, indicating that division is blocked at an early stage. FtsL is also shown to be required for septation during sporulation, and depletion of FtsL blocks the activation but not the synthesis of the prespore-specific sigma factor, F . Immunofluorescence microscopy shows that depletion of FtsL has little or no effect on FtsZ ring formation, but the assembly of other division proteins, DivIB and DivIC, at the site of division is prevented. Repression of FtsL also results in a rapid loss of DivIC protein, indicating that DivIC stability is dependent on the presence of FtsL, in turn suggesting that FtsL is intrinsically unstable. The instability of one or more components of the division apparatus may be important for the cyclic assembly/disassembly of the division apparatus.

SummaryBacteria that have a circular chromosome with a bidirectional DNA replication origin are thought to utilize a 'replication fork trap' to control termination of replication. The fork trap is an arrangement of replication pause sites that ensures that the two replication forks fuse within the terminus region of the chromosome, approximately opposite the origin on the circular map. However, the biological significance of the replication fork trap has been mysterious, as its inactivation has no obvious consequence. Here we review the research that led to the replication fork trap theory, and we aim to integrate several recent findings that contribute towards an understanding of the physiological roles of the replication fork trap. Likely roles include the prevention of over-replication, and the optimization of post-replicative mechanisms of chromosome segregation, such as that involving FtsK in Escherichia coli.

SummarySpores of a thymine-requiring strain of Bacillus subtilis 168, which is also temperature sensitive for the initiation of chromosome replication, were germinated and allowed to grow out at the permissive temperature in a minimal medium containing no added thymine. Under these conditions, there was no or very limited progression into the elongation phase of the ®rst round of replication. In a signi®cant proportion of the outgrown cells, a Z ring formed precisely at mid-cell and over the centrally positioned nucleoid, leading eventually to the formation of a mature division septum. When initiation of the ®rst round of replication was blocked through a temperature shift and with thymine present, the Z ring was positioned acentrally. The central Z ring that formed in the absence of thymine was blocked by the presence of a DNA polymerase III inhibitor. It is concluded that the very early stages of a round of replication (initiation plus possibly limited progression into the elongation phase) play a key role in the precise positioning of the Z ring at mid-cell and between replicating daughter chromosomes.

SummaryWe have shown previously that, when spores of a thymine-requiring strain of Bacillus subtilis were grown out in the absence of thymine, mid-cell Z rings formed over the nucleoid and much earlier than might be expected with respect to progression into the round of replication. It is now shown that such conditions allow no replication of oriC. Rather than replication, partial degradation of the oriC region occurs, suggesting that the status of this region is connected with the`premature' mid-cell Z ring assembly. A correlation was observed between entry into the replication elongation phase and a block to mid-cell Z rings. The conformation of the nucleoid under various conditions of DNA replication inhibition or limitation suggests that relief of nucleoid occlusion is not primarily responsible for mid-cell Z ring formation in the absence of thymine. We propose the existence of a specific structure at mid-cell that defines the Z ring nucleation site (NS). It is suggested that this NS is normally masked by the replisome upon initiation of replication or soon after entry into the elongation phase, and subsequently unmasked relatively late in the round. During spore outgrowth in the absence of thymine, this checkpoint control over mid-cell Z ring assembly breaks down prematurely.

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