Genes for cell division have been identified in Escherichia coli by the isolation of conditional lethal mutations that block cell division, but do not affect DNA replication or segregation. Of these genes, ftsZ is of great interest as it acts earliest in the division pathway, is essential, its level dictates the frequency of division, and it is thought to be the target of two cell-division inhibitors, SulA, produced in response to DNA damage, and MinCD, which prevents division at old sites. Here we have used immunoelectronmicroscopy to localize the FtsZ protein to the division site. The results suggest that FtsZ self-assembles into a ring structure at the future division site and may function as a cytoskeletal element. The formation of this ring may be the point at which division is regulated.

The positioning of a cytoskeletal element that dictates the division plane is a fundamental problem in biology. The assembly and positioning of this cytoskeletal element has to be coordinated with DNA segregation and cell growth to ensure that equal-sized progeny cells are produced, each with a copy of the chromosome. In most prokaryotes, cytokinesis involves positioning a Z ring assembled from FtsZ, the ancestral homologue of tubulin. The position of the Z ring is determined by a gradient of negative regulators of Z-ring assembly. In Escherichia coli, the Min system consists of three proteins that cooperate to position the Z ring through a fascinating oscillation, which inhibits the formation of the Z ring away from midcell. Additional gradients of negative regulators of FtsZ assembly are used by E. coli and other bacteria to achieve spatial control of Z-ring assembly.

Placement of the Z ring at midcell in Escherichia coli is assured by the action of the min system, which blocks usage of potential division sites that exist at the cell poles. This activity of min is achieved through the action of an inhibitor of division, MinC, that is activated by MinD and topologically regulated by MinE. In this study, we have used a functional GFP–MinC fusion to monitor the location of MinC. We find that GFP–MinC is a cytoplasmic protein in the absence of the other Min proteins. The addition of MinD, a peripheral membrane protein that interacts with MinC, results in GFP–MinC appearing on the membrane. In the presence of both MinD and MinE, GFP–MinC oscillates rapidly between the halves of the cell. Thus, MinC is positioned by the other Min products, but in a dynamic manner so that it is in position to inhibit Z ring assembly away from midcell.

SummaryThe cytokinetic Z ring is required for bacterial cell division. It consists of polymers of FtsZ, the bacterial ancestor of eukaryotic tubulin, linked to the cytoplasmic membrane. Formation of a Z ring in Escherichia coli occurs as long as one of two proteins, ZipA or FtsA, is present. Both of these proteins bind FtsZ suggesting that they might function to tether FtsZ filaments to the membrane. Although ZipA has a transmembrane domain and therefore can function as a membrane anchor, interaction of FtsA with the membrane has not been explored. In this study we demonstrate that FtsA, which is structurally related to eukaryotic actin, has a conserved C-terminal amphipathic helix that is essential for FtsA function. It is required to target FtsA to the membrane and subsequently to the Z ring. As FtsA is much more widely conserved in bacteria than ZipA, it is likely that FtsA serves as the principal membrane anchor for the Z ring.

FtsZ forms a cytokinetic ring, designated the Z ring, that directs cytokinesis in prokaryotes. It has limited sequence similarity to eukaryotic tubulins and, like tubulin, it has GTPase activity and the ability to assemble into various structures including protofilaments, bundles and minirings. By using both electron microscopy and sedimentation, we demonstrate that FtsZ from Escherichia coli undergoes a strictly GTPdependent polymerization and the polymers disappear as the GTP is consumed. Thus, FtsZ polymerization, like that of tubulin, is dynamic and regulated by GTP hydrolysis. These results provide the basis for the dynamics of the Z ring and favor a model in which the Z ring is formed by a nucleation event.