FtsZ and bacterial cell division
Bacterial cell division is in virtually all cases orchestrated by the tubulin-homologue FtsZ. FtsZ assembles into a cytokinetic ring, the Z ring, which marks the site of division and recruits more than ten other division proteins. They link the cytoplasmic Z ring to the enzymes for peptidoglycan assembly, which are active on the outside of the membrane. The Z ring then contracts while the septum, a crosswall of peptidoglycan, is being laid down.
Unusual control of cell division in Actinobacteria
Several proteins contribute to control Z ring assembly so that it occurs at exactly the right time and place in bacteria like Escherichia coli and Bacillus subtilis. This includes proteins that stabilize Z rings or anchor them in the membrane (e.g. FtsA, ZipA, EzrA), and others that destabilize FtsZ polymers or prevent their polymerization (e.g. MinC, SulA). Intriguingly, there are no recognizable homologues of these proteins encoded by actinobacterial genomes (Flärdh, 2003; Flärdh and van Wezel, 2003; Flärdh and Buttner, 2009). Thus, the mechanisms for regulation of cell division in the Actinobacteria differ clearly from what is known from E. coli and B. subtilis. Our aim is to clarify these mechanisms.
Model for assembly and remodelling of FtsZ polymers into a series of Z rings during sporulation ofStreptomyces coelicolor. The drawings show the conversion of the apical cell of an aerial hypha into a chain of spores. The ftsZ gene is upregulated in this cell, giving rise to an increased concentration of FtsZ (green), and subsequently polymerisation into helical fibres. They are remodelled into Z rings which then give rise to sporulation septa. Redrawn from Grantcharova et al., 2005.
Streptomyces coelicolor is a powerful model system for actinobacterial cell division and FtsZ assembly dynamics
Streptomyces coelicolor has several unique features that make it ideal for analysis of bacterial cell division. First, it is the only known FtsZ-containing bacterium in which the ftsZ gene is dispensable and can be deleted. Second, this organism has two forms of cell division, of which one is the developmentally regulated sporulation septation that converts aerial hyphae into chains of spores (Fig. 1; Flärdh et al., 2000). Third, the spore pigment works as an excellent built-in reporter in genetic analyses of this cell division. Finally, the formation of many tens of closely spaced Z rings in a single cell involves remodelling of highly dynamic helical structures into a series of rings (Fig. 1 and 2; Grantcharova et al., 2005). This makes the sporulation process highly sensitive to disturbances in FtsZ polymerisation dynamics, and it is therefore a great model to investigate this central aspect of cell division.
FtsZ polymers (green) visualized using FtsZ-EGFP in a sporulating hypha of Streptomyces coelicolor.
Our investigations of FtsZ and cell division
We have established a procedure for identification and analysis of ftsZ mutations that block or interfere with sporulation septation in Streptomyces coelicolor(Grantcharova et al., 2003; Grantcharova, 2006). In addition, a system for purification of active S. coelicolor FtsZ has been developed, and we have sensitive microscopy-methods for visualising FtsZ in live cells (Grantcharova et al., 2005). This allows us to examine both the basis for FtsZ polymerization dynamics and the mechanisms for developmental control of cell division, as well as to explore FtsZ as a drug target in Actinobacteria.