Division site selection in rod-shaped bacteria
Introduction
Cell division is a highly complex cytological process to produce viable progeny. In virtually all cells cytokinesis requires cytoskeletal elements [1••]. In bacteria the tubulin homolog FtsZ is the central cell division protein [2] that assembles into a cytoskeletal scaffold known as the Z-ring. Subsequently, other proteins that drive invagination of cell membrane and synthesis of cell wall material are recruited to the division site [3, 4]. It has long been known that cell division in rod-shaped bacteria is restricted to midcell, between the segregated nucleoids [5, 6]. In the last decades research has revealed that two inhibitory systems are involved in the spatial regulation of cytokinesis (Figure 1). The Min system has been shown to prevent aberrant cell division close to the cell poles, while nucleoid occlusion (NO) prevents cell division from occurring over the nucleoids [7, 8, 9]. Although, the textbook view on division site selection is that the inhibitory effect is due to the combined action of the Min and NO systems on FtsZ, a number of publications have provided evidence that the regulation of cytokinesis by the Min system could also occur downstream of FtsZ assembly [10•, 11].
This review will focus on division site selection during vegetative growth in the rod-shaped model organisms Escherichia coli and Bacillus subtilis. Although the Min system is widespread among bacteria, other regulatory principles have evolved in bacteria whose genomes do not encode prototypical Min/NO systems. An interesting example is the spatial control of division in the vibrioid rod Caulobacter crescentus ([12••], and see the review by M Thanbichler in this issue). We emphasize recent discoveries that challenge and extend the classical view of the Min system.
Section snippets
The cytokinetic machinery
The cytokinetic machinery, or divisome, is highly conserved in bacteria and many of the essential components are found in almost all bacterial cells. The tubulin homolog FtsZ is the first protein to be localized at the incipient division site [2, 13]. When bound to GTP, FtsZ assembles into protofilaments that can interact laterally to form the Z-ring. Associated proteins like ZapA and ZipA promote lateral bundling and help the Z-ring to coalesce into a functional ring [14, 15]. The Z-ring is
Nucleoid occlusion
One of the two identified systems that determine division site selection is the NO system. Although nucleoid occlusion was proposed long ago [7], the actual effector, Noc (yyaA) of B. subtilis was only identified recently [8]. Although a noc null mutation had no obvious phenotype, a conditional deletion with minD resulted in a severe division defect with a failure to form functional Z-rings between the nucleoids, while overproduction led to cell elongation. A GFP–Noc fusion protein localized to
The Min system
The Min system prevents aberrant division at the cell poles [21••, 22] and it consists of the actual inhibitor MinC, a membrane-associated ATPase MinD that localizes MinC to the plasma membrane and a topological factor that spatially organizes the inhibitory MinCD complex. Only the topological factor differs between E. coli (MinE) and B. subtilis (DivIVA). Interestingly, the Min system is also found in chloroplasts [23••]. Chloroplasts contain MinD and MinE; however, a MinC-like protein has not
Conclusions
Division site selection in rod-shaped bacteria is regulated by at least two negative regulators of FtsZ assembly, the NO and the Min system. Their combined action defines the site of septation in many rod-shaped bacteria. Recent discoveries have added to the knowledge about the mechanism how the division machinery is spatially regulated and opened up new, unexpected, insights into division site selection.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We thank Catriona Donovan, Inga Wadenpohl, and Frank Bürmann for critical comments on the manuscript and fruitful discussions. Work in the author's lab is funded by the Deutsche Forschungsgemeinschaft (DFG). SB was supported by the Max Planck Society and an EU Marie-Curie ADOPT fellowship.
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