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Bacterial Division: The Fellowship of The Ring
Current Biology, Vol. 13, R16–R18, January 8, 2003, ©2003 Elsevier Science Ltd. All rights reserved.
Bacterial Division: The Fellowship Of The Ring William Margolin
The Z ring, composed of the tubulin homolog FtsZ, is essential for bacterial cell division. Recently a new protein, ZapA, has been discovered that localizes to the Z ring and stabilizes it, probably by promoting the bundling of FtsZ protofilaments.
Cell division in most bacteria requires FtsZ, a tubulin homolog that assembles into the so-called Z ring at the future site of septum formation [1]. Purified FtsZ assembles into protofilaments in the presence of GTP, and its GTPase activity is important for assembly dynamics [2]. A number of conditions promote lateral interactions between protofilaments in vitro, but the physiologically relevant, assembled form of FtsZ in the Z ring is not known. A number of other proteins are also essential for division of a bacterial cell and are recruited by the Z ring. In Escherichia coli, one of the earliest of these recruits is ZipA, which binds to and promotes bundling of FtsZ protofilaments in vitro [3,4] and helps to stabilize Z rings [5]. ZipA has been proposed to be an accessory factor analogous to eukaryotic microtubule-associated proteins (MAPs), which bind to microtubules and modify their assembly properties [4]. Because ZipA is not conserved outside the gamma-proteobacteria, other stabilizing factors probably play similar roles in many species. Recently, a candidate for such a factor was discovered [6] in the Gram-positive bacterium Bacillus subtilis. The new protein, ZapA, has several functions similar to ZipA and is present in diverse bacterial species. In this new study, Gueiros-Filho and Losick [6] exploited the effects of an FtsZ assembly inhibitor, MinC, to search for additional unknown factors that would stimulate Z-ring assembly in B. subtilis. MinC promotes disassembly of FtsZ polymers in vivo and in vitro [7,8]. In the cell, this disassembly activity is largely restricted to the cell poles, serving to prevent unwanted polar division events. MinC binds to MinD protein, forming a MinCD complex, which in B. subtilis is sequestered at the poles by DivIVA protein [9]. When MinCD is overproduced, however, it overcomes the sequestration and localizes throughout the cell. This prevents Z-ring assembly at all potential division sites [10], causing a lethal block to cell division. E. coli cells can become resistant to excess MinCD by overproducing FtsZ, inhibiting FtsZ GTPase or overproducing FtsA, another essential cell division protein implicated in stabilizing Z rings [11,12]. This all suggests that disassembly of Z rings by MinCD can be reversed by tipping the balance in favor of assembly. Department of Microbiology and Molecular Genetics, University of Texas Medical School, 6431 Fannin, Houston, Texas 77030, USA. E-mail: [email protected]
PII S0960-9822(03)01381-7
Dispatch
Gueiros-Filho and Losick [6] exploited this phenomenon: using an overexpression library, they isolated a novel gene that, when overexpressed, resulted in MinCD-resistance in B. subtilis. The 85 amino-acid gene product was named ZapA, for ‘Z-ring associated protein’. ZapA has apparent orthologs in a number of species, including E. coli. To investigate whether ZapA might interact with the cell division machinery, Gueiros-Filho and Losick [6] expressed a ZapA–green fluorescent protein (GFP) fusion in B. subtilis cells. They found that ZapA–GFP localizes early to Z rings in a FtsZ-dependent manner, and can also localize to Z rings in E. coli cells. This suggested that ZapA might bind directly to FtsZ, the most conserved member of the division complex, and have a universal function. ZapA was indeed found to bind strongly to FtsZ in vitro, and to greatly stimulate GTP-dependent assembly of FtsZ as measured by light scattering. Examination of FtsZ polymers by electron microscopy indicated that addition of ZapA bundles FtsZ protofilaments in the presence of GTP and decreases the GTPase activity of FtsZ, reflecting the increased stability of the polymer network. These activities of ZapA, including early recruitment to the Z ring, stabilization of Z rings and bundling of FtsZ protofilaments, are strikingly similar to those of E. coli ZipA. But whereas inactivating ZipA in E. coli results in lethal filamentation, loss of zapA function had no detectable effect on B. subtilis cell division or viability. This raised concerns that ZapA might be a minor accessory factor for FtsZ, without an important or even defined role in cell division. A clue to ZapA’s function came from the effects of decreased FtsZ levels: within a range tolerable for a zapA+ cell, lower levels of FtsZ inhibited cell division and viability in zapA– cells. Moreover, the zapA– mutation significantly enhanced the cell division defects of divIVA or ezrA mutants. Because DivIVA pilots MinCD to the cell poles, inactivation of DivIVA causes MinCD to be distributed throughout the cell, inhibiting Z-ring assembly at midcell and explaining why the lack of ZapA further inhibits cell division. The result with ezrA, on the other hand, is puzzling. EzrA has been proposed to be an inhibitor of Z ring assembly, because ezrA– cells are viable, have extra Z rings [13] and, like cells overproducing ZapA, are resistant to the effects of MinCD overproduction [10]. It is surprising, then, that removing a Z-ring inhibitor such as EzrA would make zapAcells sicker. How can this result be explained? Several independent studies, including recent results using the technique of fluorescence recovery after photobleaching (FRAP), have shown that FtsZ in the Z ring has a high turnover rate [14–16]. Because large masses of septal membrane staining were visible in ezrA– filaments depleted of ZapA [6], it was possible that, in the mutant cells, FtsZ in existing Z rings aggregates
Current Biology R17
Figure 1. Proteins involved in the assembly–disassembly cycle of the Z ring. The assembly of FtsZ from monomers into protofilaments and protofilament bundles is shown, followed by disassembly of the ZapA bundles and then the protofilaments. This MinC ZipA cycle is active at all times, including when the Z ring appears to be stable [14]. It is EzrA? GTP FtsA? assumed that dynamic protofilament bundles are the operative structures in the Z ring, but this is not known. It is also not known if single protofilaments are an obligate intermediate in the pathway to protofilCurrent Biology ament bundles in vivo. Known factors involved and the putative steps of their involvement are shown. Question marks after a protein indicate that no biochemical activity relevant to FtsZ assembly has been demonstrated. The potential distribution of FtsZ in the cell is shown for each of the steps in the assembly pathway: generalized localization for the monomeric form, nucleated FtsZ structures for the protofilaments, and the Z ring for the protofilament bundles.
and fails to recycle efficiently into new rings at midcell. This is reminiscent of ftsZ mutants with low GTPase activity: one of these mutants, ftsZ84 of E. coli, has a mutation in the GTP-binding domain, causing the FtsZ84 protein to bind and hydrolyze GTP inefficiently. As a result, FtsZ84 has a nine-fold lower turnover rate than FtsZ in the Z ring as measured by FRAP [14]. Despite this defect, ftsZ84 cells can divide normally at all temperatures below 42°C. Along with the known viability of another GTPasedefective mutant, ftsZ2, this argues strongly that reducing FtsZ turnover does not affect Z-ring function under most conditions [17]. Nevertheless, removing the Min proteins in ftsZ84 cells causes a severe deficiency in assembly of new Z rings, and viability is restricted to temperatures below 32°C unless extra FtsZ84 is synthesized [18]. Is this because the balance has been tipped too far in favor of Z-ring stability and no new Z-ring assembly? For example, decreased turnover of FtsZ84 in a ∆min cell or of FtsZ in a ezrA– zapA– double mutant cell may result in FtsZ being largely tied up in old Z rings, preventing its recycling into new rings. This alone may not be sufficient to block cell division, as noted above. Importantly, however, the absence of a Z-ring stabilizer such as ZapA or ZipA may increase the critical concentration needed for assembly of new Z rings, compounding the problem and affecting viability. Future studies of Z-ring frequency, location and stability in filamentous cells depleted for ZapA and EzrA, as well as determining whether extra FtsZ can rescue the lethality, should address this issue. The discovery of ZapA as a possible conserved factor conferring stability on Z rings provides more evidence that FtsZ does not normally assemble in isolation, but instead, like tubulin, is regulated by a number of accessory factors (Figure 1). Some of these factors, like ZapA and ZipA, may increase stability of the Z ring by promoting lateral bonding among FtsZ protofilaments. Some, like FtsA, may increase stability of the Z ring by protofilament bundling as well as other mechanisms. Others, like MinC and EzrA, appear to decrease stability. The requirement of FtsA and ZipA, but not ZapA, EzrA and MinC, for septum formation suggests that FtsA and
ZipA may function in both Z-ring stabilization and recruitment of essential downstream division proteins [5,19]. Understanding more about how this growing group of proteins regulates assembly of the Z ring should shed important light on how the Z ring forms, contracts, and reassembles at new division sites. This is just the first installment of what should be an engrossing search to find the secret of the ring. References 1. Addinall, S.G. and Holland, B. (2002). The tubulin ancestor, FtsZ, draughtsman, designer and driving force for bacterial cytokinesis. J. Mol. Biol. 318, 219–236. 2. Mukherjee, A. and Lutkenhaus, J. (1998). Dynamic assembly of FtsZ regulated by GTP hydrolysis. EMBO J. 17, 462–469. 3. Hale, C.A., Rhee, A.C. and de Boer, P.A. (2000). ZipA-induced bundling of FtsZ polymers mediated by an interaction between Cterminal domains. J. Bacteriol. 182, 5153–5166. 4. Raychaudhuri, D. (1999). ZipA is a MAP-Tau homolog and is essential for structural integrity of the cytokinetic FtsZ ring during bacterial cell division. EMBO J. 18, 2372–2383. 5. Pichoff, S. and Lutkenhaus, J. (2002). Unique and overlapping roles for ZipA and FtsA in septal ring assembly in Escherichia coli. EMBO J. 21, 685–693. 6. Gueiros-Filho, F.J. and Losick, R. (2002). A widely conserved bacterial cell division protein that promotes assembly of the tubulin-like protein FtsZ. Genes Dev. 16, 2544–2556. 7. Hu, Z., Mukherjee, A., Pichoff, S. and Lutkenhaus, J. (1999). The MinC component of the division site selection system in Escherichia coli interacts with FtsZ to prevent polymerization. Proc. Natl. Acad. Sci. U.S.A. 96, 14819–14824. 8. Pichoff, S. and Lutkenhaus, J. (2001). Escherichia coli division inhibitor MinCD blocks septation by preventing Z-ring formation. J. Bacteriol. 183, 6630–6635. 9. Marston, A.L. and Errington, J. (1999). Selection of the midcell division site in Bacillus subtilis through MinD-dependent polar localization and activation of MinC. Mol. Microbiol. 33, 84–96. 10. Levin, P.A., Schwartz, R.L. and Grossman, A.D. (2001). Polymer stability plays an important role in the positional regulation of FtsZ. J. Bacteriol. 183, 5449–5452. 11. Bi, E. and Lutkenhaus, J. (1990). Interaction between the min locus and ftsZ. J. Bacteriol. 172, 5610–5616. 12. Justice, S.S., Garcia-Lara, J. and Rothfield, L.I. (2000). Cell division inhibitors SulA and MinC/MinD block septum formation at different steps in the assembly of the Escherichia coli division machinery. Mol. Microbiol. 37, 410–423. 13. Levin, P.A., Kurtser, I.G. and Grossman, A.D. (1999). Identification and characterization of a negative regulator of FtsZ ring formation in Bacillus subtilis. Proc. Natl. Acad. Sci. U.S.A. 96, 9642–9647. 14. Stricker, J., Maddox, P., Salmon, E.D. and Erickson, H.P. (2002). Rapid assembly dynamics of the Escherichia coli FtsZ-ring demonstrated by fluorescence recovery after photobleaching. Proc. Natl. Acad. Sci. U.S.A. 99, 3171–3175. 15. Sun, Q. and Margolin, W. (1998). FtsZ dynamics during the cell division cycle of live Escherichia coli. J. Bacteriol. 180, 2050–2056.
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Addinall, S.G., Cao, C. and Lutkenhaus, J. (1997). Temperature shift experiments with an ftsZ84(Ts) strain reveal rapid dynamics of FtsZ localization and indicate that the Z ring is required throughout septation and cannot reoccupy division sites once constriction has initiated. J. Bacteriol. 179, 4277–4284. Mukherjee, A., Saez, C. and Lutkenhaus, J. (2001). Assembly of an FtsZ mutant deficient in GTPase activity has implications for FtsZ assembly and the role of the Z ring in cell division. J. Bacteriol. 183, 7190–7197. Yu, X.C. and Margolin, W. (2000). Deletion of the min operon results in increased thermosensitivity of an ftsZ84 mutant and abnormal FtsZ ring assembly, placement and disassembly. J. Bacteriol. 182, 6203–6213. Hale, C.A. and de Boer, P.A. (2002). ZipA is required for recruitment of FtsK, FtsQ, FtsL and FtsN to the septal ring in Escherichia coli. J. Bacteriol. 184, 2552–2556.
Bacterial Division: The Fellowship Of The Ring William Margolin
The Z ring, composed of the tubulin homolog FtsZ, is essential for bacterial cell division. Recently a new protein, ZapA, has been discovered that localizes to the Z ring and stabilizes it, probably by promoting the bundling of FtsZ protofilaments.
Cell division in most bacteria requires FtsZ, a tubulin homolog that assembles into the so-called Z ring at the future site of septum formation [1]. Purified FtsZ assembles into protofilaments in the presence of GTP, and its GTPase activity is important for assembly dynamics [2]. A number of conditions promote lateral interactions between protofilaments in vitro, but the physiologically relevant, assembled form of FtsZ in the Z ring is not known. A number of other proteins are also essential for division of a bacterial cell and are recruited by the Z ring. In Escherichia coli, one of the earliest of these recruits is ZipA, which binds to and promotes bundling of FtsZ protofilaments in vitro [3,4] and helps to stabilize Z rings [5]. ZipA has been proposed to be an accessory factor analogous to eukaryotic microtubule-associated proteins (MAPs), which bind to microtubules and modify their assembly properties [4]. Because ZipA is not conserved outside the gamma-proteobacteria, other stabilizing factors probably play similar roles in many species. Recently, a candidate for such a factor was discovered [6] in the Gram-positive bacterium Bacillus subtilis. The new protein, ZapA, has several functions similar to ZipA and is present in diverse bacterial species. In this new study, Gueiros-Filho and Losick [6] exploited the effects of an FtsZ assembly inhibitor, MinC, to search for additional unknown factors that would stimulate Z-ring assembly in B. subtilis. MinC promotes disassembly of FtsZ polymers in vivo and in vitro [7,8]. In the cell, this disassembly activity is largely restricted to the cell poles, serving to prevent unwanted polar division events. MinC binds to MinD protein, forming a MinCD complex, which in B. subtilis is sequestered at the poles by DivIVA protein [9]. When MinCD is overproduced, however, it overcomes the sequestration and localizes throughout the cell. This prevents Z-ring assembly at all potential division sites [10], causing a lethal block to cell division. E. coli cells can become resistant to excess MinCD by overproducing FtsZ, inhibiting FtsZ GTPase or overproducing FtsA, another essential cell division protein implicated in stabilizing Z rings [11,12]. This all suggests that disassembly of Z rings by MinCD can be reversed by tipping the balance in favor of assembly. Department of Microbiology and Molecular Genetics, University of Texas Medical School, 6431 Fannin, Houston, Texas 77030, USA. E-mail: [email protected]
PII S0960-9822(03)01381-7
Dispatch
Gueiros-Filho and Losick [6] exploited this phenomenon: using an overexpression library, they isolated a novel gene that, when overexpressed, resulted in MinCD-resistance in B. subtilis. The 85 amino-acid gene product was named ZapA, for ‘Z-ring associated protein’. ZapA has apparent orthologs in a number of species, including E. coli. To investigate whether ZapA might interact with the cell division machinery, Gueiros-Filho and Losick [6] expressed a ZapA–green fluorescent protein (GFP) fusion in B. subtilis cells. They found that ZapA–GFP localizes early to Z rings in a FtsZ-dependent manner, and can also localize to Z rings in E. coli cells. This suggested that ZapA might bind directly to FtsZ, the most conserved member of the division complex, and have a universal function. ZapA was indeed found to bind strongly to FtsZ in vitro, and to greatly stimulate GTP-dependent assembly of FtsZ as measured by light scattering. Examination of FtsZ polymers by electron microscopy indicated that addition of ZapA bundles FtsZ protofilaments in the presence of GTP and decreases the GTPase activity of FtsZ, reflecting the increased stability of the polymer network. These activities of ZapA, including early recruitment to the Z ring, stabilization of Z rings and bundling of FtsZ protofilaments, are strikingly similar to those of E. coli ZipA. But whereas inactivating ZipA in E. coli results in lethal filamentation, loss of zapA function had no detectable effect on B. subtilis cell division or viability. This raised concerns that ZapA might be a minor accessory factor for FtsZ, without an important or even defined role in cell division. A clue to ZapA’s function came from the effects of decreased FtsZ levels: within a range tolerable for a zapA+ cell, lower levels of FtsZ inhibited cell division and viability in zapA– cells. Moreover, the zapA– mutation significantly enhanced the cell division defects of divIVA or ezrA mutants. Because DivIVA pilots MinCD to the cell poles, inactivation of DivIVA causes MinCD to be distributed throughout the cell, inhibiting Z-ring assembly at midcell and explaining why the lack of ZapA further inhibits cell division. The result with ezrA, on the other hand, is puzzling. EzrA has been proposed to be an inhibitor of Z ring assembly, because ezrA– cells are viable, have extra Z rings [13] and, like cells overproducing ZapA, are resistant to the effects of MinCD overproduction [10]. It is surprising, then, that removing a Z-ring inhibitor such as EzrA would make zapAcells sicker. How can this result be explained? Several independent studies, including recent results using the technique of fluorescence recovery after photobleaching (FRAP), have shown that FtsZ in the Z ring has a high turnover rate [14–16]. Because large masses of septal membrane staining were visible in ezrA– filaments depleted of ZapA [6], it was possible that, in the mutant cells, FtsZ in existing Z rings aggregates
Current Biology R17
Figure 1. Proteins involved in the assembly–disassembly cycle of the Z ring. The assembly of FtsZ from monomers into protofilaments and protofilament bundles is shown, followed by disassembly of the ZapA bundles and then the protofilaments. This MinC ZipA cycle is active at all times, including when the Z ring appears to be stable [14]. It is EzrA? GTP FtsA? assumed that dynamic protofilament bundles are the operative structures in the Z ring, but this is not known. It is also not known if single protofilaments are an obligate intermediate in the pathway to protofilCurrent Biology ament bundles in vivo. Known factors involved and the putative steps of their involvement are shown. Question marks after a protein indicate that no biochemical activity relevant to FtsZ assembly has been demonstrated. The potential distribution of FtsZ in the cell is shown for each of the steps in the assembly pathway: generalized localization for the monomeric form, nucleated FtsZ structures for the protofilaments, and the Z ring for the protofilament bundles.
and fails to recycle efficiently into new rings at midcell. This is reminiscent of ftsZ mutants with low GTPase activity: one of these mutants, ftsZ84 of E. coli, has a mutation in the GTP-binding domain, causing the FtsZ84 protein to bind and hydrolyze GTP inefficiently. As a result, FtsZ84 has a nine-fold lower turnover rate than FtsZ in the Z ring as measured by FRAP [14]. Despite this defect, ftsZ84 cells can divide normally at all temperatures below 42°C. Along with the known viability of another GTPasedefective mutant, ftsZ2, this argues strongly that reducing FtsZ turnover does not affect Z-ring function under most conditions [17]. Nevertheless, removing the Min proteins in ftsZ84 cells causes a severe deficiency in assembly of new Z rings, and viability is restricted to temperatures below 32°C unless extra FtsZ84 is synthesized [18]. Is this because the balance has been tipped too far in favor of Z-ring stability and no new Z-ring assembly? For example, decreased turnover of FtsZ84 in a ∆min cell or of FtsZ in a ezrA– zapA– double mutant cell may result in FtsZ being largely tied up in old Z rings, preventing its recycling into new rings. This alone may not be sufficient to block cell division, as noted above. Importantly, however, the absence of a Z-ring stabilizer such as ZapA or ZipA may increase the critical concentration needed for assembly of new Z rings, compounding the problem and affecting viability. Future studies of Z-ring frequency, location and stability in filamentous cells depleted for ZapA and EzrA, as well as determining whether extra FtsZ can rescue the lethality, should address this issue. The discovery of ZapA as a possible conserved factor conferring stability on Z rings provides more evidence that FtsZ does not normally assemble in isolation, but instead, like tubulin, is regulated by a number of accessory factors (Figure 1). Some of these factors, like ZapA and ZipA, may increase stability of the Z ring by promoting lateral bonding among FtsZ protofilaments. Some, like FtsA, may increase stability of the Z ring by protofilament bundling as well as other mechanisms. Others, like MinC and EzrA, appear to decrease stability. The requirement of FtsA and ZipA, but not ZapA, EzrA and MinC, for septum formation suggests that FtsA and
ZipA may function in both Z-ring stabilization and recruitment of essential downstream division proteins [5,19]. Understanding more about how this growing group of proteins regulates assembly of the Z ring should shed important light on how the Z ring forms, contracts, and reassembles at new division sites. This is just the first installment of what should be an engrossing search to find the secret of the ring. References 1. Addinall, S.G. and Holland, B. (2002). The tubulin ancestor, FtsZ, draughtsman, designer and driving force for bacterial cytokinesis. J. Mol. Biol. 318, 219–236. 2. Mukherjee, A. and Lutkenhaus, J. (1998). Dynamic assembly of FtsZ regulated by GTP hydrolysis. EMBO J. 17, 462–469. 3. Hale, C.A., Rhee, A.C. and de Boer, P.A. (2000). ZipA-induced bundling of FtsZ polymers mediated by an interaction between Cterminal domains. J. Bacteriol. 182, 5153–5166. 4. Raychaudhuri, D. (1999). ZipA is a MAP-Tau homolog and is essential for structural integrity of the cytokinetic FtsZ ring during bacterial cell division. EMBO J. 18, 2372–2383. 5. Pichoff, S. and Lutkenhaus, J. (2002). Unique and overlapping roles for ZipA and FtsA in septal ring assembly in Escherichia coli. EMBO J. 21, 685–693. 6. Gueiros-Filho, F.J. and Losick, R. (2002). A widely conserved bacterial cell division protein that promotes assembly of the tubulin-like protein FtsZ. Genes Dev. 16, 2544–2556. 7. Hu, Z., Mukherjee, A., Pichoff, S. and Lutkenhaus, J. (1999). The MinC component of the division site selection system in Escherichia coli interacts with FtsZ to prevent polymerization. Proc. Natl. Acad. Sci. U.S.A. 96, 14819–14824. 8. Pichoff, S. and Lutkenhaus, J. (2001). Escherichia coli division inhibitor MinCD blocks septation by preventing Z-ring formation. J. Bacteriol. 183, 6630–6635. 9. Marston, A.L. and Errington, J. (1999). Selection of the midcell division site in Bacillus subtilis through MinD-dependent polar localization and activation of MinC. Mol. Microbiol. 33, 84–96. 10. Levin, P.A., Schwartz, R.L. and Grossman, A.D. (2001). Polymer stability plays an important role in the positional regulation of FtsZ. J. Bacteriol. 183, 5449–5452. 11. Bi, E. and Lutkenhaus, J. (1990). Interaction between the min locus and ftsZ. J. Bacteriol. 172, 5610–5616. 12. Justice, S.S., Garcia-Lara, J. and Rothfield, L.I. (2000). Cell division inhibitors SulA and MinC/MinD block septum formation at different steps in the assembly of the Escherichia coli division machinery. Mol. Microbiol. 37, 410–423. 13. Levin, P.A., Kurtser, I.G. and Grossman, A.D. (1999). Identification and characterization of a negative regulator of FtsZ ring formation in Bacillus subtilis. Proc. Natl. Acad. Sci. U.S.A. 96, 9642–9647. 14. Stricker, J., Maddox, P., Salmon, E.D. and Erickson, H.P. (2002). Rapid assembly dynamics of the Escherichia coli FtsZ-ring demonstrated by fluorescence recovery after photobleaching. Proc. Natl. Acad. Sci. U.S.A. 99, 3171–3175. 15. Sun, Q. and Margolin, W. (1998). FtsZ dynamics during the cell division cycle of live Escherichia coli. J. Bacteriol. 180, 2050–2056.
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Addinall, S.G., Cao, C. and Lutkenhaus, J. (1997). Temperature shift experiments with an ftsZ84(Ts) strain reveal rapid dynamics of FtsZ localization and indicate that the Z ring is required throughout septation and cannot reoccupy division sites once constriction has initiated. J. Bacteriol. 179, 4277–4284. Mukherjee, A., Saez, C. and Lutkenhaus, J. (2001). Assembly of an FtsZ mutant deficient in GTPase activity has implications for FtsZ assembly and the role of the Z ring in cell division. J. Bacteriol. 183, 7190–7197. Yu, X.C. and Margolin, W. (2000). Deletion of the min operon results in increased thermosensitivity of an ftsZ84 mutant and abnormal FtsZ ring assembly, placement and disassembly. J. Bacteriol. 182, 6203–6213. Hale, C.A. and de Boer, P.A. (2002). ZipA is required for recruitment of FtsK, FtsQ, FtsL and FtsN to the septal ring in Escherichia coli. J. Bacteriol. 184, 2552–2556.