Characterization of a morphological checkpoint coupling cell-specific transcription to septation in Bacillus subtilis

Cell Cycle Machinery in Bacillus subtilis

Bacillus subtilis is the best described member of the Gram positive bacteria. It is a typical rod shaped bacterium and grows by elongation in its long axis, before dividing at mid cell to generate two similar daughter cells. B. subtilis is a particularly interesting model for cell cycle studies because it also carries out a modified, asymmetrical division during endospore formation, which can be simply induced by starvation. Cell growth occurs strictly by elongation of the rod, which maintains a constant diameter at all growth rates. This process involves expansion of the cell wall, requiring intercalation of new peptidoglycan and teichoic acid material, as well as controlled hydrolysis of existing wall material. Actin-like MreB proteins are the key spatial regulators that orchestrate the plethora of enzymes needed for cell elongation, many of which are thought to assemble into functional complexes called elongasomes. Cell division requires a switch in the orientation of cell wall synthesis and is organised by a tubulin-like protein FtsZ. FtsZ forms a ring-like structure at the site of impending division, which is specified by a range of mainly negative regulators. There it recruits a set of dedicated division proteins to form a structure called the divisome, which brings about the process of division. During sporulation, both the positioning and fine structure of the division septum are altered, and again, several dedicated proteins that contribute specifically to this process have been identified. This chapter summarises our current understanding of elongation and division in B. subtilis, with particular emphasis on the cytoskeletal proteins MreB and FtsZ, and highlights where the major gaps in our understanding remain.

Requirement for the cell division protein DivIB in polar cell division and engulfment during sporulation in Bacillus subtilis

During spore formation in Bacillus subtilis, cell division occurs at the cell pole and is believed to require essentially the same division machinery as vegetative division. Intriguingly, although the cell division protein DivIB is not required for vegetative division at low temperatures, it is essential for efficient sporulation under these conditions. We show here that at low temperatures in the absence of DivIB, formation of the polar septum during sporulation is delayed and less efficient. Furthermore, the polar septa that are complete are abnormally thick, containing more peptidoglycan than a normal polar septum. These results show that DivIB is specifically required for the efficient and correct formation of a polar septum. This suggests that DivIB is required for the modification of sporulation septal peptidoglycan, raising the possibility that DivIB either regulates hydrolysis of polar septal peptidoglycan or is a hydrolase itself. We also show that, despite the significant number of completed polar septa that form in this mutant, it is unable to undergo engulfment. Instead, hydrolysis of the peptidoglycan within the polar septum, which occurs during the early stages of engulfment, is incomplete, producing a similar phenotype to that of mutants defective in the production of sporulation-specific septal peptidoglycan hydrolases. We propose a role for DivIB in sporulation-specific peptidoglycan remodelling or its regulation during polar septation and engulfment.

Where asymmetry in gene expression originates

A general problem in developmental biology concerns the process by which cells of one type divide to give dissimilar daughter cells. Even though these daughter cells may be genetically identical, they can differ morphologically and physiologically and have different fates. As one of the simplest differentiation processes, Bacillus subtilis sporulation represents an excellent model system for studying cell differentiation. Several decades of study of this process have provided insight into cell cycle regulation and development. This review summarizes important advances in our understanding of asymmetric gene expression during spore formation with an emphasis on developmental stages that lead to asymmetric septum formation and especially to activation of the first compartment-specific sigma factor -sigma(F).

The Bacillus subtilis SinR and RapA developmental regulators are responsible for inhibition of spore development by alcohol

Even though there is a large body of information concerning the harmful effects of alcohol on different organisms, the mechanism(s) that affects developmental programs, at a single-cell level, has not been clearly identified. In this respect, the spore-forming bacterium Bacillus subtilis constitutes an excellent model to study universal questions of cell fate, cell differentiation, and morphogenesis. Here, we demonstrate that treatment with subinhibitory concentrations of alcohol that did not affect vegetative growth inhibited the initiation of spore development through a selective blockage of key developmental genes under the control of the master transcription factor Spo0A approximately P. Isopropyl-beta-D-thiogalactopyranoside-directed expression of a phosphorylation-independent form of Spo0A (Sad67) and the use of an in vivo mini-Tn10 insertional library permitted the identification of the developmental SinR repressor and RapA phosphatase as the effectors that mediated the inhibitory effect of alcohol on spore morphogenesis. A double rapA sinR mutant strain was completely resistant to the inhibitory effects of different-C-length alcohols on sporulation, indicating that the two cell fate determinants were the main or unique regulators responsible for the spo0 phenotype of wild-type cells in the presence of alcohol. Furthermore, treatment with alcohol produced a significant induction of rapA and sinR, while the stationary-phase induction of sinI, which codes for a SinR inhibitor, was completely turned off by alcohol. As a result, a dramatic repression of spo0A and the genes under its control occurred soon after alcohol addition, inhibiting the onset of sporulation and permitting the evaluation of alternative pathways required for cellular survival.

ftsZ mutations affecting cell division frequency, placement and morphology in Bacillus subtilis

A key event in cytokinesis in bacteria is the assembly of the essential division protein FtsZ into ring-like structures at the nascent division site. FtsZ is the prokaryotic homologue of tubulin, and is found in nearly all bacteria. In vitro, FtsZ polymerizes in the presence of GTP to form higher-ordered polymers. FtsZ consists of two domains, with the GTP-binding site located in the N-terminal domain. The less-conserved C-terminal domain contains residues important for GTP hydrolysis, but its overall function is still unclear. This paper reports the development of a simple strategy to generate mutations in the essential division gene ftsZ. Nine novel and viable ftsZ mutants of Bacillus subtilis are described. Eight of the mutations would affect the C-terminus of FtsZ. The collection of mutants exhibits a range of morphological phenotypes, ranging from normal to highly filamentous cells; some produce minicells, or divide in a twisted configuration; one mutation has a temperature-sensitive effect specifically impairing sporulation. The sites of the amino acid changes generated by the mutations could be informative about FtsZ function and its protein–protein interactions.

Cell division protein DivIB influences the Spo0J/Soj system of chromosome segregation in Bacillus subtilis

The initiation of the developmental process of sporulation in the rod-shaped bacterium Bacillus subtilis involves the activation of the Spo0A response regulator. Spo0A then drives the switch in the site of division septum formation from midcell to a polar position. Activated Spo0A is required for the transcription of key sporulation loci such as spoIIG, which are negatively regulated by the Soj protein. The transcriptional repressing activity of Soj is antagonized by Spo0J, and both proteins belong to the well-conserved Par family of partitioning proteins. Soj has been shown to jump from nucleoid to nucleoid via the cell pole. The dynamic behaviour of Soj is somehow controlled by Spo0J, which prevents the static association of Soj with the nucleoid, and presumably its transcriptional repression activity. Soj in turn is required for the proper condensation of Spo0J foci around the oriC region. The asymmetric partitioning of the sporangial cell requires DivIB and other proteins involved in vegetative (medial) division. We describe an allele of the cell division gene divIB (divIB80) that reduces the cellular levels of DivIB, and affects nucleoid structure and segregation in growing cells, yet has no major impact on cell division. In divIB80 cells Spo0J foci are not correctly condensed and Soj associates statically with the nucleoid. The divIB80 allele prevents transcription of spoIIG, and arrests sporulation prior to the formation of the asymmetric division septum. The defect in Spo0A-dependent gene expression, and the Spo- phenotype can be suppressed by expression of divIB in trans or by deletion of the soj-spo0J locus. However, deletion of the spo0J-soj region does not restore the normal cellular levels of DivIB. Therefore, the reduced levels of DivIB in the divIB80 mutant are sufficient for efficient cell division, but not to sustain a second, earlier function of DivIB related to the activity of the Spo0J/Soj system of chromosome segregation.

SpoIIE regulates sporulation but does not directly affect solventogenesis in Clostridium acetobutylicum ATCC 824

Using gene expression reporter vectors, we examined the activity of the spoIIE promoter in wild-type and spo0A-deleted strains of Clostridium acetobutylicum ATCC 824. In wild-type cells, the spoIIE promoter is active in a transient manner during late solventogenesis, but in strain SKO1, where the sporulation initiator spo0A is disrupted, no spoIIE promoter activity is detectable at any stage of growth. Strains 824(pMSpo) and 824(pASspo) were created to overexpress spoIIE and to decrease spoIIE expression via antisense RNA targeted against spoIIE, respectively. Some cultures of strains 824(pMSpo) degenerated during fermentations by losing the pSOL1 megaplasmid and hence did not produce the solvents ethanol, acetone, and butanol. The frequent degeneration event was shown to require an intact copy of spoIIE. Nondegenerate cultures of 824(pMSpo) exhibited normal growth and solvent production. Strain 824(pASspo) exhibited prolonged solventogenesis characterized by increased production of ethanol (225%), acetone (43%), and butanol (110%). Sporulation in strains harboring pASspo was significantly delayed, with sporulating cells exhibiting altered morphology. These results suggest that SpoIIE has no direct effect on the control of solventogenesis and that the changes in solvent production in spoIIE-downregulated cells are mediated by effects on the cell during sporulation.

Efficient regulation of sigmaF, the first sporulation-specific sigma factor in B.subtilis

Differential gene expression is established in the prespore and mother-cell compartments of Bacillus subtilis through the successive activation of a series of cell-type-specific sigma factors. Crucial to the success of this process is the control of the first prespore-specific sigma factor, sigmaF. sigmaF is regulated by the proteins SpoIIAB, SpoIIAA and SpoIIE. SpoIIAB forms an inhibitory complex with sigmaF, which can be dissociated by interaction with SpoIIAA. During this interaction SpoIIAA is phosphorylated. SpoIIE is a membrane-bound phosphatase that dephosphorylates SpoIIAA, thereby re-activating it. It is not understood how sigmaF is activated specifically in the prespore but not in the mother cell. Here, we use a recently developed fluorescence spectroscopy technique to follow in real time the formation of sigmaF.SpoIIAB complexes and their dissociation by SpoIIAA. We show that complete activation of sigmaF is induced by a tenfold increase in SpoIIE activity. This result demonstrates that relatively small changes in SpoIIE activity, which could arise from asymmetric septation, can achieve the all-or-nothing response in sigmaF activity required by the cell. For long-term sigmaF activation, we find that sustained SpoIIE activity is required to counteract the activity of SpoIIAB. Even though the continual phosphorylation and dephosphorylation of SpoIIAA by these two enzymes will expend some ATP, the formation of SpoIIAA.SpoIIAB.ADP complexes greatly diminishes the rate of the phosphorylation reaction, and thus minimizes the wastage of energy. These features provide a very efficient system for regulating sigmaF.

Genetic analysis of the Bacillus subtilis sigG promoter, which controls the sporulation-specific transcription factor sigma G

At the onset of sporulation in Bacillus subtilis, an asymmetric cell division gives rise to two unequal-sized compartments with distinct developmental fates. The smaller compartment, or prespore, becomes the spore, whilst the larger compartment, or mother cell, eventually lyses after contributing to spore maturation. The fate of each compartment is determined by differential gene expression, controlled by the activation of four compartment-specific sigma-factors. The expression and activity of all four sigma-factors are tightly regulated to ensure the correct sequence of morphological events. Prespore-specific genes are transcribed by two sigma-factors, sigma(F) followed by sigma(G). The gene encoding sigma(G) (sigG) is transcribed by sigma(F), but also requires the activity of one of the mother-cell-specific sigma-factors, sigma(E), for its expression. The minimal promoter required for dependence on sigma(E) was found to stretch to just upstream of the -35 site. Analysis of mutant sigG promoters generated by site-directed mutagenesis and sigG promoters from other species suggests the presence of a binding site for a transcriptional repressor within the sigG promoter region. Replacement of the wild-type promoter with sigma(E)-independent promoters resulted in impairment of sporulation. These data support the idea that sigma(E) activity is required for the transcription of sigG.

Compartmentalization of gene expression during Bacillus subtilis spore formation

Gene expression in members of the family Bacillaceae becomes compartmentalized after the distinctive, asymmetrically located sporulation division. It involves complete compartmentalization of the activities of sporulation-specific sigma factors, sigma(F) in the prespore and then sigma(E) in the mother cell, and then later, following engulfment, sigma(G) in the prespore and then sigma(K) in the mother cell. The coupling of the activation of sigma(F) to septation and sigma(G) to engulfment is clear; the mechanisms are not. The sigma factors provide the bare framework of compartment-specific gene expression. Within each sigma regulon are several temporal classes of genes, and for key regulators, timing is critical. There are also complex intercompartmental regulatory signals. The determinants for sigma(F) regulation are assembled before septation, but activation follows septation. Reversal of the anti-sigma(F) activity of SpoIIAB is critical. Only the origin-proximal 30% of a chromosome is present in the prespore when first formed; it takes approximately 15 min for the rest to be transferred. This transient genetic asymmetry is important for prespore-specific sigma(F) activation. Activation of sigma(E) requires sigma(F) activity and occurs by cleavage of a prosequence. It must occur rapidly to prevent the formation of a second septum. sigma(G) is formed only in the prespore. SpoIIAB can block sigma(G) activity, but SpoIIAB control does not explain why sigma(G) is activated only after engulfment. There is mother cell-specific excision of an insertion element in sigK and sigma(E)-directed transcription of sigK, which encodes pro-sigma(K). Activation requires removal of the prosequence following a sigma(G)-directed signal from the prespore.

Evaluation of the kinetic properties of the sporulation protein SpoIIE of Bacillus subtilis by inclusion in a model membrane

Starvation induces Bacillus subtilis to initiate a developmental process (sporulation) that includes asymmetric cell division to form the prespore and the mother cell. The integral membrane protein SpoIIE is essential for the prespore-specific activation of the transcription factor sigmaF, and it also has a morphogenic activity required for asymmetric division. An increase in the local concentration of SpoIIE at the polar septum of B. subtilis precedes dephosphorylation of the anti-anti-sigma factor SpoIIAA in the prespore. After closure and invagination of the asymmetric septum, phosphatase activity of SpoIIE increases severalfold, but the reason for this dramatic change in activity has not been determined. The central domain of SpoIIE has been seen to self-associate (I. Lucet et al., EMBO J. 19:1467-1475, 2000), suggesting that activation of the C-terminal PP2C-like phosphatase domain might be due to conformational changes brought about by the increased local concentration of SpoIIE in the sporulating septum. Here we report the inclusion of purified SpoIIE protein into a model membrane as a method for studying the effect of local concentration in a lipid bilayer on activity. In vitro assays indicate that the membrane-bound enzyme maintains dephosphorylation rates similar to the highly active micellar state at all molar ratios of protein to lipid. Atomic force microscopy images indicate that increased local concentration does not lead to self-association.

A Threshold Mechanism Governing Activation of the Developmental Regulatory Protein σF in Bacillus subtilis

The RNA polymerase sigma factor sigma(F) is a developmental regulatory protein that is activated in a cell-specific manner following the formation of the polar septum during the process of spore formation in the bacterium Bacillus subtilis. Activation of sigma(F) depends on the membrane-bound phosphatase SpoIIE, which localizes to the septum, and on the formation of the polar septum itself. SpoIIE is responsible for dephosphorylating and thereby activating the phosphoprotein SpoIIAA, which, in turn, triggers the release of sigma(F) from the anti-sigma(F) factor SpoIIAB. Paradoxically, however, the presence of unphosphorylated SpoIIAA is insufficient to cause sigma(F) activation as SpoIIAA reaches substantial levels in mutants blocked in polar septation. We now describe mutants of SpoIIE, SpoIIAA, and SpoIIAB that break the dependence of sigma(F) activation on polar division. Analysis of these mutants indicates that unphosphorylated SpoIIAA must reach a threshold concentration in order to trigger the release of sigma(F) from SpoIIAB. Evidence is presented that this threshold is created by the action of SpoIIAB, which can form an alternative, long lived complex with SpoIIAA. We propose that formation of the SpoIIAA-SpoIIAB complex serves as a sink that traps SpoIIAA in an inactive state and that only when unphosphorylated SpoIIAA is in excess to the sink does activation of sigma(F) take place.

Temporal and spatial regulation in prokaryotic cell cycle progression and development

Bacteria exhibit a high degree of intracellular organization, both in the timing of essential processes and in the placement of the chromosome, the division site, and individual structural and regulatory proteins. We examine the temporal and spatial regulation of the Caulobacter cell cycle, bacterial chromosome segregation and cytokinesis, and Bacillus subtilis sporulation. Mechanisms that control timing of cell cycle and developmental events include transcriptional cascades, regulated phosphorylation and proteolysis of signal transduction proteins, transient genetic asymmetry, and intercellular communication. Surprisingly, many signal transduction proteins are dynamically localized to specific subcellular addresses during the cell division cycle and sporulation, and proper localization is essential for their function. The Min proteins that govern division site selection in Escherichia coli may be the first example of a system that generates positional information de novo.

Regulation of endospore formation in Bacillus subtilis

Spore formation in bacteria poses a number of biological problems of fundamental significance. Asymmetric cell division at the onset of sporulation is a powerful model for studying basic cell-cycle problems, including chromosome segregation and septum formation. Sporulation is one of the best understood examples of cellular development and differentiation. Fascinating problems posed by sporulation include the temporal and spatial control of gene expression, intercellular communication and various aspects of cell morphogenesis.

Characterization of a novel inhibitory feedback of the anti-anti-sigma SpoIIAA on Spo0A activation during development in Bacillus subtilis

Compartmentalized gene expression during sporulation is initiated after asymmetric division by cell-specific activation of the transcription factors sigmaF and sigmaE. Synthesis of these sigma factors, and their regulatory proteins, requires the activation (phosphorylation) of Spo0A by the phosphorelay signalling system. We report here a novel regulatory function of the anti-anti-sigmaF SpoIIAA as inhibitor of Spo0A activation. This effect did not require sigmaF activity, and it was abolished by expression of the phosphorelay-independent form Spo0A-Sad67 indicating that SpoIIAA directly interfered with Spo0A approximately P generation. IPTG-directed synthesis of the SpoIIE phosphatase in a strain carrying a multicopy plasmid coding for SpoIIAA and its specific inhibitory kinase SpoIIAB blocked Spo0A activation suggesting that the active form of the inhibitor was SpoIIAA and not SpoIIAA-P. Furthermore, expression of the non-phosphorylatable mutant SpoIIAAS58A (SpoIIAA-like), but not SpoIIAAS58D (SpoIIAA-P-like), completely blocked Spo0A-dependent gene expression. Importantly, SpoIIAA expressed from the chromosome under the control of its normal spoIIA promoter showed the same negative effect regulated not only by SpoIIAB and SpoIIE but also by septum morphogenesis. These findings are discussed in relation to the potential contribution of this novel inhibitory feedback with the proper activation of sigmaF and sigmaE during development.

Cytokinesis in bacteria

Work on two diverse rod-shaped bacteria, Escherichia coli and Bacillus subtilis, has defined a set of about 10 conserved proteins that are important for cell division in a wide range of eubacteria. These proteins are directed to the division site by the combination of two negative regulatory systems. Nucleoid occlusion is a poorly understood mechanism whereby the nucleoid prevents division in the cylindrical part of the cell, until chromosome segregation has occurred near midcell. The Min proteins prevent division in the nucleoid-free spaces near the cell poles in a manner that is beginning to be understood in cytological and biochemical terms. The hierarchy whereby the essential division proteins assemble at the midcell division site has been worked out for both E. coli and B. subtilis. They can be divided into essentially three classes depending on their position in the hierarchy and, to a certain extent, their subcellular localization. FtsZ is a cytosolic tubulin-like protein that polymerizes into an oligomeric structure that forms the initial ring at midcell. FtsA is another cytosolic protein that is related to actin, but its precise function is unclear. The cytoplasmic proteins are linked to the membrane by putative membrane anchor proteins, such as ZipA of E. coli and possibly EzrA of B. subtilis, which have a single membrane span but a cytoplasmic C-terminal domain. The remaining proteins are either integral membrane proteins or transmembrane proteins with their major domains outside the cell. The functions of most of these proteins are unclear with the exception of at least one penicillin-binding protein, which catalyzes a key step in cell wall synthesis in the division septum.

Novel spoIIE mutation that causes uncompartmentalized sigmaF activation in Bacillus subtilis

During sporulation, Bacillus subtilis undergoes an asymmetric division that results in two cells with different fates, the larger mother cell and the smaller forespore. The protein phosphatase SpoIIE, which is required for activation of the forespore-specific transcription factor sigma(F), is also required for optimal efficiency and timing of asymmetric division. We performed a genetic screen for spoIIE mutants that were impaired in sporulation but not sigma(F) activity and isolated a strain with the mutation spoIIEV697A. The mutant exhibited a 10- to 40-fold reduction in sporulation and a sixfold reduction in asymmetric division compared to the parent. Transcription of the sigma(F)-dependent spoIIQ promoter was increased more than 10-fold and was no longer confined to the forespore. The excessive sigma(F) activity persisted even when asymmetric division was prevented. Disruption of spoIIGB did not restore asymmetric division to the spoIIEV697A mutant, indicating that the deficiency is not a consequence of predivisional activation of the mother cell-specific transcription factor sigma(E). Deletion of the gene encoding sigma(F) (spoIIAC) restored asymmetric division; however, a mutation that dramatically reduced the number of promoters responsive to sigma(F), spoIIAC561 (spoIIACV233 M), failed to do so. This result suggests that the block is due to expression of one of the small subset of sigma(F)-dependent genes expressed in this background or to unregulated interaction of sigmaF with some other factor. Our results indicate that regulation of SpoIIE plays a critical role in coupling asymmetric division to sigma(F) activation in order to ensure proper spatial and temporal expression of forespore-specific genes.

Generating and exploiting polarity in bacteria

Bacteria are often highly polarized, exhibiting specialized structures at or near the ends of the cell. Among such structures are actin-organizing centers, which mediate the movement of certain pathogenic bacteria within the cytoplasm of an animal host cell; organized arrays of membrane receptors, which govern chemosensory behavior in swimming bacteria; and asymmetrically positioned septa, which generate specialized progeny in differentiating bacteria. This polarization is orchestrated by complex and dynamic changes in the subcellular localization of signal transduction and cytoskeleton proteins as well as of specific regions of the chromosome. Recent work has provided information on how dynamic subcellular localization occurs and how it is exploited by the bacterial cell. The main task of a bacterial cell is to survive and duplicate itself. The bacterium must replicate its genetic material and divide at the correct site in the cell and at the correct time in the cell cycle with high precision. Each kind of bacterium also executes its own strategy to find nutrients in its habitat and to cope with conditions of stress from its environment. This involves moving toward food, adapting to environmental extremes, and, in many cases, entering and exploiting a eukaryotic host. These activities often involve processes that take place at or near the poles of the cell. Here we explore some of the schemes bacteria use to orchestrate dynamic changes at their poles and how these polar events execute cellular functions. In spite of their small size, bacteria have a remarkably complex internal organization and external architecture. Bacterial cells are inherently asymmetric, some more obviously so than others. The most easily recognized asymmetries involve surface structures, e.g., flagella, pili, and stalks that are preferentially assembled at one pole by many bacteria. "New" poles generated at the cell division plane differ from old poles from the previous round of cell division. Even in Escherichia coli, which is generally thought to be symmetrical, old poles are more static than new poles with respect to cell wall assembly (1), and they differ in the deposition of phospholipid domains (2). There are many instances of differential polar functions; among these is the preferential use of old poles when attaching to host cells as in the interaction of Bradyrhizobium with plant root hairs (3) or the polar pili-mediated attachment of the Pseudomonas aeruginosa pathogen to tracheal epithelia (4). An unusual polar organelle that mediates directed motility on solid surfaces is found in the nonpathogenic bacterium Myxococcus xanthus. The gliding motility of this bacterium is propelled by a nozzle-like structure that squirts a polysaccharide-containing slime from the pole of the cell (5). Interestingly, M. xanthus, which has nozzles at both poles, can reverse direction by closing one nozzle and opening the other in response to end-to-end interactions between cells.

The cell differentiation protein SpoIIE contains a regulatory site that controls its phosphatase activity in response to asymmetric septation

Starvation induces Bacillus subtilis to initiate a -simple, two-cell developmental process that begins with an asymmetric cell division. Activation of the first compartment-specific transcription factor, sigmaF, is coupled to this morphological event. SpoIIE, a bifunctional protein, is essential for the compartment-specific activation of sigmaF and also has a morphogenic activity required for asymmetric cell division. SpoIIE consists of three domains: a hydrophobic N-terminal domain, which targets the protein to the membrane; a central domain, involved in oligomerization of SpoIIE and interaction with the cell division protein FtsZ; and a C-terminal domain comprising a PP2C protein phosphatase. Here, we report the isolation of mutations at the very beginning of the central domain of spoIIE, which are capable of activating sigmaF inde-pendently of septum formation. Purified mutant proteins showed the same phosphatase activity as the wild-type protein in vitro. The mutant proteins were fully functional in respect of their localization to sites of asymmetric septation and support of asymmetric division. The data provide strong evidence that the phosphatase domain of SpoIIE is tightly regulated in a way that makes it respond to the formation of the asymmetric septum.

FtsA mutants of Bacillus subtilis impaired in sporulation

Spore formation in Bacillus subtilis involves a switch in the site of cell division from the midcell to a polar position. Both medial division and polar division are mediated in part by the actin-like, cytokinetic protein FtsA. We report the isolation of an FtsA mutant (FtsA(D265G)) that is defective in sporulation but is apparently unimpaired in vegetative growth. Sporulating cells of the mutant reach the stage of asymmetric division but are partially blocked in the subsequent morphological process of engulfment. As judged by fluorescence microscopy and electron microscopy, the FtsA(D265G) mutant produces normal-looking medial septa but immature (abnormally thin) polar septa. The mutant was unimpaired in transcription under the control of Spo0A, the master regulator for entry into sporulation, but was defective in transcription under the control of sigmaF, a regulatory protein whose activation is known to depend on polar division. An amino acid substitution at a residue (Y264) adjacent to D265 also caused a defect in sporulation. D265 and Y264 are conserved among endospore-forming bacteria, raising the possibility that these residues are involved in a sporulation-specific protein interaction that facilitates maturation of the sporulation septum and the activation of sigmaF.

Developmental biology: regulation by selective gene localization

Recent studies have shown that, early during sporulation in Bacillus subtilis, the temporary exclusion of 70% of the chromosome from the forespore compartment is critical to the regulated activation of two major transcription factors, sigma(F) and sigma(E).

Characterization of the parB-like yyaA gene of Bacillus subtilis

We have characterized the yyaA gene of Bacillus subtilis, located near the origin of chromosome replication (oriC). Its protein product is similar to the Spo0J protein, which belongs to the ParB family of chromosome- and plasmid-partitioning proteins. Insertional inactivation of the yyaA gene had no apparent effect on chromosome organization and partitioning during vegetative growth or sporulation. Subcellular localization of YyaA by immunofluorescence microscopy indicated that it colocalizes with the nucleoid, and gel retardation studies confirmed that YyaA binds relatively nonspecifically to DNA. Overexpression of yyaA caused a sporulation defect characterized by the formation of multiple septa within the cell. This phenotype indicates that YyaA may have a regulatory role at the onset of sporulation.

Morphological coupling in development: lessons from prokaryotes

At certain junctures in development, gene transcription is coupled to the completion of landmark morphological events. We refer to this dependence on morphogenesis for gene expression as "morphological coupling." Three examples of morphological coupling in prokaryotes are reviewed in which the activation of a transcription factor is tied to the assembly of a critically important structure in development.

Differential gene expression governed by chromosomal spatial asymmetry

The activity of the transcription factor sigmaF is confined to one (the forespore) of two cells created by asymmetric division during sporulation in B. subtilis. We show that sigmaF activation is partly governed by the position of the gene for the unstable anti-sigmaF factor SpoIIAB. Because cytokinesis precedes chromosome segregation, most of the chromosome is translocated into the forespore after division. We hypothesize that because spoIIAB enters the forespore late, SpoIIAB lost to proteolysis is temporarily not replenished. Thus, chromosome asymmetry would be translated into the asymmetric distribution of SpoIIAB. Supporting this idea, transposition of spoIIAB to sites present in the forespore at the time of division impaired sporulation when a second pathway that participates in sigmaF activation was disabled.

Division site selection protein DivIVA of Bacillus subtilis has a second distinct function in chromosome segregation during sporulation

DivIVA is a coiled-coil, tropomyosin-like protein of Gram-positive bacteria. Previous work showed that this protein is targeted to division sites and retained at the cell poles after division. In vegetative cells, DivIVA sequesters the MinCD division inhibitor to the cell poles, thereby helping to direct cell division to the correct midcell site. We now show that DivIVA has a second, quite separate role in sporulating cells of Bacillus subtilis. It again acts at the cell pole but in this case interacts with the chromosome segregation machinery to help position the oriC region of the chromosome at the cell pole, in preparation for polar division. We isolated mutations in divIVA that separate the protein's role in sporulation from its vegetative function in cell division. DivIVA therefore appears to be a bifunctional protein with distinct roles in division-site selection and chromosome segregation.

Cytological and biochemical characterization of the FtsA cell division protein of Bacillus subtilis

The actin-like protein FtsA is present in many eubacteria, and genetic experiments have shown that it plays an important, sometimes essential, role in cell division. Here, we show that Bacillus subtilis FtsA is targeted to division sites in both vegetative and sporulating cells. As in other organisms FtsA is probably recruited immediately after FtsZ. In sporulating cells of B. subtilis FtsZ is recruited to potential division sites at both poles of the cell, but asymmetric division occurs at only one pole. We have now found that FtsA is recruited to only one cell pole, suggesting that it may play an important role in the generation of asymmetry in this system. FtsA is present in much higher quantities in B. subtilis than in Escherichia coli, with approximately one molecule of FtsA for five of FtsZ. This means that there is sufficient FtsA to form a complete circumferential ring at the division site. Therefore, FtsA may have a direct structural role in cell division. We have purified FtsA and shown that it behaves as a dimer and that it has both ATP-binding and ATP-hydrolysis activities. This suggests that ATP hydrolysis by FtsA is required, together with GTP hydrolysis by FtsZ, for cell division in B. subtilis (and possibly in most eubacteria).

Regulation of sigma factor activity during Bacillus subtilis development

Progression of Bacillus subtilis through a series of morphological changes is driven by a cascade of sigma (sigma) factors and results in formation of a spore. Recent work has provided new insights into the location and function of proteins that control sigma factor activity, and has suggested that multiple mechanisms allow one sigma factor to replace another in the cascade.

The Bacillus subtilis cell division protein FtsL localizes to sites of septation and interacts with DivIC

FtsL is a small bitopic membrane protein required for vegetative cell division and sporulation in Bacillus subtilis. We investigated its localization by fluorescence microscopy using a green fluorescent protein (GFP) fusion. GFP-FtsL was localized at mid-cell in vegetative cells and at the asymmetric septum in sporulating cells. We also show that FtsL forms a ring-like structure at the division site and that it remains localized at mid-cell during the whole septation process. By yeast two-hybrid analysis and non-denaturing polyacrylamide gel electrophoresis (PAGE) with purified proteins, FtsL was found to interact with another membrane-bound division protein, the FtsL-like DivIC protein.

Direct interaction between the cell division protein FtsZ and the cell differentiation protein SpoIIE

SpoIIE is a bifunctional protein with two critical roles in the establishment of cell fate in Bacillus subtilis. First, SpoIIE is needed for the normal formation of the asymmetrically positioned septum that forms early in sporulation and separates the mother cell from the prespore compartment. Secondly, SpoIIE is essential for the activation of the first compartment-specific transcription factor sigma(F) in the prespore. After initiation of sporulation, SpoIIE localizes to the potential asymmetric cell division sites near one or both cell poles. Localization of SpoIIE was shown to be dependent on the essential cell division protein FtsZ. To understand how SpoIIE is targeted to the asymmetric septum we have now analysed its interaction with FtsZ in vitro. Using the yeast two-hybrid system and purified FtsZ, and full-length and truncated SpoIIE proteins, we demonstrate that the two proteins interact directly and that domain II and possibly domain I of SpoIIE are required for the interaction. Moreover, we show that SpoIIE interacts with itself and suggest that this self-interaction plays a role in assembly of SpoIIE into the division machinery.

Intrinsic instability of the essential cell division protein FtsL of Bacillus subtilis and a role for DivIB protein in FtsL turnover

Cell division in most eubacteria is driven by an assembly of about eight conserved division proteins. These proteins form a ring structure that constricts in parallel with the formation of the division septum. Here, we show that one of the division proteins, FtsL, is highly unstable. We also show that the protein is targeted to the ring structure and that targeting occurs in concert with the recruitment of several other membrane-associated division proteins. FtsL stability is further reduced in the absence of DivIB protein (probably homologous to E. coli FtsQ) at high temperature, suggesting that DivIB is involved in the control of FtsL turnover. The reduced stability of FtsL may explain the temperature dependence of divIB mutants, because their phenotype can be suppressed by overexpression of FtsL. The results provide new insights into the roles of the FtsL and DivIB proteins in bacterial cell division.

Cell-cycle checkpoints that ensure coordination between nuclear and cytoplasmic events in Saccharomyces cerevisiae

Cytoskeletal organization is crucial for several aspects of cell-cycle progression but cytoskeletal elements are quite sensitive to environmental perturbations. Two novel checkpoint controls monitor the function of the actin and microtubule systems in budding yeast and operate to delay cell-cycle progression in response to cytoskeletal perturbations. In cells whose actin cytoskeleton has been perturbed, bud formation is frequently delayed and the morphogenesis checkpoint introduces a compensatory delay of nuclear division until a bud has been formed. In cells whose microtubule cytoskeleton has been perturbed, anaphase spindle elongation often occurs entirely within the mother cell, and the post-anaphase nuclear migration checkpoint introduces a compensatory delay of cytokinesis until one pole of the anaphase nucleus enters the bud. Recent studies indicate that regulators of entry into mitosis are localized to the daughter side of the mother-bud neck whereas regulators of exit from mitosis are localized to the spindle pole bodies. Thus, specific cell-cycle regulators are well-placed to monitor whether a cell has formed a bud and whether a daughter nucleus has been delivered accurately to the bud following mitosis.