Septation, dephosphorylation, and the activation of sigmaF during sporulation in Bacillus subtilis

Cell division machinery drives cell-specific gene activation during differentiation in Bacillus subtilis

When faced with starvation, the bacterium Bacillus subtilis transforms itself into a dormant cell type called a "spore". Sporulation initiates with an asymmetric division event, which requires the relocation of the core divisome components FtsA and FtsZ, after which the sigma factor σF is exclusively activated in the smaller daughter cell. Compartment-specific activation of σF requires the SpoIIE phosphatase, which displays a biased localization on one side of the asymmetric division septum and associates with the structural protein DivIVA, but the mechanism by which this preferential localization is achieved is unclear. Here, we isolated a variant of DivIVA that indiscriminately activates σF in both daughter cells due to promiscuous localization of SpoIIE, which was corrected by overproduction of FtsA and FtsZ. We propose that the core components of the redeployed cell division machinery drive the asymmetric localization of DivIVA and SpoIIE to trigger the initiation of the sporulation program.

Cell division machinery drives cell-specific gene activation during bacterial differentiation

When faced with starvation, the bacterium Bacillus subtilis transforms itself into a dormant cell type called a "spore". Sporulation initiates with an asymmetric division event, which requires the relocation of the core divisome components FtsA and FtsZ, after which the sigma factor σF is exclusively activated in the smaller daughter cell. Compartment specific activation of σF requires the SpoIIE phosphatase, which displays a biased localization on one side of the asymmetric division septum and associates with the structural protein DivIVA, but the mechanism by which this preferential localization is achieved is unclear. Here, we isolated a variant of DivIVA that indiscriminately activates σF in both daughter cells due to promiscuous localization of SpoIIE, which was corrected by overproduction of FtsA and FtsZ. We propose that a unique feature of the sporulation septum, defined by the cell division machinery, drives the asymmetric localization of DivIVA and SpoIIE to trigger the initiation of the sporulation program.

Single-molecule optical microscopy of protein dynamics and computational analysis of images to determine cell structure development in differentiating Bacillus subtilis

Here we use singe-molecule optical proteomics and computational analysis of live cell bacterial images, using millisecond super-resolved tracking and quantification of fluorescently labelled protein SpoIIE in single live Bacillus subtilis bacteria to understand its crucial role in cell development. Asymmetric cell division during sporulation in Bacillus subtilis presents a model system for studying cell development. SpoIIE is a key integral membrane protein phosphatase that couples morphological development to differential gene expression. However, the basic mechanisms behind its operation remain unclear due to limitations of traditional tools and technologies. We instead used advanced single-molecule imaging of fluorescently tagged SpoIIE in real time on living cells to reveal vital changes to the patterns of expression, localization, mobility and stoichiometry as cells undergo asymmetric cell division then engulfment of the smaller forespore by the larger mother cell. We find, unexpectedly, that SpoIIE forms tetramers capable of cell- and stage-dependent clustering, its copy number rising to ~ 700 molecules as sporulation progresses. We observed that slow moving SpoIIE clusters initially located at septa are released as mobile clusters at the forespore pole as phosphatase activity is manifested and compartment-specific RNA polymerase sigma factor, σF, becomes active. Our findings reveal that information captured in its quaternary organization enables one protein to perform multiple functions, extending an important paradigm for regulatory proteins in cells. Our findings more generally demonstrate the utility of rapid live cell single-molecule optical proteomics for enabling mechanistic insight into the complex processes of cell development during the cell cycle.

Comparison of expression of key sporulation, solventogenic and acetogenic genes in C. beijerinckii NRRL B-598 and its mutant strain overexpressing spo0A

The production of acetone, butanol and ethanol by fermentation of renewable biomass has potential to become a valuable industrial process. Mechanisms of solvent production and sporulation involve some common regulators in some ABE-producing clostridia, although details of the links between the pathways are not clear. In this study, we compare a wild-type (WT) Clostridium beijerinckii NRRL B-598 with its mutant strain OESpo0A, in which the gene encoding Spo0A, an important regulator of both sporulation and solventogenesis, is overexpressed in terms of solvent and acid production. We also compare morphologies during growth on two different media: TYA broth, where the WT culture sporulates, and RCM, where the WT culture does not. In addition, RT-qPCR-based analysis of expression profiles of spo0A, spoIIE, sigG, spoVD, ald and buk1 genes involved in sporulation or solvent production in these strains, were compared. The OESpo0A mutant did not produce spores and butanol titre was lower compared to the WT, but increased amounts of butyric acid and ethanol were produced. The gene spo0A had high levels of expression in the WT under non-sporulating culture conditions while other selected genes for sporulation factors were downregulated significantly. Similar observations were obtained for OESpo0A where spo0A overexpression and downregulation of other sporulation genes were demonstrated. Higher expression of spo0A led to higher expression of buk1 and ald, which could confirm the role of spo0A in activation of the solventogenic pathway, although solvent production was not affected significantly in the WT and was weakened in the OESpo0A mutant.

Analysis of Spo0M function in Bacillus subtilis

Spo0M has been previously reported as a regulator of sporulation in Bacillus subtilis; however, little is known about the mechanisms through which it participates in sporulation, and there is no information to date that relates this protein to other processes in the bacterium. In this work we present evidence from proteomic, protein-protein interaction, morphological, subcellular localization microscopy and bioinformatics studies which indicate that Spo0M function is not necessarily restricted to sporulation, and point towards its involvement in other stages of the vegetative life cycle. In the current study, we provide evidence that Spo0M interacts with cytoskeletal proteins involved in cell division, which suggest a function additional to that previously described in sporulation. Spo0M expression is not restricted to the transition phase or sporulation; rather, its expression begins during the early stages of growth and Spo0M localization in B. subtilis depends on the bacterial life cycle and could be related to an additional proposed function. This is supported by our discovery of homologs in a broad distribution of bacterial genera, even in non-sporulating species. Our work paves the way for re-evaluation of the role of Spo0M in bacterial cell.

A Novel Cell Type Enables B. subtilis to Escape from Unsuccessful Sporulation in Minimal Medium

Sporulation is the most enduring survival strategy developed by several bacterial species. However, spore development of the model organism Bacillus subtilis has mainly been studied by means of media or conditions optimized for the induction of sporogenesis. Here, I show that during prolonged growth during stationary phase in minimal medium, B. subtilis undergoes an asymmetric cell division that produces small and round-shaped, DNA containing cells. In contrast to wild-type cells, mutants harboring spo0A or spoIIIE/sftA double mutations neither sporulate nor produce this special cell type, providing evidence that the small round cells emerge from the abortion of endospore formation. In most cases observed, the small round cells arise in the presence of sigma H but absence of sigma F activity, different from cases of abortive sporulation described for rich media. These data suggest that in minimal media, many cells are able to initiate but fail to complete spore development, and therefore return to normal growth as rods. This work reveals that the continuation of asymmetric cell division, which results in the formation of the small round cells, is a way for cells to delay or escape from—unsuccessful—sporulation. Based on these findings, I suggest to name the here described cell type as “dwarf cells” to distinguish them from the well-known minicells observed in mutants defective in septum placement or proper chromosome partitioning.

GerM is required to assemble the basal platform of the SpoIIIA-SpoIIQ transenvelope complex during sporulation in Bacillus subtilis

Sporulating Bacillus subtilis cells assemble a multimeric membrane complex connecting the mother cell and developing spore that is required to maintain forespore differentiation. An early step in the assembly of this transenvelope complex (called the A-Q complex) is an interaction between the extracellular domains of the forespore membrane protein SpoIIQ and the mother cell membrane protein SpoIIIAH. This interaction provides a platform onto which the remaining components of the complex assemble and also functions as an anchor for cell-cell signalling and morphogenetic proteins involved in spore development. SpoIIQ is required to recruit SpoIIIAH to the sporulation septum on the mother cell side; however, the mechanism by which SpoIIQ specifically localizes to the septal membranes on the forespore side has remained enigmatic. Here, we identify GerM, a lipoprotein previously implicated in spore germination, as the missing factor required for SpoIIQ localization. Our data indicate that GerM and SpoIIIAH, derived from the mother cell, and SpoIIQ, from the forespore, have reciprocal localization dependencies suggesting they constitute a tripartite platform for the assembly of the A-Q complex and a hub for the localization of mother cell and forespore proteins.

Protein Targeting during Bacillus subtilis Sporulation

The Gram-positive bacterium Bacillus subtilis initiates the formation of an endospore in response to conditions of nutrient limitation. The morphological differentiation that spores undergo initiates with the formation of an asymmetric septum near to one pole of the cell, forming a smaller compartment, the forespore, and a larger compartment, the mother cell. This process continues with the complex morphogenesis of the spore as governed by an intricate series of interactions between forespore and mother cell proteins across the inner and outer forespore membranes. Given that these interactions occur at a particular place in the cell, a critical question is how the proteins involved in these processes get properly targeted, and we discuss recent progress in identifying mechanisms responsible for this targeting.

The Clostridium sporulation programs: diversity and preservation of endospore differentiation

SUMMARY

Bacillus and Clostridium organisms initiate the sporulation process when unfavorable conditions are detected. The sporulation process is a carefully orchestrated cascade of events at both the transcriptional and posttranslational levels involving a multitude of sigma factors, transcription factors, proteases, and phosphatases. Like Bacillus genomes, sequenced Clostridium genomes contain genes for all major sporulation-specific transcription and sigma factors (spo0A, sigH, sigF, sigE, sigG, and sigK) that orchestrate the sporulation program. However, recent studies have shown that there are substantial differences in the sporulation programs between the two genera as well as among different Clostridium species. First, in the absence of a Bacillus-like phosphorelay system, activation of Spo0A in Clostridium organisms is carried out by a number of orphan histidine kinases. Second, downstream of Spo0A, the transcriptional and posttranslational regulation of the canonical set of four sporulation-specific sigma factors (σ(F), σ(E), σ(G), and σ(K)) display different patterns, not only compared to Bacillus but also among Clostridium organisms. Finally, recent studies demonstrated that σ(K), the last sigma factor to be activated according to the Bacillus subtilis model, is involved in the very early stages of sporulation in Clostridium acetobutylicum, C. perfringens, and C. botulinum as well as in the very late stages of spore maturation in C. acetobutylicum. Despite profound differences in initiation, propagation, and orchestration of expression of spore morphogenetic components, these findings demonstrate not only the robustness of the endospore sporulation program but also the plasticity of the program to generate different complex phenotypes, some apparently regulated at the epigenetic level.

A love affair with Bacillus subtilis

My career in science was launched when I was an undergraduate at Princeton University and reinforced by graduate training at the Massachusetts Institute of Technology. However, it was only after I moved to Harvard University as a junior fellow that my affections were captured by a seemingly mundane soil bacterium. What Bacillus subtilis offered was endless fascinating biological problems (alternative sigma factors, sporulation, swarming, biofilm formation, stochastic cell fate switching) embedded in a uniquely powerful genetic system. Along the way, my career in science became inseparably interwoven with teaching and mentoring, which proved to be as rewarding as the thrill of discovery.

Asymmetric division and differential gene expression during a bacterial developmental program requires DivIVA

Sporulation in the bacterium Bacillus subtilis is a developmental program in which a progenitor cell differentiates into two different cell types, the smaller of which eventually becomes a dormant cell called a spore. The process begins with an asymmetric cell division event, followed by the activation of a transcription factor, σ^F, specifically in the smaller cell. Here, we show that the structural protein DivIVA localizes to the polar septum during sporulation and is required for asymmetric division and the compartment-specific activation of σ^F. Both events are known to require a protein called SpoIIE, which also localizes to the polar septum. We show that DivIVA copurifies with SpoIIE and that DivIVA may anchor SpoIIE briefly to the assembling polar septum before SpoIIE is subsequently released into the forespore membrane and recaptured at the polar septum. Finally, using super-resolution microscopy, we demonstrate that DivIVA and SpoIIE ultimately display a biased localization on the side of the polar septum that faces the smaller compartment in which σ^F is activated.

A central feature of developmental programs is the establishment of asymmetry and the production of genetically identical daughter cells that display different cell fates. Sporulation in the bacterium Bacillus subtilis is a simple developmental program in which the cell divides asymmetrically to produce two daughter cells, after which the transcription factor σ^F is activated specifically in the smaller cell. Here we investigated DivIVA, which localizes to highly negatively curved membranes, and discovered that it localizes at the asymmetric division site. In the absence of DivIVA, cells failed to asymmetrically divide and prematurely activated σ^F in the predivisional cell, largely unreported phenotypes for any deletion mutant in a sporulation gene. We found that DivIVA copurifies with SpoIIE, a protein that is required for asymmetric division and σ^F activation, and that both proteins preferentially localize on the side of the septum facing the smaller daughter cell. DivIVA is therefore a previously overlooked structural factor that is required at the onset of sporulation to mediate both asymmetric division and compartment-specific transcription.

Spo0A links de novo fatty acid synthesis to sporulation and biofilm development in Bacillus subtilis

During sporulation in Bacillus subtilis, the committed-cell undergoes substantial membrane rearrangements to generate two cells of different sizes and fates: the mother cell and the forespore. Here, we demonstrate that the master transcription factor Spo0A reactivates lipid synthesis during development. Maximal Spo0A-dependent lipid synthesis occurs during the key stages of asymmetric division and forespore engulfment. Spo0A reactivates the accDA operon that encodes the carboxylase component of the acetyl-CoA carboxylase enzyme, which catalyses the first and rate-limiting step in de novo lipid biosynthesis, malonyl-CoA formation. The disruption of the Spo0A-binding box in the promoter region of accDA impairs its transcriptional reactivation and blocks lipid synthesis. The Spo0A-insensitive accDA(0A) cells were proficient in planktonic growth but defective in sporulation (σ(E) activation) and biofilm development (cell cluster formation and water repellency). Exogenous fatty acid supplementation to accDA(0A) cells overcomes their inability to synthesize lipids during development and restores sporulation and biofilm proficiencies. The transient exclusion of the lipid synthesis regulon from the forespore and the known compartmentalization of Spo0A and ACP in the mother cell suggest that de novo lipid synthesis is confined to the mother cell. The significance of the Spo0A-controlled de novo lipid synthesis during B. subtilis development is discussed.

Inferring Biological Mechanisms by Data-Based Mathematical Modelling: Compartment-Specific Gene Activation during Sporulation in Bacillus subtilis as a Test Case

Biological functionality arises from the complex interactions of simple components. Emerging behaviour is difficult to recognize with verbal models alone, and mathematical approaches are important. Even few interacting components can give rise to a wide range of different responses, that is, sustained, transient, oscillatory, switch-like responses, depending on the values of the model parameters. A quantitative comparison of model predictions and experiments is therefore important to distinguish between competing hypotheses and to judge whether a certain regulatory behaviour is at all possible and plausible given the observed type and strengths of interactions and the speed of reactions. Here I will review a detailed model for the transcription factor σ(F), a regulator of cell differentiation during sporulation in Bacillus subtilis. I will focus in particular on the type of conclusions that can be drawn from detailed, carefully validated models of biological signaling networks. For most systems, such detailed experimental information is currently not available, but accumulating biochemical data through technical advances are likely to enable the detailed modelling of an increasing number of pathways. A major challenge will be the linking of such detailed models and their integration into a multiscale framework to enable their analysis in a larger biological context.

SpoIIE is necessary for asymmetric division, sporulation, and expression of sigmaF, sigmaE, and sigmaG but does not control solvent production in Clostridium acetobutylicum ATCC 824

In order to better characterize the initial stages of sporulation past Spo0A activation and the associated solventogenesis in the important industrial and model organism Clostridium acetobutylicum, the spoIIE gene was successfully disrupted and its expression was silenced. By silencing spoIIE, sporulation was blocked prior to asymmetric division, and no mature spores or any distinguishable morphogenetic changes developed. Upon plasmid-based complementation of spoIIE, sporulation was restored, although the number of spores formed was below that of the plasmid control strain. To investigate the impact of silencing spoIIE on the regulation of sporulation, transcript levels of sigF, sigE, and sigG were examined by semiquantitative reverse transcription-PCR, and the corresponding σF, σE, and σG protein levels were determined by Western analysis. Expression of sigF was significantly reduced in the inactivation strain, and this resulted in very low σF protein levels. Expression of sigE was barely detected, and no sigG transcript was detected at all; consequently, no σE or σG proteins were detected. These data suggest an autostimulatory role for σF in C. acetobutylicum, in contrast to the model organism for endospore formation, Bacillus subtilis, and confirm that high-level expression of σF is required for expression of σE and σG. Unlike the σF and σE inactivation strains, the SpoIIE inactivation strain did not exhibit inoculum-dependent solvent formation and produced good levels of solvents from both exponential- and stationary-phase inocula. Thus, we concluded that SpoIIE does not control solvent formation.

Inactivation of σF in Clostridium acetobutylicum ATCC 824 blocks sporulation prior to asymmetric division and abolishes σE and σG protein expression but does not block solvent formation

Clostridium acetobutylicum is both a model organism for the understanding of sporulation in solventogenic clostridia and its relationship to solvent formation and an industrial organism for anaerobic acetone-butanol-ethanol (ABE) fermentation. How solvent production is coupled to endospore formation--both stationary-phase events--remains incompletely understood at the molecular level. Specifically, it is unclear how sporulation-specific sigma factors affect solvent formation. Here the sigF gene in C. acetobutylicum was successfully disrupted and silenced. Not only σ(F) but also the sigma factors σ(E) and σ(G) were not detected in the sigF mutant (FKO1), and differentiation was stopped prior to asymmetric division. Since plasmid expression of the spoIIA operon (spoIIAA-spoIIAB-sigF) failed to complement FKO1, the operon was integrated into the FKO1 chromosome to generate strain FKO1-C. In FKO1-C, σ(F) expression was restored along with sporulation and σ(E) and σ(G) protein expression. Quantitative reverse transcription-PCR (RT-PCR) analysis of a select set of genes (csfB, gpr, spoIIP, sigG, lonB, and spoIIR) that could be controlled by σ(F), based on the Bacillus subtilis model, indicated that sigG may be under the control of σ(F), but spoIIR, an important activator of σ(E) in B. subtilis, is not, and neither are the rest of the genes investigated. FKO1 produced solvents at a level similar to that of the parent strain, but solvent levels were dependent on the physiological state of the inoculum. Finally, the complementation strain FKO1-C is the first reported instance of purposeful integration of multiple functional genes into a clostridial chromosome--here, the C. acetobutylicum chromosome--with the aim of altering cell metabolism and differentiation.

The spoIIE homolog of Epulopiscium sp. type B is expressed early in intracellular offspring development

Epulopiscium sp. type B is an enormous intestinal symbiont of the surgeonfish Naso tonganus. Intracellular offspring production in Epulopiscium shares features with endospore formation. Here, we characterize the spoIIE homolog in Epulopiscium. The timing of spoIIE gene expression and presence of interacting partners suggest that the activation of σ(F) occurs early in Epulopiscium offspring development.

SpoIIQ anchors membrane proteins on both sides of the sporulation septum in Bacillus subtilis

During the process of spore formation in Bacillus subtilis, many membrane proteins localize to the polar septum where they participate in morphogenesis and signal transduction. The forespore membrane protein SpoIIQ plays a central role in anchoring several mother-cell membrane proteins in the septal membrane. Here, we report that SpoIIQ is also responsible for anchoring a membrane protein on the forespore side of the sporulation septum. Co-immunoprecipitation experiments reveal that SpoIIQ resides in a complex with the polytopic membrane protein SpoIIE. During the early stages of sporulation, SpoIIE participates in the switch from medial to polar division and co-localizes with FtsZ at the polar septum. We show that after cytokinesis, SpoIIE is released from the septum and transiently localizes to all membranes in the forespore compartment. Upon the initiation of engulfment, it specifically re-localizes to the septal membrane on the forespore side. Importantly, the re-localization of SpoIIE to the engulfing septum requires SpoIIQ. These results indicate that SpoIIQ is required to anchor membrane proteins on both sides of the division septum. Moreover, our data suggest that forespore membrane proteins can localize to the septal membrane by diffusion-and-capture as has been described for membrane proteins in the mother cell. Finally, our results raise the intriguing possibility that SpoIIE has an uncharacterized function at a late stage of sporulation.

Signalling network with a bistable hysteretic switch controls developmental activation of the sigma transcription factor in Bacillus subtilis

The sporulation process of the bacterium Bacillus subtilis unfolds by means of separate but co-ordinated programmes of gene expression within two unequal cell compartments, the mother cell and the smaller forespore. sigmaF is the first compartment-specific transcription factor activated during this process, and it is controlled at the post-translational level by a partner-switching mechanism that restricts sigmaF activity to the forespore. The crux of this mechanism lies in the ability of the anti-sigma factor SpoIIAB (AB) to form alternative complexes either with sigmaF, holding it in an inactive form, or with the anti-anti-sigma factor SpoIIAA (AA) and a nucleotide, either ATP or ADP. In the complex with AB and ATP, AA is phosphorylated on a serine residue and released, making AB available to capture sigmaF in an inactive complex. Subsequent activation of sigmaF requires the intervention of the SpoIIE serine phosphatase to dephosphorylate AA, which can then attack the AB-sigmaF complex to induce the release of sigmaF. By incorporating biochemical, biophysical and genetic data from the literature we have constructed an integrative mathematical model of this partner-switching network. The model predicts that the self-enhancing formation of a long-lived complex of AA, AB and ADP transforms the network into an essentially irreversible hysteretic switch, thereby explaining the sharp, robust and irreversible activation of sigmaF in the forespore compartment. The model also clarifies the contributions of the partly redundant mechanisms that ensure correct spatial and temporal activation of sigmaF, reproduces the behaviour of various mutants and makes strong, testable predictions.

The mechanism of cell differentiation in Bacillus subtilis

Sporulation in Bacillus subtilis serves as a model for the development of two different cell types from a single cell. Although much information has been accumulated about the mechanisms that initiate the developmental programmes, important questions remain that can be answered only by quantitative analysis. Here we develop, with the help of existing and new experimental results, a mathematical model that reproduces published in vitro experiments and explains how the activation of the key transcription factor is regulated. The model identifies the difference in volume between the two cell types as the primary trigger for determining cell fate. It shows that this effect depends on the allosteric behaviour of a key protein kinase and on a low rate of dephosphorylation by the corresponding phosphatase; both predicted effects are confirmed experimentally.

Contributions of protein structure and gene position to the compartmentalization of the regulatory proteins sigma(E) and SpoIIE in sporulating Bacillus subtilis

At an early stage in endospore formation Bacillus subtilis partitions itself into two dissimilar compartments with unique developmental fates. Transcription appropriate to each compartment is initiated by the activation of compartment-specific RNA polymerase sigma subunits, sigma(E) in the mother cell and sigma(F) in the forespore. Among the possible factors contributing to the compartment specificity of sigma(E) and sigma(F) is the selective accumulation of the sigma(E) protein in the mother cell and that of SpoIIE, a regulatory phosphatase essential to the activation of sigma(F), in the forespore. In the current work, fluorescent microscopy is used to investigate the contributions of sigma(E) and SpoIIE's protein structures, expression and the genetic asymmetry that develops during chromosome translocation into the forespore on their abundance in each compartment. Time of entry of the spoIIE and sigE genes into the forespore was found to have a significant effect on the enrichment of their products in one or the other compartment. In contrast, the structures of the proteins themselves do not appear to promote their transfer to a particular compartment, but nonetheless contribute to compartmentalization by facilitating degradation in the compartment where each protein's activity would be inappropriate.

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).

Differential gene expression in genetically identical sister cells: the initiation of sporulation in Bacillus subtilis

Early in sporulation, the cell divides asymmetrically to give two sister compartments, a smaller prespore and a larger mother cell. Differential gene expression in these compartments depends on the regulation of the first sporulation-specific sigma factor, sigma(F), which is activated only in the prespore. Regulation relies on the interactions of four proteins -sigma(F), its antisigma SpoIIAB (which also has protein kinase activity), the anti-antisigma SpoIIAA and the protein phosphatase SpoIIE. Before asymmetric division, and in the mother cell after division, sigma(F) is held in an inactive complex with SpoIIAB and ATP; SpoIIAA is in its phosphorylated form. To disrupt the complex so as to liberate sigma(F) in the prespore, dephosphorylated SpoIIAA is needed, and this is made available by SpoIIE. Thereafter, SpoIIAB and SpoIIE are active simultaneously in the prespore, cycling SpoIIAA through phosphorylated and non-phosphorylated forms. This cycle detains SpoIIAB in a state in which it cannot inhibit sigma(F). Results from biophysical techniques, mathematical simulations and enzyme kinetics have now helped to elucidate the dynamics of the protein-protein interactions involved. An understanding of these dynamics largely accounts for the regulation of sigma(F). We show that the system is tuned to be highly efficient in its use of components and extremely economical in conserving ATP.

Genetic dissection of the sporulation protein SpoIIE and its role in asymmetric division in Bacillus subtilis

SpoIIE is a dual-function protein in Bacillus subtilis that contributes to the switch from medial to polar cell division during sporulation and is responsible for activating the cell-specific transcription factor sigma(F). SpoIIE consists of an N-terminal domain with 10 membrane-spanning segments (region I), a C-terminal phosphatase domain (region III), and a central domain (region II) of uncertain function. To investigate the role of SpoIIE in polar division, we took advantage of a system for efficiently producing polar septa during growth in a SpoIIE-dependent manner using cells engineered to produce the sporulation protein in response to an inducer. The results show that regions II and III play a critical role in polar septum formation and that specific amino acid substitutions in those regions affect the abilities of SpoIIE both to promote polar division and to localize to the division machinery. Additionally, we show that neither the phosphatase function of SpoIIE nor the N-terminal, membrane-spanning region is needed for the switch to asymmetric division.

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.

Zipper-like interaction between proteins in adjacent daughter cells mediates protein localization

Protein localization is crucial for cellular morphogenesis and intracellular signal transduction cascades. Here we describe an interaction between two membrane proteins expressed in different cells of the Bacillus subtilis sporangium, the mother cell protein SpoIIIAH and the forespore protein SpoIIQ. We used affinity chromatography, coimmunoprecipitation, and the yeast two-hybrid system to demonstrate that the extracellular domains of these proteins interact, tethering SpoIIIAH to the sporulation septum, and directing its assembly with SpoIIQ into helical arcs and foci around the forespore. We also demonstrate that this interaction can direct proteins made in the same cell to active division sites, as when SpoIIQ is made in the mother cell, it localizes to nascent septa in a SpoIIIAH-dependent manner. Both SpoIIIAH and SpoIIQ are necessary for activation of the second forespore-specific transcription factor (sigma(G)) after engulfment, and we propose that the SpoIIIAH-SpoIIQ complex contributes to a morphological checkpoint coupling sigma(G) activation to engulfment. In keeping with this hypothesis, SpoIIIAH localization depends on the first step of engulfment, septal thinning. The SpoIIQ-SpoIIIAH complex reaches from the mother cell cytoplasm to the forespore cytoplasm and is ideally positioned to govern the activity of engulfment-dependent transcription factors.

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.

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.

Contrasting effects of sigmaE on compartmentalization of sigmaF activity during sporulation of Bacillus subtilis

Spore formation by Bacillus subtilis is a primitive form of development. In response to nutrient starvation and high cell density, B. subtilis divides asymmetrically, resulting in two cells with different sizes and cell fates. Immediately after division, the transcription factor sigmaF becomes active in the smaller prespore, which is followed by the activation of sigmaE in the larger mother cell. In this report, we examine the role of the mother cell-specific transcription factor sigmaE in maintaining the compartmentalization of gene expression during development. We have studied a strain with a deletion of the spoIIIE gene, encoding a DNA translocase, that exhibits uncompartmentalized sigmaF activity. We have determined that the deletion of spoIIIE alone does not substantially impact compartmentalization, but in the spoIIIE mutant, the expression of putative peptidoglycan hydrolases under the control of sigmaE in the mother cell destroys the integrity of the septum. As a consequence, small proteins can cross the septum, thereby abolishing compartmentalization. In addition, we have found that in a mutant with partially impaired control of sigmaF, the activation of sigmaE in the mother cell is important to prevent the activation of sigmaF in this compartment. Therefore, the activity of sigmaE can either maintain or abolish the compartmentalization of sigmaF, depending upon the genetic makeup of the strain. We conclude that sigmaE activity must be carefully regulated in order to maintain compartmentalization of gene expression during development.

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.

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.

Transient genetic asymmetry and cell fate in a bacterium

Certain species of Gram-positive bacteria can initiate a developmental program that results in the formation of two daughter cells with different fates. One cell develops into a spore and the other cell undergoes programmed lysis, with each process being mediated by a cascade of cell-type-specific transcription factors. An early and critical step in this developmental pathway is the formation of a division septum near one pole, creating two compartments of different sizes. But how is this morphological asymmetry translated into the transcriptional asymmetry of the two compartments? Recent results suggest that the chromosomal position of the genes encoding several key components of the transcriptional regulatory network has an important role in this process.

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.

Mutation in yaaT leads to significant inhibition of phosphorelay during sporulation in Bacillus subtilis

In the course of a Bacillus subtilis functional genomics project which involved screening for sporulation genes, we identified an open reading frame, yaaT, whose disruptant exhibits a sporulation defect. Twenty-four hours after the initiation of sporulation, most cells of the yaaT mutant exhibited stage 0 of sporulation, indicating that the yaaT mutation blocks sporulation at an early stage. Furthermore, the mutation in yaaT led to a significant decrease in transcription from a promoter controlled by Spo0A, a key response regulator required for the initiation of sporulation. However, neither the level of transcription of spo0A, the activity of sigma(H), which transcribes spo0A, nor the amount of Spo0A protein was severely affected by the mutation in yaaT. Bypassing the phosphorelay by introducing an spo0A mutation (sof-1) into the yaaT mutant suppressed the sporulation defect, suggesting that the yaaT mutation interferes with the phosphorelay process comprising Spo0F, Spo0B, and histidine kinases. We also observed that mutation of spo0E, which encodes the phosphatase that dephosphorylates Spo0A-P, suppressed the sporulation defect in the yaaT mutant. These results strongly suggest that yaaT plays a significant role in the transduction of signals to the phosphorelay for initiation of sporulation. Micrographs indicated that YaaT-green fluorescent protein localizes to the peripheral membrane, as well as to the septum, during sporulation.

A widely conserved bacterial cell division protein that promotes assembly of the tubulin-like protein FtsZ

Cell division in bacteria is mediated by the tubulin-like protein FtsZ, which assembles into a structure known as the Z ring at the future site of cytokinesis. We report the discovery of a Z-ring-associated protein in Bacillus subtilis called ZapA. ZapA was found to colocalize with the Z ring in vivo and was capable of binding to FtsZ and stimulating the formation of higher-order assemblies of the cytokinetic protein in vitro. The absence of ZapA alone did not impair cell viability, but the absence of ZapA in combination with the absence of a second, dispensable division protein EzrA caused a severe block in cytokinesis. The absence of ZapA also caused lethality in cells producing lower than normal levels of FtsZ or lacking the division-site-selection protein DivIVA. Conversely, overproduction of ZapA reversed the toxicity of excess levels of the division inhibitor MinD. In toto, the evidence indicates that ZapA is part of the cytokinetic machinery of the cell and acts by promoting Z-ring formation. Finally, ZapA is widely conserved among bacteria with apparent orthologs in many species, including Escherichia coli, in which the orthologous protein exhibited a strikingly similar pattern of subcellular localization to that of ZapA. Members of the ZapA family of proteins are likely to be a common feature of the cytokinetic machinery in bacteria.

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.

Asymmetric cell division in B. subtilis involves a spiral-like intermediate of the cytokinetic protein FtsZ

A fundamental feature of development in the spore-forming bacterium Bacillus subtilis is the switch from medial to asymmetric division. The switch is brought about by a change in the location of the cytokinetic Z ring, which is composed of the tubulin-like protein FtsZ, from the cell middle to the poles during sporulation. We report that the medial Z ring is replaced by a spiral-like filament of FtsZ that grows along the long axis of the cell. We propose that the filament mediates the switch by redeploying FtsZ to the poles. Spiral formation and the switch to polar Z rings are largely caused by a sporulation-specific increase in transcription of the gene for FtsZ and activation of the gene for the FtsZ-associated protein SpoIIE.

A sporulation membrane protein tethers the pro-sigmaK processing enzyme to its inhibitor and dictates its subcellular localization

The developmental transcription factor sigmaK is derived from the inactive precursor protein pro-sigmaK by regulated proteolysis during the process of sporulation in the bacterium Bacillus subtilis. The putative pro-sigmaK processing enzyme SpoIVFB is a member of a family of membrane-embedded metalloproteases and is held inactive by two other integral membrane proteins, SpoIVFA and BofA. Herein we show that the processing enzyme and its two regulators exist in a multimeric complex that localizes to the membrane surrounding the developing spore (the forespore). We further show that one of the regulators, SpoIVFA, plays a central role in both the formation of this complex and its subcellular localization. Evidence is presented in support of a model in which SpoIVFA acts as a platform for bringing BofA and SpoIVFB together, whereby BofA inhibits pro-sigmaK processing until a signal has been received from the forespore.

Analysis of the Bacillus subtilis spoIIIJ gene and its Paralogue gene, yqjG

The Bacillus subtilis spoIIIJ gene, which has been proven to be vegetatively expressed, has also been implicated as a sporulation gene. Recent genome sequencing information in many organisms reveals that spoIIIJ and its paralogous gene, yqjG, are conserved from prokaryotes to humans. A homologue of SpoIIIJ/YqjG, the Escherichia coli YidC is involved in the insertion of membrane proteins into the lipid bilayer. On the basis of this similarity, it was proposed that the two homologues act as translocase for the membrane proteins. We studied the requirements for spoIIIJ and yqjG during vegetative growth and sporulation. In rich media, the growth of spoIIIJ and yqjG single mutants were the same as that of the wild type, whereas spoIIIJ yqjG double inactivation was lethal, indicating that together these B. subtilis translocase homologues play an important role in maintaining the viability of the cell. This result also suggests that SpoIIIJ and YqjG probably control significantly overlapping functions during vegetative growth. spoIIIJ mutations have already been established to block sporulation at stage III. In contrast, disruption of yqjG did not interfere with sporulation. We further show that high level expression of spoIIIJ during vegetative phase is dispensable for spore formation, but the sporulation-specific expression of spoIIIJ is necessary for efficient sporulation even at the basal level. Using green fluorescent protein reporter to monitor SpoIIIJ and YqjG localization, we found that the proteins localize at the cell membrane in vegetative cells and at the polar and engulfment septa in sporulating cells. This localization of SpoIIIJ at the sporulation-specific septa may be important for the role of spoIIIJ during sporulation.

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).

An investigation into the compartmentalization of the sporulation transcription factor sigmaE in Bacillus subtilis

Sporulation in Bacillus subtilis involves the formation of a polar septum, which divides the sporangium into a mother cell and a forespore. The sigmaE factor, which is encoded within the spoIIG operon, is a cell-specific regulatory protein that directs gene transcription in the mother cell. SigmaE is synthesized as an inactive proprotein pro-sigmaE, which is converted to the mature factor by the putative processing enzyme SpoIIGA. Processing of pro-sigmaE does not commence until after asymmetric division when sigmaE is largely confined to the mother cell. Processing depends on the signalling protein SpoIIR, which delays proteolysis until after polar septation, but the mechanism by which sigmaE is confined to the mother cell is not understood. Previous work favoured a model in which pro-sigmaE localizes to the mother cell face of the polar septum, such that sigmaE would be selectively released into mother cell cytoplasm. Based on the use of green fluorescent protein (GFP) fusions, we now report that pro-sigmaE is distributed approximately uniformly along all membrane surfaces and is not confined to the mother- cell face of the septum. Rather, our results are consistent with a model in which preferential and persistent transcription of the spoIIG operon in the mother cell and degradation of sigmaE in the forespore contribute to the selective accumulation of sigmaE in the mother cell. Persistent transcription of spoIIG after polar septation also contributes to the proper timing of pro-sigmaE processing.

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.

A three-protein inhibitor of polar septation during sporulation in Bacillus subtilis

We present evidence for a three-protein inhibitor of polar division that locks in asymmetry after the formation of a polar septum during sporulation in Bacillus subtilis. Asymmetric division involves the formation of cytokinetic Z-rings near both poles of the developing cell. Next, a septum is formed at one of the two polar Z-rings, thereby generating a small, forespore cell and a mother cell. Gene expression under the control of the mother-cell transcription factor sigmaE is needed to block cytokinesis at the pole distal to the newly formed septum. We report that this block in polar cytokinesis is mediated partly by sigmaE-directed transcription of spoIID, spoIIM and spoIIP, sporulation genes that were known to be involved in the subsequent process of forespore engulfment. We find that a spoIID, spoIIM and spoIIP triple mutant substantially mimicked the bipolar division phenotype of a sigmaE mutant and that cells engineered to produce SpoIID, SpoIIM and SpoIIP prematurely were inhibited in septum formation at both poles. Consistent with the hypothesis that SpoIID, SpoIIM and SpoIIP function at both poles of the sporangium, a GFP--SpoIIM fusion localized to the membrane that surrounds the engulfed forespore and to the potential division site at the distal pole.