Septal localization of forespore membrane proteins during engulfment in Bacillus subtilis

Ribosomes translocation into the spore of Bacillus subtilis is highly organised and requires peptidoglycan rearrangements

In the spore-forming bacterium Bacillus subtilis transcription and translation are uncoupled and the translational machinery is located at the cell poles. During sporulation, the cell undergoes morphological changes including asymmetric division and chromosome translocation into the forespore. However, the fate of translational machinery during sporulation has not been described. Here, using microscopy and mass spectrometry, we show the localisation of ribosomes during sporulation in wild type and mutant Bacillus subtilis. We demonstrate that ribosomes are associated with the asymmetric septum, a functionally important organelle in the cell's developmental control, and that SpoIIDMP-driven peptidoglycan rearrangement is crucial for ribosomes packing into the forespore. We also show that the SpoIIIA-SpoIIQ 'feeding-tube' channel is not required for ribosome translocation. Our results demonstrate that translation and translational machinery are temporally and spatially organised in B. subtilis during sporulation and that the forespore 'inherits' ribosomes from the mother cell. We propose that the movement of ribosomes in the cell may be mediated by the bacterial homologs of cytoskeletal proteins and that the cues for asymmetric division localisation may be translation-dependent. We anticipate our findings to elicit more sophisticated structural and mechanistic studies of ribosome organisation during bacterial cell development.

Remodeling of the Enterococcal Cell Envelope during Surface Penetration Promotes Intrinsic Resistance to Stress

Enterococcus faecalis is a normal commensal of the human gastrointestinal tract (GIT). However, upon disruption of gut homeostasis, this nonmotile bacterium can egress from its natural niche and spread to distal organs. While this translocation process can lead to life-threatening systemic infections, the underlying mechanisms remain largely unexplored. Our prior work showed that E. faecalis migration across diverse surfaces requires the formation of matrix-covered multicellular aggregates and the synthesis of exopolysaccharides, but how enterococcal cells are reprogrammed during this process is unknown. Whether surface penetration endows E. faecalis with adaptive advantages is also uncertain. Here, we report that surface penetration promotes the generation of a metabolically and phenotypically distinct E. faecalis population with an enhanced capacity to endure various forms of extracellular stress. Surface-invading enterococci demonstrated major ultrastructural alterations in their cell envelope characterized by increased membrane glycolipid content. These changes were accompanied by marked induction of specific transcriptional programs enhancing cell envelope biogenesis and glycolipid metabolism. Notably, the surface-invading population demonstrated superior tolerance to membrane-damaging antimicrobials, including daptomycin and β-defensins produced by epithelial cells. Genetic mutations impairing glycolipid biosynthesis sensitized E. faecalis to envelope stressors and reduced the ability of this bacterium to penetrate semisolid surfaces and translocate through human intestinal epithelial cell monolayers. Our study reveals that surface penetration induces distinct transcriptional, metabolic, and ultrastructural changes that equip E. faecalis with enhanced capacity to resist external stressors and thrive in its surrounding environment. IMPORTANCE Enterococcus faecalis inhabits the GIT of multiple organisms, where its establishment could be mediated by the formation of biofilm-like aggregates. In susceptible individuals, this bacterium can overgrow and breach intestinal barriers, a process that may lead to lethal systemic infections. While the formation of multicellular aggregates promotes E. faecalis migration across surfaces, little is known about the metabolic and physiological states of the enterococci encased in these surface-penetrating structures. The present study reveals that E. faecalis cells capable of migrating through semisolid surfaces genetically reprogram their metabolism toward increased cell envelope and glycolipid biogenesis, which confers superior tolerance to membrane-damaging agents. E. faecalis's success as a pathobiont depends on its antimicrobial resistance, as well as on its rapid adaptability to overcome multiple environmental challenges. Thus, targeting adaptive genetic and/or metabolic pathways induced during E. faecalis surface penetration may be useful to better confront infections by this bacterium in the clinic.

Shaping an Endospore: Architectural Transformations During Bacillus subtilis Sporulation

Endospore formation in Bacillus subtilis provides an ideal model system for studying development in bacteria. Sporulation studies have contributed a wealth of information about the mechanisms of cell-specific gene expression, chromosome dynamics, protein localization, and membrane remodeling, while helping to dispel the early view that bacteria lack internal organization and interesting cell biological phenomena. In this review, we focus on the architectural transformations that lead to a profound reorganization of the cellular landscape during sporulation, from two cells that lie side by side to the endospore, the unique cell within a cell structure that is a hallmark of sporulation in B. subtilis and other spore-forming Firmicutes. We discuss new insights into the mechanisms that drive morphogenesis, with special emphasis on polar septation, chromosome translocation, and the phagocytosis-like process of engulfment, and also the key experimental advances that have proven valuable in revealing the inner workings of bacterial cells.

The engulfasome in C. difficile: Variations on protein machineries

Clostridioides difficile infection (CDI) continues to be a substantial healthcare burden, and the changing disease profile raises new challenges in CDI management, both in clinical settings and in the community. CDI is transmitted by spores, which are formed by a subset of the cell population where an asymmetric septum is formed. A full copy of the chromosome is transported into the smaller compartment which is then engulfed by the mother cell. After engulfment, multiple metabolic and morphological changes occur, eventually resulting in the release of the mature spore. Whilst studies in the model organism Bacillus subtilis have demonstrated the importance of the DMP and Q:AH machineries in engulfment, it is becoming clear that there are fundamental differences in the way the two organisms organise these machineries. As spores are the infectious agent in CDI, it is crucial to understand how these dormant cells are formed, and how sporulation can be prevented or disrupted with the view of reducing CDI. Here, we review the current literature on the DMP and Q:AH machineries in C. difficile, and how they compare and contrast to those of B. subtilis.

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Overview of the DMP and Q:AH engulfment machineries in C. difficile.

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Analyses of the conservation of DMP across Bacilli, Clostridia and other bacteria.

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Proposes a multi-protein complex required for engulfment: the engulfasome.

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Highlights differential arrangements of engulfasome in B. subtilis and C. difficile.

Asymmetric cell division during Bacillus subtilis sporulation

Bacillus subtilis is a rod-shaped bacterium which divides precisely at mid-cell during vegetative growth. Unlike Escherichia coli, another model organism used for studying cell division, B. subtilis can also divide asymmetrically during sporulation, the simplest cell differentiation process. The asymmetrically positioned sporulation septum serves as a morphological foundation for establishing differential gene expression in the smaller forespore and larger mother cell. Both vegetative and sporulation septation events are fine-tuned with cell cycle, and placement of both septa are highly precise. We understand in some detail how this is achieved during vegetative growth but have limited information about how the asymmetric septation site is determined during sporulation.

Microdomain formation is a general property of bacterial membrane proteins and induces heterogeneity of diffusion patterns

Background

Proteins within the cytoplasmic membrane display distinct localization patterns and arrangements. While multiple models exist describing the dynamics of membrane proteins, to date, there have been few systematic studies, particularly in bacteria, to evaluate how protein size, number of transmembrane domains, and temperature affect their diffusion, and if conserved localization patterns exist.

Results

We have used fluorescence microscopy, single-molecule tracking (SMT), and computer-aided visualization methods to obtain a better understanding of the three-dimensional organization of bacterial membrane proteins, using the model bacterium Bacillus subtilis. First, we carried out a systematic study of the localization of over 200 B. subtilis membrane proteins, tagged with monomeric mVenus-YFP at their original gene locus. Their subcellular localization could be discriminated in polar, septal, patchy, and punctate patterns. Almost 20% of membrane proteins specifically localized to the cell poles, and a vast majority of all proteins localized in distinct structures, which we term microdomains. Dynamics were analyzed for selected membrane proteins, using SMT. Diffusion coefficients of the analyzed transmembrane proteins did not correlate with protein molecular weight, but correlated inversely with the number of transmembrane helices, i.e., transmembrane radius. We observed that temperature can strongly influence diffusion on the membrane, in that upon growth temperature upshift, diffusion coefficients of membrane proteins increased and still correlated inversely to the number of transmembrane domains, following the Saffman–Delbrück relation.

Conclusions

The vast majority of membrane proteins localized to distinct multimeric assemblies. Diffusion of membrane proteins can be suitably described by discriminating diffusion coefficients into two protein populations, one mobile and one immobile, the latter likely constituting microdomains. Our results show there is high heterogeneity and yet structural order in the cell membrane, and provide a roadmap for our understanding of membrane organization in prokaryotes.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0561-0) contains supplementary material, which is available to authorized users.

Localization of a red fluorescence protein adsorbed on wild type and mutant spores of Bacillus subtilis

BACKGROUND

Bacterial spores have been proposed as vehicles to display heterologous proteins for the development of mucosal vaccines, biocatalysts, bioremediation and diagnostic tools. Two approaches have been developed to display proteins on the spore surface: a recombinant approach, based on the construction of gene fusions between DNA molecules coding for a spore surface protein (carrier) and for the heterologous protein to be displayed (passenger); and a non-recombinant approach based on spore adsorption, a spontaneous interaction between negatively charged, hydrophobic spores and purified proteins. The molecular details of spore adsorption have not been fully clarified yet.

RESULTS

We used the monomeric Red Fluorescent Protein (mRFP) of the coral Discosoma sp. and Bacillus subtilis spores of a wild type and an isogenic mutant strain lacking the CotH protein to clarify the adsorption process. Mutant spores, characterized by a strongly altered coat, were more efficient than wild type spores in adsorbing mRFP but the interaction was less stable and mRFP could be in part released by raising the pH of the spore suspension. A collection of isogenic strains carrying GFP fused to proteins restricted in different compartments of the B. subtilis spore was used to localize adsorbed mRFP molecules. In wild type spores mRFP infiltrated through crust and outer coat, localized in the inner coat and was not surface exposed. In mutant spores mRFP was present in all surface layers, inner, outer coat and crust and was exposed on the spore surface.

CONCLUSIONS

Our results indicate that different spores can be selected for different applications. Wild type spores are preferable when a very tight protein-spore interaction is needed, for example to develop reusable biocatalysts or bioremediation systems for field applications. cotH mutant spores are instead preferable when the heterologous protein has to be displayed on the spore surface or has to be released, as could be the case in mucosal delivery systems for antigens and drugs, respectively.

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.

The SpoIIQ-SpoIIIAH complex of Clostridium difficile controls forespore engulfment and late stages of gene expression and spore morphogenesis

Engulfment of the forespore by the mother cell is a universal feature of endosporulation. In Bacillus subtilis, the forespore protein SpoIIQ and the mother cell protein SpoIIIAH form a channel, essential for endosporulation, through which the developing spore is nurtured. The two proteins also form a backup system for engulfment. Unlike in B. subtilis, SpoIIQ of Clostridium difficile has intact LytM zinc‐binding motifs. We show that spoIIQ or spoIIIAH deletion mutants of C. difficile result in anomalous engulfment, and that disruption of the SpoIIQ LytM domain via a single amino acid substitution (H120S) impairs engulfment differently. SpoIIQ and SpoIIQ^H120S interact with SpoIIIAH throughout engulfment. SpoIIQ, but not SpoIIQ^H120S, binds Zn²⁺, and metal absence alters the SpoIIQ‐SpoIIIAH complex in vitro. Possibly, SpoIIQ^H120S supports normal engulfment in some cells but not a second function of the complex, required following engulfment completion. We show that cells of the spoIIQ or spoIIIAH mutants that complete engulfment are impaired in post‐engulfment, forespore and mother cell‐specific gene expression, suggesting a channel‐like function. Both engulfment and a channel‐like function may be ancestral functions of SpoIIQ‐SpoIIIAH while the requirement for engulfment was alleviated through the emergence of redundant mechanisms in B. subtilis and related organisms.

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.

Regulation of Clostridium difficile Spore Formation by the SpoIIQ and SpoIIIA Proteins

Sporulation is an ancient developmental process that involves the formation of a highly resistant endospore within a larger mother cell. In the model organism Bacillus subtilis, sporulation-specific sigma factors activate compartment-specific transcriptional programs that drive spore morphogenesis. σ^G activity in the forespore depends on the formation of a secretion complex, known as the “feeding tube,” that bridges the mother cell and forespore and maintains forespore integrity. Even though these channel components are conserved in all spore formers, recent studies in the major nosocomial pathogen Clostridium difficile suggested that these components are dispensable for σ^G activity. In this study, we investigated the requirements of the SpoIIQ and SpoIIIA proteins during C. difficile sporulation. C. difficile spoIIQ, spoIIIA, and spoIIIAH mutants exhibited defects in engulfment, tethering of coat to the forespore, and heat-resistant spore formation, even though they activate σ^G at wildtype levels. Although the spoIIQ, spoIIIA, and spoIIIAH mutants were defective in engulfment, metabolic labeling studies revealed that they nevertheless actively transformed the peptidoglycan at the leading edge of engulfment. In vitro pull-down assays further demonstrated that C. difficile SpoIIQ directly interacts with SpoIIIAH. Interestingly, mutation of the conserved Walker A ATP binding motif, but not the Walker B ATP hydrolysis motif, disrupted SpoIIIAA function during C. difficile spore formation. This finding contrasts with B. subtilis, which requires both Walker A and B motifs for SpoIIIAA function. Taken together, our findings suggest that inhibiting SpoIIQ, SpoIIIAA, or SpoIIIAH function could prevent the formation of infectious C. difficile spores and thus disease transmission.

The bacterial spore-forming pathogen Clostridium difficile is a leading cause of nosocomial infections in the United States and represents a significant threat to healthcare systems around the world. As an obligate anaerobe, C. difficile must form spores in order to survive exit from the gastrointestinal tract. Accordingly, spore formation is essential for C. difficile disease transmission. Since the mechanisms controlling this process remain poorly characterized, we analyzed the importance of highly conserved secretion channel components during C. difficile sporulation. In the model organism Bacillus subtilis, this channel had previously been shown to function as a “feeding tube” that allows the mother cell to nurture the developing forespore and sustain transcription in the forespore. We show here that conserved components of this structure in C. difficile are dispensable for forespore transcription, although they are important for completing forespore engulfment and retaining the protective spore coat around the forespore, in contrast with B. subtilis. The results of our study suggest that targeting these conserved proteins could prevent C. difficile spore formation and thus disease transmission.

A mother cell-to-forespore channel: current understanding and future challenges

Formation of endospores allows some bacteria to survive extreme nutrient limitation. The resulting dormant cell, the spore, persists in the environment and is highly resistant to physical and chemical stresses. During spore formation, cells divide asymmetrically and the mother cell engulfs the developing spore, encasing it within a double membrane and isolating it from the medium. Communication between mother cell and isolated forespore involves a specialised connection system that allows nurturing of the forespore and continued macromolecular synthesis, required to finalise spore maturation. Here, we review current understanding of this feeding channel formed by a forespore protein, SpoIIQ, and a mother cell protein, SpoIIIAH, in the model organism Bacillus subtilis and the important human pathogen Clostridium difficile. We also analyse the presence of this channel across endospore-forming bacteria and highlight the main questions still remaining.

Stably bridging a great divide: localization of the SpoIIQ landmark protein in Bacillus subtilis

Many bacterial proteins involved in fundamental processes such as cell shape maintenance, cell cycle regulation, differentiation, division and motility localize dynamically to specific subcellular regions. However, the mechanisms underlying dynamic protein localization are incompletely understood. Using the SpoIIQ protein in Bacillus subtilis as a case study, two reports present important novel insights into how a protein finds its right place at the right time and remains stably bound. During sporulation, SpoIIQ localizes in clusters in the forespore membrane at the interface that separates the forespore and mother cell and functions as a landmark protein for SpoIIIAH in the mother cell membrane. The extracellular domains of SpoIIQ and SpoIIIAH interact directly effectively bridging the gap between the two membranes. Here, SpoIIQ localization is shown to depend on two pathways, one involves SpoIIIAH, the second involves two peptidoglycan-degrading enzymes SpoIIP and SpoIID; and, SpoIIQ is only delocalized in the absence of all three proteins. Importantly, in the absence of SpoIIIAH, SpoIIQ apparently localizes normally. However, FRAP experiments demonstrated that SpoIIQ is not stably maintained in the clusters in this mutant. Thus, a second targeting pathway can mask significant changes in the localization of a protein.

The SpoIIQ landmark protein has different requirements for septal localization and immobilization

Bacillus subtilis sporulation depends on the forespore membrane protein SpoIIQ, which interacts with the mother cell protein SpoIIIAH at the septum to localize other sporulation proteins. It has remained unclear how SpoIIQ localizes. We demonstrate that localization of SpoIIQ is achieved by two pathways: SpoIIIAH and the SpoIID, SpoIIM, SpoIIP engulfment proteins. SpoIIQ shows diffuse localization only in a mutant lacking both pathways. Super-resolution imaging shows that in the absence of SpoIIIAH, SpoIIQ forms fewer, slightly larger foci than in wild type. Surprisingly, photobleaching experiments demonstrate that, although SpoIIQ localizes without SpoIIIAH, it is no longer immobilized, and is therefore able to exchange subunits within a localized pool. SpoIIQ mobility is further increased by the additional absence of the engulfment proteins. However an enzymatically inactive SpoIID protein immobilizes SpoIIQ even in the absence of SpoIIIAH, indicating that complete septal thinning is not required for SpoIIQ localization. This suggests that SpoIIQ interacts with both SpoIIIAH and the engulfment proteins or their peptidoglycan cleavage products. They further demonstrate that apparently normal localization of a protein without a binding partner can mask dramatic alterations in protein mobility. We speculate that SpoIIQ assembles foci along the path defined by engulfment proteins degrading peptidoglycan.

Global analysis of the sporulation pathway of Clostridium difficile

The Gram-positive, spore-forming pathogen Clostridium difficile is the leading definable cause of healthcare-associated diarrhea worldwide. C. difficile infections are difficult to treat because of their frequent recurrence, which can cause life-threatening complications such as pseudomembranous colitis. The spores of C. difficile are responsible for these high rates of recurrence, since they are the major transmissive form of the organism and resistant to antibiotics and many disinfectants. Despite the importance of spores to the pathogenesis of C. difficile, little is known about their composition or formation. Based on studies in Bacillus subtilis and other Clostridium spp., the sigma factors σ(F), σ(E), σ(G), and σ(K) are predicted to control the transcription of genes required for sporulation, although their specific functions vary depending on the organism. In order to determine the roles of σ(F), σ(E), σ(G), and σ(K) in regulating C. difficile sporulation, we generated loss-of-function mutations in genes encoding these sporulation sigma factors and performed RNA-Sequencing to identify specific sigma factor-dependent genes. This analysis identified 224 genes whose expression was collectively activated by sporulation sigma factors: 183 were σ(F)-dependent, 169 were σ(E)-dependent, 34 were σ(G)-dependent, and 31 were σ(K)-dependent. In contrast with B. subtilis, C. difficile σ(E) was dispensable for σ(G) activation, σ(G) was dispensable for σ(K) activation, and σ(F) was required for post-translationally activating σ(G). Collectively, these results provide the first genome-wide transcriptional analysis of genes induced by specific sporulation sigma factors in the Clostridia and highlight that diverse mechanisms regulate sporulation sigma factor activity in the Firmicutes.

Peptidoglycan hydrolysis is required for assembly and activity of the transenvelope secretion complex during sporulation in Bacillus subtilis

Sporulating Bacillus subtilis cells assemble a transenvelope secretion complex that connects the mother cell and developing spore. The forespore protein SpoIIQ and the mother-cell protein SpoIIIAH interact across the double membrane septum and are thought to assemble into a channel that serves as the basement layer of this specialized secretion system. SpoIIQ is absolutely required to recruit SpoIIIAH to the sporulation septum on the mother-cell side, however the mechanism by which SpoIIQ is localized has been unclear. Here, we show that SpoIIQ localization requires its partner protein SpoIIIAH and degradation of the septal peptidoglycan (PG) by the two cell wall hydrolases SpoIID and SpoIIP. Our data suggest that PG degradation enables a second mother-cell-produced protein to interact with SpoIIQ. Cells in which both mother-cell anchoring mechanisms have been disabled have a synergistic sporulation defect suggesting that both localization factors function in the secretion complex. Finally, we show that septal PG degradation is critical for the assembly of an active complex. Altogether, these results suggest that the specialized secretion system that links the mother cell and forespore has a complexity approaching those found in Gram-negative bacteria and reveal that the sporulating cell must overcome similar challenges in assembling a transenvelope complex.

Structure of components of an intercellular channel complex in sporulating Bacillus subtilis

Following asymmetric cell division during spore formation in Bacillus subtilis, a forespore expressed membrane protein SpoIIQ, interacts across an intercellular space with a mother cell-expressed membrane protein, SpoIIIAH. Their interaction can serve as a molecular "ratchet" contributing to the migration of the mother cell membrane around that of the forespore in a phagocytosis-like process termed engulfment. Upon completion of engulfment, SpoIIQ and SpoIIIAH are integral components of a recently proposed intercellular channel allowing passage from the mother cell into the forespore of factors required for late gene expression in this compartment. Here we show that the extracellular domains of SpoIIQ and SpoIIIAH form a heterodimeric complex in solution. The crystal structure of this complex reveals that SpoIIQ has a LytM-like zinc-metalloprotease fold but with an incomplete zinc coordination sphere and no metal. SpoIIIAH has an α-helical subdomain and a protruding β-sheet subdomain, which mediates interactions with SpoIIQ. SpoIIIAH has sequence and structural homology to EscJ, a type III secretion system protein that forms a 24-fold symmetric ring. Superposition of the structures of SpoIIIAH and EscJ reveals that the SpoIIIAH protomer overlaps with two adjacent protomers of EscJ, allowing us to generate a dodecameric SpoIIIAH ring by using structural homology. Following this superposition, the SpoIIQ chains also form a closed dodecameric ring abutting the SpoIIIAH ring, producing an assembly surrounding a 60 Å channel. The dimensions and organization of the proposed complex suggest it is a plausible model for the extracellular component of a gap junction-like intercellular channel.

Vectorial signalling mechanism required for cell-cell communication during sporulation in Bacillus subtilis

Spore formation in Bacillus subtilis takes place in a sporangium consisting of two chambers, the forespore and the mother cell, which are linked by pathways of cell-cell communication. One pathway, which couples the proteolytic activation of the mother cell transcription factor σ(E) to the action of a forespore synthesized signal molecule, SpoIIR, has remained enigmatic. Signalling by SpoIIR requires the protein to be exported to the intermembrane space between forespore and mother cell, where it will interact with and activate the integral membrane protease SpoIIGA. Here we show that SpoIIR signal activity as well as the cleavage of its N-terminal extension is strictly dependent on the prespore fatty acid biosynthetic machinery. We also report that a conserved threonine residue (T27) in SpoIIR is required for processing, suggesting that signalling of SpoIIR is dependent on fatty acid synthesis probably because of acylation of T27. In addition, SpoIIR localization in the forespore septal membrane depends on the presence of SpoIIGA. The orchestration of σ(E) activation in the intercellular space by an acylated signal protein provides a new paradigm to ensure local transmission of a weak signal across the bilayer to control cell-cell communication during development.

Recent progress in Bacillus subtilis sporulation

The Gram-positive bacterium Bacillus subtilis can initiate the process of sporulation under conditions of nutrient limitation. Here, we review some of the last 5 years of work in this area, with a particular focus on the decision to initiate sporulation, DNA translocation, cell-cell communication, protein localization and spore morphogenesis. The progress we describe has implications not only just for the study of sporulation but also for other biological systems where homologs of sporulation-specific proteins are involved in vegetative growth.

Loss of compartmentalization of σ(E) activity need not prevent formation of spores by Bacillus subtilis

Compartmentalization of the activities of RNA polymerase sigma factors is a hallmark of formation of spores by Bacillus subtilis. It is initiated soon after the asymmetrically located sporulation division takes place with the activation of σ(F) in the smaller cell, the prespore. σ(F) then directs a signal via the membrane protease SpoIIGA to activate σ(E) in the larger mother cell by processing of pro-σ(E). Here, we show that σ(E) can be activated in the prespore with little effect on sporulation efficiency, implying that complete compartmentalization of σ(E) activity is not essential for spore formation. σ(E) activity in the prespore can be obtained by inducing transcription in the prespore of spoIIGA or of sigE*, which encodes a constitutively active form of σ(E), but not of spoIIGB, which encodes pro-σ(E). We infer that σ(E) compartmentalization is partially attributed to a competition between the compartments for the activation signaling protein SpoIIR. Normally, SpoIIGA is predominantly located in the mother cell and as a consequence confines σ(E) activation to it. In addition, we find that CsfB, previously shown to inhibit σ(G), is independently inhibiting σ(E) activity in the prespore. CsfB thus appears to serve a gatekeeper function in blocking the action of two sigma factors in the prespore: it prevents σ(G) from becoming active before completion of engulfment and helps prevent σ(E) from becoming active at all.

Interactions between late-acting proteins required for peptidoglycan synthesis during sporulation

The requirement of peptidoglycan synthesis for growth complicates the analysis of interactions between proteins involved in this pathway. In particular, the latter steps that involve membrane-linked substrates have proven largely recalcitrant to in vivo analysis. Here, we have taken advantage of the peptidoglycan synthesis that occurs during sporulation in Bacillus subtilis to examine the interactions between SpoVE, a nonessential, sporulation-specific homolog of the well-conserved and essential SEDS (shape elongation, division, and sporulation) proteins, and SpoVD, a nonessential class B penicillin binding protein. We found that localization of SpoVD is dependent on SpoVE and that SpoVD protects SpoVE from in vivo proteolysis. Co-immunoprecipitations and fluorescence resonance energy transfer experiments indicated that SpoVE and SpoVD interact, and co-affinity purification in Escherichia coli demonstrated that this interaction is direct. Finally, we generated a functional protein consisting of an SpoVE-SpoVD fusion and found that a loss-of-function point mutation in either part of the fusion resulted in loss of function of the entire fusion that was not complemented by a wild-type protein. Thus, SpoVE has a direct and functional interaction with SpoVD, and this conclusion will facilitate understanding the essential function that SpoVE and related SEDS proteins, such as FtsW and RodA, play in bacterial growth and division.

Novel secretion apparatus maintains spore integrity and developmental gene expression in Bacillus subtilis

Sporulation in Bacillus subtilis involves two cells that follow separate but coordinately regulated developmental programs. Late in sporulation, the developing spore (the forespore) resides within a mother cell. The regulation of the forespore transcription factor σ^G that acts at this stage has remained enigmatic. σ^G activity requires eight mother-cell proteins encoded in the spoIIIA operon and the forespore protein SpoIIQ. Several of the SpoIIIA proteins share similarity with components of specialized secretion systems. One of them resembles a secretion ATPase and we demonstrate that the ATPase motifs are required for σ^G activity. We further show that the SpoIIIA proteins and SpoIIQ reside in a multimeric complex that spans the two membranes surrounding the forespore. Finally, we have discovered that these proteins are all required to maintain forespore integrity. In their absence, the forespore develops large invaginations and collapses. Importantly, maintenance of forespore integrity does not require σ^G. These results support a model in which the SpoIIIA-SpoIIQ proteins form a novel secretion apparatus that allows the mother cell to nurture the forespore, thereby maintaining forespore physiology and σ^G activity during spore maturation.

During development, cell–cell signaling pathways coordinate programs of gene expression in neighboring cells. Cell–cell signaling is typically achieved by the secretion of a signaling ligand followed by its binding to a membrane receptor on the surface of a neighboring cell (or cells). The ligand-bound receptor directly or indirectly triggers transcription factor activation. Here we present evidence for a non-canonical signaling pathway that links developmental gene expression in the forespore and mother cell during spore formation in Bacillus subtilis. Our data support a model in which a novel secretion system is assembled in the double membrane that encases the spore within the mother cell. This apparatus is not involved in transducing a specific activating signal but rather allows the mother cell to nurture the spore and thereby maintain the spore program of developmental gene expression.

Bacterial development: evidence for very short umbilical cords

Higher eukaryotes have channels, such as gap junctions and plasmodesmata, that allow intercellular communication. Recent studies on endospore formation in Bacillus subtilis suggest that an analogous structure may exist in prokaryotes.

Processing of a membrane protein required for cell-to-cell signaling during endospore formation in Bacillus subtilis

Activation of the late prespore-specific RNA polymerase sigma factor sigma(G) during Bacillus subtilis sporulation coincides with completion of the engulfment process, when the prespore becomes a protoplast fully surrounded by the mother cell cytoplasm and separated from it by a double membrane system. Activation of sigma(G) also requires expression of spoIIIJ, coding for a membrane protein translocase of the YidC/Oxa1p/Alb3 family, and of the mother cell-specific spoIIIA operon. Here we present genetic and biochemical evidence indicating that SpoIIIAE, the product of one of the spoIIIA cistrons, and SpoIIIJ interact in the membrane, thereby linking the function of the spoIIIJ and spoIIIA loci in the activation of sigma(G). We also show that SpoIIIAE has a functional Sec-type signal peptide, which is cleaved during sporulation. Furthermore, mutations that reduce or eliminate processing of the SpoIIIAE signal peptide arrest sporulation following engulfment completion and prevent activation of sigma(G). SpoIIIJ-type proteins can function in cooperation with or independently of the Sec system. In one model, SpoIIIJ interacts with SpoIIIAE in the context of the Sec translocon to promote its correct localization and/or topology in the membrane, so that it can signal the activation of sigma(G) following engulfment completion.

Getting organized--how bacterial cells move proteins and DNA

In recent years, the subcellular organization of prokaryotic cells has become a focal point of interest in microbiology. Bacteria have evolved several different mechanisms to target protein complexes, membrane vesicles and DNA to specific positions within the cell. This versatility allows bacteria to establish the complex temporal and spatial regulatory networks that couple morphological and physiological differentiation with cell-cycle progression. In addition to stationary localization factors, dynamic cytoskeletal structures also have a fundamental role in many of these processes. In this Review, we summarize the current knowledge on localization mechanisms in bacteria, with an emphasis on the role of polymeric protein assemblies in the directed movement and positioning of macromolecular complexes.

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.

Dual localization pathways for the engulfment proteins during Bacillus subtilis sporulation

Engulfment in Bacillus subtilis is mediated by two complementary systems, SpoIID, SpoIIM and SpoIIP (DMP), which are essential for engulfment, and the SpoIIQ-SpoIIIAGH (Q-AH) zipper, which provides a secondary engulfment mechanism and recruits other proteins to the septum. We here identify two mechanisms by which DMP localizes to the septum. The first depends on SpoIIB, which is recruited to the septum during division and provides a septal landmark for efficient DMP localization. However, sporangia lacking SpoIIB ultimately localize DMP and complete engulfment, suggesting a second mechanism for DMP localization. This secondary targeting pathway depends on SpoIVFA and SpoIVFB, which are recruited to the septum by the Q-AH zipper. The absence of a detectable localization phenotype in mutants lacking only SpoIVFAB (or Q-AH) suggests that SpoIIB provides the primary DMP localization pathway while SpoIVFAB provides a secondary pathway. In keeping with this hypothesis, the spoIIB spoIVFAB mutant strain has a synergistic engulfment defect at septal thinning (which requires DMP) and is completely defective in DMP localization. Thus, the Q-AH zipper both provides a compensatory mechanism for engulfment when DMP activity is reduced, and indirectly provides a compensatory mechanism for septal localization of DMP when its primary targeting pathway is disrupted.

Essential internal promoter in the spoIIIA locus of Bacillus subtilis

The Bacillus subtilis spoIIIA locus encodes eight proteins, SpoIIIAA to SpoIIIAH, which are expressed in the mother cell during endospore formation and which are essential for the activation of sigma(G) in the forespore. Complementation studies indicated that this locus may be transcribed from two promoters, one promoter upstream from the first gene and possibly a second unidentified promoter within the locus. Fragments of the spoIIIA locus were expressed at an ectopic site to complement the sporulation-defective phenotype of a spoIIIAH deletion, and we determined that complementation required a fragment of DNA that extended into spoIIIAF. To confirm that there was a promoter located in spoIIIAF, we constructed transcriptional fusions to lacZ and found strong sporulation-induced promoter activity. Primer extension assays were used to determine the transcription start site, and point mutations introduced into the -10 and -35 regions of the promoter reduced its activity. This promoter is transcribed by sigma(E)-RNA polymerase and is repressed by SpoIIID. Therefore, we concluded that the spoIIIA locus is transcribed from two promoters, one at the start of the locus (P1(spoIIIA)) and the other within the locus (P2(spoIIIA)). Based on Campbell integrations and reverse transcription-PCR analysis of the P2(spoIIIA) region, we determined that P2(spoIIIA) is sufficient for transcription of spoIIIAG and spoIIIAH. Inactivation of P2(spoIIIA) blocked spore formation, indicating that P2(spoIIIA) is essential for expression of spoIIIAG and spoIIIAH. The P2(spoIIIA) activity is twice the P1(spoIIIA) activity; therefore, larger amounts of SpoIIIAG and SpoIIIAH than of proteins encoded at the upstream end of the locus may be required.

Exploration into the spatial and temporal mechanisms of bacterial polarity

The recognition of bacterial asymmetry is not new: the first high-resolution microscopy studies revealed that bacteria come in a multitude of shapes and sometimes carry asymmetrically localized external structures such as flagella on the cell surface. Even so, the idea that bacteria could have an inherent overall polarity, which affects not only their outer appearance but also many of their vital processes, has only recently been appreciated. In this review, we focus on recent advances in our understanding of the molecular mechanisms underlying the establishment of polarized functions and cell polarity in bacteria.

Forespore engulfment mediated by a ratchet-like mechanism

A key step in bacterial endospore formation is engulfment, during which one bacterial cell engulfs another in a phagocytosis-like process that normally requires SpoIID, SpoIIM, and SpoIIP (DMP). We here describe a second mechanism involving the zipper-like interaction between the forespore protein SpoIIQ and its mother cell ligand SpoIIIAH, which are essential for engulfment when DMP activity is reduced or SpoIIB is absent. They are also required for the rapid engulfment observed during the enzymatic removal of peptidoglycan, a process that does not require DMP. These results suggest the existence of two separate engulfment machineries that compensate for one another in intact cells, thereby rendering engulfment robust. Photobleaching analysis demonstrates that SpoIIQ assembles a stationary structure, suggesting that SpoIIQ and SpoIIIAH function as a ratchet that renders forward membrane movement irreversible. We suggest that ratchet-mediated engulfment minimizes the utilization of chemical energy during this dramatic cellular reorganization, which occurs during starvation.

Impact of membrane fusion and proteolysis on SpoIIQ dynamics and interaction with SpoIIIAH

The onset of engulfment-dependent gene expression during Bacillus subtilis sporulation requires the forespore membrane protein SpoIIQ, which recruits mother cell proteins involved in late gene expression to the outer forespore membrane. Engulfment activates the late forespore transcription factor sigmaG, which produces high levels of the secreted SpoIVB protease that is required for activation of the late mother cell transcription factor sigmaK. Engulfment also triggers the proteolytic cleavage of SpoIIQ, an event that depends on the SpoIVB protease but not on sigmaG activity. To determine if SpoIVB directly cleaves SpoIIQ and to determine if this event participates in the onset of late gene expression, we purified SpoIVB, SpoIIQ, and SpoIVFA (another SpoIVB substrate). SpoIVB directly cleaved SpoIIQ at the same site in vitro and in vivo and cleaved SpoIVFA in at least three different locations. SpoIIQ cleavage depends on membrane fusion, but not on sigmaG activity, suggesting that the ability of SpoIVB to cleave substrates is regulated by membrane fusion. We isolated SpoIVB-resistant SpoIIQ proteins by random mutagenesis of codons at the cleavage site and demonstrated that SpoIIQ processing is dispensable for spore formation and for activation of late forespore and mother cell gene expression. Fluorescence recovery after photobleaching analysis demonstrated that membrane fusion releases SpoIIQ from an immobile complex, an event that could allow SpoIVB to cleave SpoIIQ. We propose that this membrane fusion-dependent reorganization in the complex, rather than SpoIIQ proteolysis itself, is necessary for the onset of late transcription.

Nuclear export modulates the cytoplasmic Sir2 homologue Hst2

Modulating transcription factors is crucial to executing sophisticated gene expression programs. The silent information regulator 2 (Sir2) family of NAD-dependent protein deacetylases influences transcription by targeting proteins such as histones, p53 and forkhead-box family transcription factors. Although apparently cytoplasmic, both mammalian SIRT2 and its yeast orthologue Hst2 have been implicated in transcriptional regulation. Here, we show that Hst2 moves between the nucleus and cytoplasm, but is largely cytoplasmic owing to efficient nuclear export. This nuclear exclusion is mediated by the exportin chromosomal region maintenance 1 (Crm1) and a putative leucine-rich nuclear export sequence in Hst2, which overlaps a unique autoregulatory helix. Disruption of Hst2 export shows that nuclear exclusion inhibits the activity of Hst2 as a transcriptional repressor. Our identification of putative nuclear export sequences in numerous vertebrate SIRT2 proteins shows that active nuclear export can be a conserved mechanism for regulating Sir2 homologues.

Blocking chromosome translocation during sporulation of Bacillus subtilis can result in prespore-specific activation of sigmaG that is independent of sigmaE and of engulfment

Formation of spores by Bacillus subtilis is characterized by cell compartment-specific gene expression directed by four RNA polymerase sigma factors, which are activated in the order sigma(F)-sigma(E)-sigma(G)-sigma(K). Of these, sigma(G) becomes active in the prespore upon completion of engulfment of the prespore by the mother cell. Transcription of the gene encoding sigma(G), spoIIIG, is directed in the prespore by RNA polymerase containing sigma(F) but also requires the activity of sigma(E) in the mother cell. When first formed, sigma(G) is not active. Its activation requires expression of additional sigma(E)-directed genes, including the genes required for completion of engulfment. Here we report conditions in which sigma(G) becomes active in the prespore in the absence of sigma(E) activity and of completion of engulfment. The conditions are (i) having an spoIIIE mutation, so that only the origin-proximal 30% of the chromosome is translocated into the prespore, and (ii) placing spoIIIG in an origin-proximal location on the chromosome. The main function of the sigma(E)-directed regulation appears to be to coordinate sigma(G) activation with the completion of engulfment, not to control the level of sigma(G) activity. It seems plausible that the role of sigma(E) in sigma(G) activation is to reverse some inhibitory signal (or signals) in the engulfed prespore, a signal that is not present in the spoIIIE mutant background. It is not clear what the direct activator of sigma(G) in the prespore is. Competition for core RNA polymerase between sigma(F) and sigma(G) is unlikely to be of major importance.

Targeting proteins to the cell wall of sporulating Bacillus anthracis

Dormant spores of Bacillus anthracis germinate during host infection and their vegetative growth and dissemination precipitate anthrax disease. Upon host death, bacilli engage a developmental programme to generate infectious spores within carcasses. Hallmark of sporulation in Bacillus spp. is the formation of an asymmetric division septum between mother cell and forespore compartments. We show here that sortase C (SrtC) cleaves the LPNTA sorting signal of BasH and BasI, thereby targeting both polypeptides to the cell wall of sporulating bacilli. Sortase substrates are initially produced in different cell compartments and at different developmental stages but penultimately decorate the envelope of the maturing spore. srtC mutants appear to display no defect during the initial stages of infection and precipitate lethal anthrax disease in guinea pigs at a similar rate as wild-type B. anthracis strain Ames. Unlike wild-type bacilli, srtC mutants do not readily form spores in guinea pig tissue or sheep blood unless their vegetative forms are exposed to air.

Engulfment-regulated proteolysis of SpoIIQ: evidence that dual checkpoints control sigma activity

During Bacillus subtilis sporulation, the engulfment checkpoint is thought to directly regulate late forespore transcription but to indirectly regulate late mother cell transcription, via the sigmaG-produced protease SpoIVB. We here demonstrate that SpoIIQ is subject to sigmaG-independent, but engulfment-dependent, proteolysis that depends on SpoIVB. Thus, SpoIVB produced before engulfment supports some SpoIVB-dependent events, suggesting that its activity or access to substrates must be regulated by engulfment. Furthermore, a mutation (bofA) that allows sigmaK to be active without sigmaG does not allow sigmaK activity in engulfment mutants, although the pro-sigmaK processing enzyme (SpoIVFB) is localized to the septum in engulfment mutants, suggesting that engulfment comprises a second checkpoint for sigmaK Finally, we find that SpoIIQ and another protein required for sigmaG activity (SpoIIIAH), which directly interact and assemble helical structures around the forespore, recruit the sigmaK-processing enzyme SpoIVFB to the forespore and these structures. We suggest that these foci serve a synapse-like role, allowing engulfment to simultaneously control both sigmaG and sigmaK, and integrating multiple checkpoints and signalling pathways.

Mechanism of polarization of Listeria monocytogenes surface protein ActA

The polar distribution of the ActA protein on the surface of the Gram-positive intracellular bacterial pathogen, Listeria monocytogenes, is required for bacterial actin-based motility and successful infection. ActA spans both the bacterial membrane and the peptidoglycan cell wall. We have directly examined the de novo ActA polarization process in vitro by using an ActA-RFP (red fluorescent protein) fusion. After induction of expression, ActA initially appeared at distinct sites along the sides of bacteria and was then redistributed over the entire cylindrical cell body through helical cell wall growth. The accumulation of ActA at the bacterial poles displayed slower kinetics, occurring over several bacterial generations. ActA accumulated more efficiently at younger, less inert poles, and proper polarization required an optimal balance between protein secretion and bacterial growth rates. Within infected host cells, younger generations of L. monocytogenes initiated motility more quickly than older ones, consistent with our in vitro observations of de novo ActA polarization. We propose a model in which the polarization of ActA, and possibly other Gram-positive cell wall-associated proteins, may be a direct consequence of the differential cell wall growth rates along the bacterium and dependent on the relative rates of protein secretion, protein degradation and bacterial growth.

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.

Localization of translocation complex components in Bacillus subtilis: enrichment of the signal recognition particle receptor at early sporulation septa

We here demonstrate that in Bacillus subtilis, the signal recognition particle receptor, FtsY, transiently localizes to early sporulation septa, whereas three SecYEG translocase-associated membrane proteins (SecDF, SpoIIIJ, and YqjG) are uniformly distributed. These results suggest FtsY delivers secreted proteins to SecYEG at the septum, consistent with initial septal localization of forespore membrane proteins.

Subcellular localization of a sporulation membrane protein is achieved through a network of interactions along and across the septum

During the process of spore formation in Bacillus subtilis many membrane proteins localize to the sporulation septum where they play key roles in morphogenesis and cell-cell signalling. However, the mechanism by which these proteins are anchored at this site is not understood. In this report we have defined the localization requirements for the mother-cell membrane protein SpoIVFA, which anchors a signalling complex in the septal membrane on the mother cell side. We have identified five proteins (SpoIID, SpoIIP, SpoIIM, BofA and SpoIIIAH) synthesized in the mother cell under the control of sigma(E) and one protein (SpoIIQ) synthesized in the forespore under the control of sigma(F) that are all required for the proper localization of SpoIVFA. Surprisingly, these proteins appear to have complementary and overlapping anchoring roles suggesting that SpoIVFA is localized in the septal membrane through a web of protein interactions. Furthermore, we demonstrate a direct biochemical interaction between the extracellular domains of two of the proteins required to anchor SpoIVFA: the forespore protein SpoIIQ and the mother-cell protein SpoIIIAH. This result supports the idea that the web of interactions that anchors SpoIVFA is itself held in the septal membrane through a zipper-like interaction across the sporulation septum. Importantly, our results suggest that a second mechanism independent of forespore proteins participates in anchoring SpoIVFA. Finally, we show that the dynamic localization of SpoIIQ in the forespore is impaired in the absence of SpoIVFA but not SpoIIIAH. Thus, a complex web of interactions among mother cell and forespore proteins is responsible for static and dynamic protein localization in both compartments of the sporangium. We envision that this proposed network is involved in anchoring other sporulation proteins in the septum and that protein networks with overlapping anchoring capacity is a feature of protein localization in all bacteria.

Dynamic localization of penicillin-binding proteins during spore development in Bacillus subtilis

During Bacillus subtilis spore formation, many membrane proteins that function in spore development localize to the prespore septum and, subsequently, to the outer prespore membrane. Recently, it was shown that the cell-division-specific penicillin-binding proteins (PBPs) 1 and 2b localize to the asymmetric prespore septum. Here, the author studied the localization of other PBPs, fused to green fluorescent protein (GFP), during spore formation. Fusions to PBPs 4, 2c, 2d, 2a, 3, H, 4b, 5, 4a, 4* and X were expressed during vegetative growth, and their localization was monitored during sporulation. Of these PBPs, 2c, 2d, 4b and 4* have been implicated as having a function in sporulation. It was found that PBP2c, 2d and X changed their localization, while the other PBPs tested were not affected. The putative endopeptidase PbpX appears to spiral out in a pattern that resembles FtsZ redistribution during sporulation, but a pbpX knockout strain had no distinguishable phenotype. PBP2c and 2d localize to the prespore septum and follow the membrane during engulfment, and so are redistributed to the prespore membrane. A similar pattern was observed when GFP-PBP2c was expressed in the mother cell from a sporulation-specific promoter. This work shows that various PBPs known to function during sporulation are redistributed from the cytoplasmic membrane to the prespore.

Developmental Commitment in a Bacterium

We investigated developmental commitment during sporulation in Bacillus subtilis. Sporulation is initiated by nutrient limitation and involves division of the developing cell into two progeny, the forespore and the mother cell, with different fates. Differentiation becomes irreversible following division when neither the forespore nor the mother cell can resume growth when provided with nutrients. We show that commitment is governed by the transcription factors sigma(F) and sigma(E), which are activated in the forespore and the mother cell, respectively. We further show that commitment involves spoIIQ, which is under the control of sigma(F), and spoIIP, which is under the control of both sigma(F) and sigma(E). In the presence of nutrients, the forespore can exhibit rodlike, longitudinal growth when SpoIIQ and SpoIIP are absent, whereas the mother cell can do so when SpoIIP alone is absent. Thus, developmental commitment of this single-celled organism, like that of the cells of complex, multicellular organisms, ensures that differentiation is maintained despite changes in the extracellular milieu.

The new bacterial cell biology: moving parts and subcellular architecture

Recent advances have demonstrated that bacterial cells have an exquisitely organized and dynamic subcellular architecture. Like their eukaryotic counterparts, bacteria employ a full complement of cytoskeletal proteins, localize proteins and DNA to specific subcellular addresses at specific times, and use intercellular signaling to coordinate multicellular events. The striking conceptual and molecular similarities between prokaryotic and eukaryotic cell biology thus make bacteria powerful model systems for studying fundamental cellular questions.

Protein Localization: Reach out and Touch the Forespore

Bacterial proteins are typically sorted to subcellular regions with distinct physical characteristics that serve as cellular 'addresses', but many proteins are evidently sorted to specific areas that lack any apparent unique identity. Recent work in Bacillus subtilis suggests that such proteins may be localized by interacting with extracellular domains of proteins in an adjacent cellular compartment.

Sporulation of Bacillus subtilis

Differentiation of vegetative Bacillus subtilis into heat resistant spores is initiated by the activation of the key transcription regulator Spo0A through the phosphorelay. Subsequent events depend on the cell compartment-specific action of a series of RNA polymerase sigma factors. Analysis of genes in the Spo0A regulon has helped delineate the mechanisms of axial chromatin formation and asymmetric division. There have been considerable advances in our understanding of critical controls that act to regulate the phosphorelay and to activate the sigma factors.

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.