Expression of spoIIIJ in the prespore is sufficient for activation of sigma G and for sporulation in Bacillus subtilis

Construction of a Conditionally Asporogenous Bacillus thuringiensis Recombinant Strain Overproducing Cry Protein by Deletion of the leuB Gene

One of the common shortcomings with Bacillus thuringiensis (Bt) biopesticides in field application is their instability under UV irradiation. In Bt, the leuB gene encodes the 3-isopropylmalate dehydrogenase. In addition to its role in leucine biosynthesis, LeuB would be likely recruited to catalyze the dehydrogenation of malate in the final step of tricarboxylic acid cycle during sporulation. In this study, we constructed a Bt recombinant strain in which the gene leuB was deleted by using the markerless gene deletion system. The ΔleuB mutant strain showed a conditionally asporogenous phenotype while overproducing insecticidal crystal proteins and retaining its insecticidal activity well in both fermentation and LB media. Furthermore, the metabolic regulation mechanisms of LeuB was elucidated by iTRAQ-based quantitative proteomics approach. Evidences from proteomics data suggested that the inhibited supply of pyruvate (carbon source) was an important factor related to the conditionally asporogenous feature of the mutant. Consistently, the mutant regained its ability to sporulate in LB medium by adding 1% glucose or 1% sodium pyruvate. Taken together, our study demonstrated that deletion of the leuB gene resulted in delayed or completely blocked mother cell lysis, allowing the crystals encapsulated within cells, which makes this recombinant strain a good candidate for developing Bt preparations with better UV-stability.

Membrane Proteomes and Ion Transporters in Bacillus anthracis and Bacillus subtilis Dormant and Germinating Spores

Bacterial endospores produced by Bacillus and Clostridium species can remain dormant and highly resistant to environmental insults for long periods, but they can also rapidly germinate in response to a nutrient-rich environment. Multiple proteins involved in sensing and responding to nutrient germinants, initiating solute and water transport, and accomplishing spore wall degradation are associated with the membrane surrounding the spore core. In order to more fully catalog proteins that may be involved in spore germination, as well as to identify protein changes taking place during germination, unbiased proteomic analyses of membrane preparations isolated from dormant and germinated spores of Bacillus anthracis and Bacillus subtilis were undertaken. Membrane-associated proteins were fractionated by SDS-PAGE, gel slices were trypsin digested, and extracted peptides were fractionated by liquid chromatography and analyzed by matrix-assisted laser desorption ionization-tandem time of flight mass spectrometry. More than 500 proteins were identified from each preparation. Bioinformatic methods were used to characterize proteins with regard to membrane association, cellular function, and conservation across species. Numerous proteins not previously known to be spore associated, 6 in B. subtilis and 68 in B. anthracis, were identified. Relative quantitation based on spectral counting indicated that the majority of spore membrane proteins decrease in abundance during the first 20 min of germination. The spore membranes contained several proteins thought to be involved in the transport of metal ions, a process that plays a major role in spore formation and germination. Analyses of mutant strains lacking these transport proteins implicated YloB in the accumulation of calcium within the developing forespore.IMPORTANCE Bacterial endospores can remain dormant and highly resistant to environmental insults for long periods but can also rapidly germinate in response to a nutrient-rich environment. The persistence and subsequent germination of spores contribute to their colonization of new environments and to the spread of certain diseases. Proteins of Bacillus subtilis and Bacillus anthracis were identified that are associated with the spore membrane, a position that can allow them to contribute to germination. A set of identified proteins that are predicted to carry out ion transport were examined for their contributions to spore formation, stability, and germination. Greater knowledge of spore formation and germination can contribute to the development of better decontamination strategies.

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.

Cell physiology of the biotechnological relevant bacterium Bacillus pumilus-an omics-based approach

Members of the species Bacillus pumilus get more and more in focus of the biotechnological industry as potential new production strains. Based on exoproteome analysis, B. pumilus strain Jo2, possessing a high secretion capability, was chosen for an omics-based investigation. The proteome and metabolome of B. pumilus cells growing either in minimal or complex medium was analyzed. In total, 1542 proteins were identified in growing B. pumilus cells, among them 1182 cytosolic proteins, 297 membrane and lipoproteins and 63 secreted proteins. This accounts for about 43% of the 3616 proteins encoded in the B. pumilus Jo2 genome sequence. By using GC-MS, IP-LC/MS and H NMR methods numerous metabolites were analyzed and assigned to reconstructed metabolic pathways. In the genome sequence a functional secretion system including the components of the Sec- and Tat-secretion machinery was found. Analysis of the exoproteome revealed secretion of about 70 proteins with predicted secretion signals. In addition, selected production-relevant genome features such as restriction modification systems and NRPS clusters of B. pumilus Jo2 are discussed.

A conserved cysteine residue of Bacillus subtilis SpoIIIJ is important for endospore development

During sporulation in Bacillus subtilis, the onset of activity of the late forespore-specific sigma factor σ^G coincides with completion of forespore engulfment by the mother cell. At this stage, the forespore becomes a free protoplast, surrounded by the mother cell cytoplasm and separated from it by two membranes that derive from the asymmetric division septum. Continued gene expression in the forespore, isolated from the surrounding medium, relies on the SpoIIIA-SpoIIQ secretion system assembled from proteins synthesised both in the mother cell and in the forespore. The membrane protein insertase SpoIIIJ, of the YidC/Oxa1/Alb3 family, is involved in the assembly of the SpoIIIA-SpoIIQ complex. Here we show that SpoIIIJ exists as a mixture of monomers and dimers stabilised by a disulphide bond. We show that residue Cys134 within transmembrane segment 2 (TM2) of SpoIIIJ is important to stabilise the protein in the dimeric form. Labelling of Cys134 with a Cys-reactive reagent could only be achieved under stringent conditions, suggesting a tight association at least in part through TM2, between monomers in the membrane. Substitution of Cys134 by an Ala results in accumulation of the monomer, and reduces SpoIIIJ function in vivo. Therefore, SpoIIIJ activity in vivo appears to require dimer formation.

Defining the region of Bacillus subtilis SpoIIIJ that is essential for its sporulation-specific function

Proteins of the YidC/OxaI/Alb3 family play a crucial role in the insertion, folding, and/or assembly of membrane proteins in prokaryotes and eukaryotes. Bacillus subtilis has two YidC-like proteins, denoted SpoIIIJ and YqjG. SpoIIIJ and YqjG are largely exchangeable in function, but SpoIIIJ has a unique role in sporulation, while YqjG stimulates competence development. To obtain more insight into the regions important for the sporulation specificity of SpoIIIJ, a series of SpoIIIJ/YqjG chimeras was constructed. These chimeras were tested for functionality during vegetative growth and for their ability to complement the sporulation defect of a spoIIIJ deletion strain. The data suggest an important role for the domain comprising transmembrane segment 2 (TMS2) and its flanking loops in sporulation specificity, with lesser contributions to specificity by TMS1 and TMS3.

The membrane insertase YidC

The membrane insertases YidC-Oxa1-Alb3 provide a simple cellular system that catalyzes the transmembrane topology of newly synthesized membrane proteins. The insertases are composed of a single protein with 5 to 6 transmembrane (TM) helices that contact hydrophobic segments of the substrate proteins. Since YidC also cooperates with the Sec translocase it is widely involved in the assembly of many different membrane proteins including proteins that obtain complex membrane topologies. Homologues found in mitochondria (Oxa1) and thylakoids (Alb3) point to a common evolutionary origin and also demonstrate the general importance of this cellular process. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

A negative feedback loop that limits the ectopic activation of a cell type-specific sporulation sigma factor of Bacillus subtilis

Two highly similar RNA polymerase sigma subunits, σ^F and σ^G, govern the early and late phases of forespore-specific gene expression during spore differentiation in Bacillus subtilis. σ^F drives synthesis of σ^G but the latter only becomes active once engulfment of the forespore by the mother cell is completed, its levels rising quickly due to a positive feedback loop. The mechanisms that prevent premature or ectopic activation of σ^G while discriminating between σ^F and σ^G in the forespore are not fully comprehended. Here, we report that the substitution of an asparagine by a glutamic acid at position 45 of σ^G (N45E) strongly reduced binding by a previously characterized anti-sigma factor, CsfB (also known as Gin), in vitro, and increased the activity of σ^G in vivo. The N45E mutation caused the appearance of a sub-population of pre-divisional cells with strong activity of σ^G. CsfB is normally produced in the forespore, under σ^F control, but sigGN45E mutant cells also expressed csfB and did so in a σ^G-dependent manner, autonomously from σ^F. Thus, a negative feedback loop involving CsfB counteracts the positive feedback loop resulting from ectopic σ^G activity. N45 is invariant in the homologous position of σ^G orthologues, whereas its functional equivalent in σ^F proteins, E39, is highly conserved. While CsfB does not bind to wild-type σ^F, a E39N substitution in σ^F resulted in efficient binding of CsfB to σ^F. Moreover, under certain conditions, the E39N alteration strongly restrains the activity of σ^F in vivo, in a csfB-dependent manner, and the efficiency of sporulation. Therefore, a single amino residue, N45/E39, is sufficient for the ability of CsfB to discriminate between the two forespore-specific sigma factors in B. subtilis.

Positive auto-regulation of a transcriptional activator during cell differentiation or development often allows the rapid and robust deployment of cell- and stage-specific genes and the routing of the differentiating cell down a specific path. Positive auto-regulation however, raises the potential for inappropriate activity of the transcription factor. Here we unravel the role of a previously characterized anti-sigma factor, CsfB, in a negative feedback loop that prevents ectopic expression of the sporulation-specific sigma factor σ^G of Bacillus subtilis. σ^G is activated in the forespore, one of the two chambers of the developing cell, at an intermediate stage in spore development. Once active, a positive feedback loop allows the rapid accumulation of σ^G. Synthesis of both σ^G and CsfB is under the control of the early forespore regulator σ^F, and CsfB may help prevent the premature activity of σ^G in the forespore. However, CsfB is also produced under σ^G control in non-sporulating cells, setting a negative feedback loop that we show limits its ectopic activation. We further show that an asparagine residue conserved among σ^G orthologues is critical for binding and inhibition by CsfB, whereas the exclusion of asparagine from the homologous position in σ^F confers immunity to CsfB.

Bacillus subtilis YqjG is required for genetic competence development

Members of the evolutionary conserved Oxa1/Alb3/YidC family have been shown to play an important role in membrane protein insertion, folding and/or assembly. Bacillus subtilis contains two YidC-like proteins, denoted as SpoIIIJ and YqjG. SpoIIIJ and YqjG are largely exchangeable, but SpoIIIJ is essential for spore formation and YqjG cannot complement this activity. To elucidate the role of YqjG, we determined the membrane proteome and functional aspects of B. subtilis cells devoid of SpoIIIJ, YqjG or both. The data show that SpoIIIJ and YqjG have complementary functions in membrane protein insertion and assembly. The reduced levels of F(1)F(O) ATP synthase in cells devoid of both SpoIIIJ and YqjG are due to a defective assembly of the F(1)-domain onto the F(0)-domain. Importantly, for the first time, a specific function is demonstrated for YqjG in genetic competence development.

Bacillus subtilis SpoIIIJ and YqjG function in membrane protein biogenesis

In all domains of life Oxa1p-like proteins are involved in membrane protein biogenesis. Bacillus subtilis, a model organism for gram-positive bacteria, contains two Oxa1p homologs: SpoIIIJ and YqjG. These molecules appear to be mutually exchangeable, although SpoIIIJ is specifically required for spore formation. SpoIIIJ and YqjG have been implicated in a posttranslocational stage of protein secretion. Here we show that the expression of either spoIIIJ or yqjG functionally compensates for the defects in membrane insertion due to YidC depletion in Escherichia coli. Both SpoIIIJ and YqjG complement the function of YidC in SecYEG-dependent and -independent membrane insertion of subunits of the cytochrome o oxidase and F(1)F(o) ATP synthase complexes. Furthermore, SpoIIIJ and YqjG facilitate membrane insertion of F(1)F(o) ATP synthase subunit c from both E. coli and B. subtilis into inner membrane vesicles of E. coli. When isolated from B. subtilis cells, SpoIIIJ and YqjG were found to be associated with the entire F(1)F(o) ATP synthase complex, suggesting that they have a role late in the membrane assembly process. These data demonstrate that the Bacillus Oxa1p homologs have a role in membrane protein biogenesis rather than in protein secretion.

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.

A novel pathway of intercellular signalling in Bacillus subtilis involves a protein with similarity to a component of type III secretion channels

During spore formation in Bacillus subtilis, sigma(E)-directed gene expression in the mother-cell compartment of the sporangium triggers the activation of sigma(G) in the forespore by a pathway of intercellular signalling that is composed of multiple proteins of unknown function. Here, we confirm that the vegetative protein SpoIIIJ, the forespore protein SpoIIQ and eight membrane proteins (SpoIIIAA through SpoIIIAH) produced in the mother cell under the control of sigma(E) are ordinarily required for intercellular signalling. In contrast, an anti-sigma(G) factor previously implicated in the pathway is shown to be dispensable. We also present evidence suggesting that SpoIIIJ is a membrane protein translocase that facilitates the insertion of SpoIIIAE into the membrane. In addition, we report the isolation of a mutation that partially bypasses the requirement for SpoIIIJ and for SpoIIIAA through SpoIIIAG, but not for SpoIIIAH or SpoIIQ, in the activation of sigma(G). We therefore propose that under certain genetic conditions, SpoIIIAH and SpoIIQ can constitute a minimal pathway for the activation of sigma(G). Finally, based on the similarity of SpoIIIAH to a component of type III secretion systems, we speculate that signalling is mediated by a channel that links the mother cell to the forespore.

Expression of the sigmaF-directed csfB locus prevents premature appearance of sigmaG activity during sporulation of Bacillus subtilis

During sporulation, sigma(G) becomes active in the prespore upon the completion of engulfment. We show that the inactivation of the sigma(F)-directed csfB locus resulted in premature activation of sigma(G). CsfB exerted control distinct from but overlapping with that exerted by LonA to prevent inappropriate sigma(G) activation. The artificial induction of csfB severely compromised spore formation.

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.

Assembly and function of a spore coat-associated transglutaminase of Bacillus subtilis

The assembly of a multiprotein coat around the Bacillus subtilis spore confers resistance to lytic enzymes and noxious chemicals and ensures normal germination. Part of the coat is cross-linked and resistant to solubilization. The coat contains epsilon-(gamma-glutamyl)lysyl cross-links, and the expression of the gene (tgl) for a spore-associated transglutaminase was shown before to be required for the cross-linking of coat protein GerQ. Here, we have investigated the assembly and function of Tgl. We found that Tgl associates, albeit at somewhat reduced levels, with the coats of mutants that are unable to assemble the outer coat (cotE), that are missing the inner coat and with a greatly altered outer coat (gerE), or that are lacking discernible inner and outer coat structures (cotE gerE double mutant). This suggests that Tgl is present at various levels within the coat lattice. The assembly of Tgl occurs independently of its own activity, as a single amino acid substitution of a cysteine to an alanine (C116A) at the active site of Tgl does not affect its accumulation or assembly. However, like a tgl insertional mutation, the tglC116A allele causes increased extractability of polypeptides of about 40, 28, and 16 kDa in addition to GerQ (20 kDa) and affects the structural integrity of the coat. We show that most Tgl is assembled onto the spore surface soon after its synthesis in the mother cell under sigma(K) control but that the complete insolubilization of at least two of the Tgl-controlled polypeptides occurs several hours later. We also show that a multicopy allele of tgl causes increased assembly of Tgl and affects the assembly, structure, and functional properties of the coat.

Control of the expression and compartmentalization of (sigma)G activity during sporulation of Bacillus subtilis by regulators of (sigma)F and (sigma)E

During formation of spores by Bacillus subtilis the RNA polymerase factor sigma(G) ordinarily becomes active during spore formation exclusively in the prespore upon completion of engulfment of the prespore by the mother cell. Formation and activation of sigma(G) ordinarily requires prior activity of sigma(F) in the prespore and sigma(E) in the mother cell. Here we report that in spoIIA mutants lacking both sigma(F) and the anti-sigma factor SpoIIAB and in which sigma(E) is not active, sigma(G) nevertheless becomes active. Further, its activity is largely confined to the mother cell. Thus, there is a switch in the location of sigma(G) activity from prespore to mother cell. Factors contributing to the mother cell location are inferred to be read-through of spoIIIG, the structural gene for sigma(G), from the upstream spoIIG locus and the absence of SpoIIAB, which can act in the mother cell as an anti-sigma factor to sigma(G). When the spoIIIG locus was moved away from spoIIG to the distal amyE locus, sigma(G) became active earlier in sporulation in spoIIA deletion mutants, and the sporulation septum was not formed, suggesting that premature sigma(G) activation can block septum formation. We report a previously unrecognized control in which SpoIIGA can prevent the appearance of sigma(G) activity, and pro-sigma(E) (but not sigma(E)) can counteract this effect of SpoIIGA. We find that in strains lacking sigma(F) and SpoIIAB and engineered to produce active sigma(E) in the mother cell without the need for SpoIIGA, sigma(G) also becomes active in the mother cell.

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.

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.

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

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

Compartmentalization of gene expression during Bacillus subtilis spore formation

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

Role of the anti-sigma factor SpoIIAB in regulation of sigmaG during Bacillus subtilis sporulation

RNA polymerase sigma factor sigma(F) initiates the prespore-specific program of gene expression during Bacillus subtilis sporulation. sigma(F) governs transcription of spoIIIG, encoding the late prespore-specific regulator sigma(G). However, transcription of spoIIIG is delayed relative to other genes under the control of sigma(F), and after synthesis, sigma(G) is initially kept in an inactive form. Activation of sigma(G) requires the complete engulfment of the prespore by the mother cell and expression of the spoIIIA and spoIIIJ loci. We screened for random mutations in spoIIIG that bypassed the requirement for spoIIIA for the activation of sigma(G). We found a mutation (spoIIIGE156K) that resulted in an amino acid substitution at position 156, which is adjacent to the position of a mutation (E155K) previously shown to prevent interaction of SpoIIAB with sigma(G). Comparative modelling techniques and in vivo studies suggested that the spoIIIGE156K mutation interferes with the interaction of SpoIIAB with sigma(G). The sigma(GE156K) isoform restored sigma(G)-directed gene expression to spoIIIA mutant cells. However, expression of sspE-lacZ in the spoIIIA spoIIIGE156K double mutant was delayed relative to completion of the engulfment process and was not confined to the prespore. Rather, beta-galactosidase accumulated throughout the entire cell at late times in development. This suggests that the activity of sigma(GE156K) is still regulated in the prespore of a spoIIIA mutant, but not by SpoIIAB. In agreement with this suggestion, we also found that expression of spoIIIGE156K from the promoter for the early prespore-specific gene spoIIQ still resulted in sspE-lacZ induction at the normal time during sporulation, coincidently with completion of the engulfment process. In contrast, transcription of spoIIIGE156K, but not of the wild-type spoIIIG gene, from the mother cell-specific spoIID promoter permitted the rapid induction of sspE-lacZ expression. Together, the results suggest that SpoIIAB is either redundant or has no role in the regulation of sigma(G) in the prespore.

Septal localization of forespore membrane proteins during engulfment in Bacillus subtilis

In Bacillus subtilis, many membrane proteins localize to the sporulation septum, where they play key roles in spore morphogenesis and cell-specific gene expression, but the mechanism for septal targeting is not well understood. SpoIIQ, a forespore-expressed protein, is involved in engulfment and forespore-specific gene expression. We find that SpoIIQ dynamically localizes to the sporulation septum, tracks the engulfing mother cell membrane, assembles into helical arcs around the forespore and is finally degraded. Retention of SpoIIQ in the septum requires one or more mother cell-expressed proteins. We also observed that any forespore-expressed membrane protein initially localizes to the septum and later spreads throughout the forespore membrane, suggesting that membrane protein insertion occurs at the forespore septal region. This possibility provides an attractive mechanism for how activation of mother cell-specific gene expression is restricted to adjacent sister cells, since direct insertion of the signaling protein SpoIIR into the septum would spatially restrict its activity. In keeping with this hypothesis, we find that SpoIIR localizes to the septum and is transiently expressed.

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.