Adjacent and divergently oriented operons under the control of the sporulation regulatory protein GerE in Bacillus subtilis

Identification of CgeA as a glycoprotein that anchors polysaccharides to the spore surface in Bacillus subtilis

The Bacillus subtilis spore is composed of a core, containing chromosomal DNA, surrounded by a cortex layer made of peptidoglycan, and a coat composed of concentric proteinaceous layers. A polysaccharide layer is added to the spore surface, and likely anchored to the crust, the coat outermost layer. However, the identity of the coat protein(s) to which the spore polysaccharides (SPS) are attached is uncertain. First, we showed that the crust proteins CotVWXYZ and CgeA were all contained in the peeled SPS layer obtained from a strain missing CotE, the outer coat morphogenetic protein, suggesting that the SPS is indeed bound to at least one of the spore surface proteins. Second, CgeA is known to be located at the most downstream position in the crust assembly pathway. An analysis of truncated variants of CgeA suggested that its N-terminal half is required for localization to the spore surface, while its C-terminal half is necessary for SPS addition. Third, an amino acid substitution strategy revealed that SPS was anchored at threonine 112 (T112), which constitutes a probable O-glycosylation site on CgeA. Our results indicated that CgeA is a glycoprotein required to initiate SPS assembly and serves as an anchor protein linking the crust and SPS layers.

Sporulation conditions influence the surface and adhesion properties of Bacillus subtilis spores

Spore-forming bacteria of the Bacillus subtilis group are responsible for recurrent contamination of processing lines in the food industry which can lead to food spoilage. The persistence of B. subtilis would be due to the high resistance of spores to extreme environmental condition and their propensity to contaminate surfaces. While it is well known that sporulation conditions modulate spore resistance properties, little is known about their effect on surface and adhesion properties. Here, we studied the impact of 13 sporulation conditions on the surface and adhesion properties of B. subtilis 168 spores. We showed that Ca2+ or Mg2+ depletion, lower oxygen availability, acidic pH as well as oxidative stresses during sporulation lead to the release of more hydrophobic and adherent spores. The consequences of these sporulation conditions on crust composition in carbohydrates and proteins were also evaluated. The crust glycans of spores produced in a sporulation medium depleted in Ca2+ or Mg2+ or oxygen-limited conditions were impaired and contained lower amounts of rhamnose and legionaminic acid. In addition, we showed that lower oxygen availability or addition of hydrogen peroxide during sporulation decreases the relative amount of two crust proteins (CgeA and CotY) and the changes observed in these conditions could be due to transcriptional repression of genes involved in crust synthesis in late stationary phase. The fact that sporulation conditions affect the ease with which spores can contaminate surfaces could explain the frequent and recurrent presence of B. subtilis spores in food processing lines.

Discovery of 3'-O-β-Glucosyltubercidin and the Nucleoside Specific Glycosyltransferase AvpGT through Genome Mining

A genome mining approach was used to identify a hybrid tubercidin-nucleocidin biosynthetic gene cluster (BGC) in Streptomyces sp. AVP053U2. Analysis of culture extracts by liquid chromatography-mass spectrometry revealed the presence of a glucosylated tubercidin derivative. A gene, avpGT, was identified within the hybrid cluster that has homology to the glucosyltransferase that is responsible for 3'-O-β-glucosylation of the fluorinated natural product nucleocidin. AvpGT was heterologously expressed and purified from Escherichia coli for in vitro characterization. AvpGT is active toward UDP-glucose and UDP-galactose as glycosyl donors and several nucleosides as acceptors. Kinetic analysis revealed that AvpGT is most specific for UDP-glucose [k_cat/K_M^app = (1.1 ± 0.3) × 10⁵ M^-1·s^-1] as the glycosyl donor and tubercidin [k_cat/K_M^app = (5.3 ± 1.8) × 10⁴ M^-1·s^-1] as the glycosyl acceptor. NMR spectroscopic analysis revealed the product of this reaction to be 3'-O-β-glucopyranosyl tubercidin. A sequence analysis of AvpGT reveals a family of nucleoside-specific GTs, which may be used as markers of BGCs that produce glycosylated nucleosides.

Expansion of the Spore Surface Polysaccharide Layer in Bacillus subtilis by Deletion of Genes Encoding Glycosyltransferases and Glucose Modification Enzymes

Polysaccharides (PS) decorate the surface of dormant endospores (spores). In the model organism for sporulation, Bacillus subtilis, the composition of the spore PS is not known in detail. Here, we have assessed how PS synthesis enzymes produced during the late stages of sporulation affect spore surface properties. Using four methods, bacterial adhesion to hydrocarbons (BATH) assays, India ink staining, transmission electron microscopy (TEM) with ruthenium red staining, and scanning electron microscopy (SEM), we characterized the contributions of four sporulation gene clusters, spsABCDEFGHIJKL, yfnHGF-yfnED, ytdA-ytcABC, and cgeAB-cgeCDE, on the morphology and properties of the crust, the outermost spore layer. Our results show that all mutations in the sps operon result in the production of spores that are more hydrophobic and lack a visible crust, presumably because of reduced PS deposition, while mutations in cgeD and the yfnH-D cluster noticeably expand the PS layer. In addition, yfnH-D mutant spores exhibit a crust with an unusual weblike morphology. The hydrophobic phenotype from sps mutant spores was partially rescued by a second mutation inactivating any gene in the yfnHGF operon. While spsI, yfnH, and ytdA are paralogous genes, all encoding glucose-1-phosphate nucleotidyltransferases, each paralog appears to contribute in a distinct manner to the spore PS. Our data are consistent with the possibility that each gene cluster is responsible for the production of its own respective deoxyhexose. In summary, we found that disruptions to the PS layer modify spore surface hydrophobicity and that there are multiple saccharide synthesis pathways involved in spore surface properties.IMPORTANCE Many bacteria are characterized by their ability to form highly resistant spores. The dormant spore state allows these species to survive even the harshest treatments with antimicrobial agents. Spore surface properties are particularly relevant because they influence spore dispersal in various habitats from natural to human-made environments. The spore surface in Bacillus subtilis (crust) is composed of a combination of proteins and polysaccharides. By inactivating the enzymes responsible for the synthesis of spore polysaccharides, we can assess how spore surface properties such as hydrophobicity are modulated by the addition of specific carbohydrates. Our findings indicate that several sporulation gene clusters are responsible for the assembly and allocation of surface polysaccharides. Similar mechanisms could be modulating the dispersal of infectious spore-forming bacteria.

Contributions of crust proteins to spore surface properties in Bacillus subtilis

Surface properties, such as adhesion and hydrophobicity, constrain dispersal of bacterial spores in the environment. In Bacillus subtilis, these properties are influenced by the outermost layer of the spore, the crust. Previous work has shown that two clusters, cotVWXYZ and cgeAB, encode the protein components of the crust. Here, we characterize the respective roles of these genes in surface properties using Bacterial Adherence to Hydrocarbons assays, negative staining of polysaccharides by India ink and Transmission Electron Microscopy. We showed that inactivation of crust genes caused increases in spore relative hydrophobicity, disrupted the spore polysaccharide layer, and impaired crust structure and attachment to the rest of the coat. We also found that cotO, previously identified for its role in outer coat formation, is necessary for proper encasement of the spore by the crust. In parallel, we conducted fluorescence microscopy experiments to determine the full network of genetic dependencies for subcellular localization of crust proteins. We determined that CotZ is required for the localization of most crust proteins, while CgeA is at the bottom of the genetic interaction hierarchy.

Sporobeads: The Utilization of the Bacillus subtilis Endospore Crust as a Protein Display Platform

Upon starvation, the soil bacterium Bacillus subtilis forms an intracellular, metabolically inactive endospore. Its core contains the DNA, encased by three protein layers protecting it against a multitude of environmental threats. The outermost layer, the crust, harbors great potential as a protein-displaying platform: a gene of interest can be translationally fused to a crust protein gene, resulting in endospores displaying the desired protein on their surface. To unlock this potential in a standardized fashion, we designed a suite of 12 vectors (Sporovectors), based on the BioBrick cloning standard. With these vectors, proteins can easily be fused N- or C-terminally to the six crust proteins CotV, CotW, CotX, CotY, CotZ, and CgeA under the control of the strongest crust gene promoter P_cotYZ. All Sporovectors were evaluated with GFP and two different laccases. On the basis of our data, CotY and CotZ represent the best anchor proteins. But there are significant differences in activity and functional stability between the two tested laccases. Our vector suite is a powerful tool to generate and evaluate a vast variety of functionalized endospores. It allows quickly identifying the best anchor and fusion site for the protein of interest. Our findings demonstrate that the crust of B. subtilis endospores is an inexpensive and easy platform for displaying different proteins of interest.

The Spore Coat

Spores of Clostridiales and Bacillales are encased in a complex series of concentric shells that provide protection, facilitate germination, and mediate interactions with the environment. Analysis of diverse spore-forming species by thin-section transmission electron microscopy reveals that the number and morphology of these encasing shells vary greatly. In some species, they appear to be composed of a small number of discrete layers. In other species, they can comprise multiple, morphologically complex layers. In addition, spore surfaces can possess elaborate appendages. For all their variability, there is a consistent architecture to the layers encasing the spore. A hallmark of all Clostridiales and Bacillales spores is the cortex, a layer made of peptidoglycan. In close association with the cortex, all species examined possess, at a minimum, a series of proteinaceous layers, called the coat. In some species, including Bacillus subtilis, only the coat is present. In other species, including Bacillus anthracis, an additional layer, called the exosporium, surrounds the coat. Our goals here are to review the present understanding of the structure, composition, assembly, and functions of the coat, primarily in the model organism B. subtilis, but also in the small but growing number of other spore-forming species where new data are showing that there is much to be learned beyond the relatively well-developed basis of knowledge in B. subtilis. To help summarize this large field and define future directions for research, we will focus on key findings in recent years.

The Exosporium Layer of Bacterial Spores: a Connection to the Environment and the Infected Host

Much of what we know regarding bacterial spore structure and function has been learned from studies of the genetically well-characterized bacterium Bacillus subtilis. Molecular aspects of spore structure, assembly, and function are well defined. However, certain bacteria produce spores with an outer spore layer, the exosporium, which is not present on B. subtilis spores. Our understanding of the composition and biological functions of the exosporium layer is much more limited than that of other aspects of the spore. Because the bacterial spore surface is important for the spore's interactions with the environment, as well as being the site of interaction of the spore with the host's innate immune system in the case of spore-forming bacterial pathogens, the exosporium is worthy of continued investigation. Recent exosporium studies have focused largely on members of the Bacillus cereus family, principally Bacillus anthracis and Bacillus cereus. Our understanding of the composition of the exosporium, the pathway of its assembly, and its role in spore biology is now coming into sharper focus. This review expands on a 2007 review of spore surface layers which provided an excellent conceptual framework of exosporium structure and function (A. O. Henriques and C. P. Moran, Jr., Annu Rev Microbiol 61:555-588, 2007, http://dx.doi.org/10.1146/annurev.micro.61.080706.093224). That review began a process of considering outer spore layers as an integrated, multilayered structure rather than simply regarding the outer spore components as independent parts.

Transcriptomic responses of germinating Bacillus subtilis spores exposed to 1.5 years of space and simulated martian conditions on the EXPOSE-E experiment PROTECT

Because of their ubiquity and resistance to spacecraft decontamination, bacterial spores are considered likely potential forward contaminants on robotic missions to Mars. Thus, it is important to understand their global responses to long-term exposure to space or martian environments. As part of the PROTECT experiment, spores of B. subtilis 168 were exposed to real space conditions and to simulated martian conditions for 559 days in low-Earth orbit mounted on the EXPOSE-E exposure platform outside the European Columbus module on the International Space Station. Upon return, spores were germinated, total RNA extracted, fluorescently labeled, and used to probe a custom Bacillus subtilis microarray to identify genes preferentially activated or repressed relative to ground control spores. Increased transcript levels were detected for a number of stress-related regulons responding to DNA damage (SOS response, SPβ prophage induction), protein damage (CtsR/Clp system), oxidative stress (PerR regulon), and cell envelope stress (SigV regulon). Spores exposed to space demonstrated a much broader and more severe stress response than spores exposed to simulated martian conditions. The results are discussed in the context of planetary protection for a hypothetical journey of potential forward contaminant spores from Earth to Mars and their subsequent residence on Mars.

Proteins involved in formation of the outermost layer of Bacillus subtilis spores

To investigate the outermost structure of the Bacillus subtilis spore, we analyzed the accessibility of antibodies to proteins on spores of B. subtilis. Anti-green fluorescent protein (GFP) antibodies efficiently accessed GFP fused to CgeA or CotZ, which were previously assigned to the outermost layer termed the spore crust. However, anti-GFP antibodies did not bind to spores of strains expressing GFP fused to 14 outer coat, inner coat, or cortex proteins. Anti-CgeA antibodies bound to spores of wild-type and CgeA-GFP strains but not cgeA mutant spores. These results suggest that the spore crust covers the spore coat and is the externally exposed, outermost layer of the B. subtilis spore. We found that CotZ was essential for the spore crust to surround the spore but not for spore coat formation, indicating that CotZ plays a critical role in spore crust formation. In addition, we found that CotY-GFP was exposed on the surface of the spore, suggesting that CotY is an additional component of the spore crust. Moreover, the localization of CotY-GFP around the spore depended on CotZ, and CotY and CotZ depended on each other for spore assembly. Furthermore, a disruption of cotW affected the assembly of CotV-GFP, and a disruption of cotX affected the assembly of both CotV-GFP and CgeA-GFP. These results suggest that cgeA and genes in the cotVWXYZ cluster are involved in spore crust formation.

Nucleoid occlusion prevents cell division during replication fork arrest in Bacillus subtilis

How bacteria respond to chromosome replication stress has been traditionally studied using temperature-sensitive mutants and chemical inhibitors. These methods inevitably arrest all replication and lead to induction of transcriptional responses and inhibition of cell division. Here, we used repressor proteins bound to operator arrays to generate a single stalled replication fork. These replication roadblocks impeded replisome progression on one arm, leaving replication of the other arm and re-initiation unaffected. Remarkably, despite robust generation of RecA-GFP filaments and a strong block to cell division during the roadblock, patterns of gene expression were not significantly altered. Consistent with these findings, division inhibition was not mediated by the SOS-induced regulator YneA nor by RecA-independent repression of ftsL. In support of the idea that nucleoid occlusion prevents inappropriate cell division during fork arrest, immature FtsZ-rings formed adjacent to the DNA mass but rarely on top of it. Furthermore, mild alterations in chromosome compaction resulted in cell division that guillotined the DNA. Strikingly, the nucleoid occlusion protein Noc had no discernable role in division inhibition. Our data indicate that Noc-independent nucleoid occlusion prevents inappropriate cell division during replication fork arrest. They further suggest that Bacillus subtilis normally manages replication stress rather than inducing a stress response.

Localization of proteins to different layers and regions of Bacillus subtilis spore coats

Bacterial spores are encased in a multilayered proteinaceous shell known as the coat. In Bacillus subtilis, over 50 proteins are involved in spore coat assembly but the locations of these proteins in the spore coat are poorly understood. Here, we describe methods to estimate the positions of protein fusions to fluorescent proteins in the spore coat by using fluorescence microscopy. Our investigation suggested that CotD, CotF, CotT, GerQ, YaaH, YeeK, YmaG, YsnD, and YxeE are present in the inner coat and that CotA, CotB, CotC, and YtxO reside in the outer coat. In addition, CotZ and CgeA appeared in the outermost layer of the spore coat and were more abundant at the mother cell proximal pole of the forespore, whereas CotA and CotC were more abundant at the mother cell distal pole of the forespore. These polar localizations were observed both in sporangia prior to the release of the forespore from the mother cell and in mature spores after release. Moreover, CotB was observed at the middle of the spore as a ring- or spiral-like structure. Formation of this structure required cotG expression. Thus, we conclude not only that the spore coat is a multilayered assembly but also that it exhibits uneven spatial distribution of particular proteins.

The effect of calcium on the transcriptome of sporulating B. subtilis cells

Bacterial spores formed in the presence of high concentrations of minerals are a major problem in the food industry because of their extreme heat resistance. In order to enhance our insight in the molecular mechanisms underlying this phenomenon we have performed a detailed time-resolved analysis of the genome-wide transcriptome pattern of Bacillus subtilis sporulated both in the absence and presence of high calcium concentrations. The data was analysed in two ways. First, we determined the influence of the presence of high calcium levels during sporulation on the expression of gene groups as defined in Subtilist and KEGG pathways database. Second, we assessed the differential expression at the level of individual genes. When analysing groups and pathways, we found that those annotated as being involved in sporulation were significantly affected. Also, groups and pathways involved in flagella formation and biofilm matrix production were affected by the presence of calcium in the sporulation medium. When we analysed the behaviour of individual genes we found 305 genes influenced by calcium, including all known spore coat polysaccharide biosynthesis genes (10 induced and 1 repressed). A number of the calcium affected genes were also involved in biofilm formation. Minimal overlap with other stress outputs like sigma B activation and weak acid stress response was noted. Those genes that did overlap were unique to that combination which corroborates the notion that the cells sense these conditions differently.

Two-step assembly dynamics of the Bacillus subtilis divisome

Cell division in bacteria is carried out by about a dozen proteins which assemble at midcell and form a complex known as the divisome. To study the dynamics and temporal hierarchy of divisome assembly in Bacillus subtilis, we have examined the in vivo localization pattern of a set of division proteins fused to green fluorescent protein in germinating spores and vegetative cells. Using time series and time-lapse microscopy, we show that the FtsZ ring assembles early and concomitantly with FtsA, ZapA, and EzrA. After a time delay of at least 20% of the cell cycle, a second set of division proteins, including GpsB, FtsL, DivIB, FtsW, Pbp2B, and DivIVA, are recruited to midcell. Together, our data provide in vivo evidence for two-step assembly of the divisome. Interestingly, overproduction of FtsZ advances the temporal assembly of EzrA but not of DivIVA, suggesting that a signal different from that of FtsZ polymerization drives the assembly of late divisome proteins. Microarray analysis shows that FtsZ depletion or overexpression does not significantly alter the transcription of division genes, supporting the hypothesis that cell division in B. subtilis is mainly regulated at the posttranscriptional level.

Expression of yeeK during Bacillus subtilis sporulation and localization of YeeK to the inner spore coat using fluorescence microscopy

The yeeK gene of Bacillus subtilis is predicted to encode a protein of 145 amino acids composed of 28% glycine, 23% histidine, and 12% tyrosine residues. Previous studies were unable to detect YeeK in wild-type spores; however, the 18-kDa YeeK polypeptide has been identified in yabG mutant spores. In this study, we analyze the expression and localization of YeeK to explore the relationship between YeeK and YabG. Northern hybridization analysis of wild-type RNA indicated that transcription of the yeeK gene, which was initiated 5 h after the onset of sporulation, was dependent on a SigK-containing RNA polymerase and the GerE protein. Genetic disruption of yeeK did not impair vegetative growth, development of resistant spores, or germination. Fluorescent microscopy of in-frame fusions of YeeK with green fluorescent protein (YeeK-GFP) and red fluorescent protein (YeeK-RFP) confirmed that YeeK assembles into the spore integument. CotE, SafA, and SpoVID were required for the proper localization of YeeK-GFP. Comparative analysis of YeeK-RFP and an in-frame GFP fusion of YabG indicated that YeeK colocalized with YabG in the spore coat. This is the first use of fluorescent proteins to show localization to different layers of the spore coat. Immunoblotting with anti-GFP antiserum indicated that YeeK-GFP was primarily synthesized as a 44-kDa molecule, which was then digested into a 29-kDa fragment that corresponded to the molecular size of GFP in wild-type spores. In contrast, a minimal amount of 44-kDa YeeK-GFP was digested in yabG mutant spores. Our findings demonstrate that YeeK is guided into the spore coat by CotE, SafA, and SpoVID. We conclude that YabG is directly or indirectly involved in the digestion of YeeK.

Identification of a second collagen-like glycoprotein produced by Bacillus anthracis and demonstration of associated spore-specific sugars

Certain carbohydrates (rhamnose, 3-O-methyl rhamnose, and galactosamine) have been demonstrated to be present in Bacillus anthracis spores but absent in vegetative cells. Others have demonstrated that these spore-specific sugars are constituents of the glycoprotein BclA. In the current work, spore extracts were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A second collagen-like glycoprotein, BclB, was identified in B. anthracis. The protein moiety of this glycoprotein was identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MS) and the carbohydrate components by gas chromatography-mass spectrometry and tandem mass spectrometry. Spore-specific sugars were also demonstrated to be components of BclB.

The ylbO gene product of Bacillus subtilis is involved in the coat development and lysozyme resistance of spore

The Bacillus subtilis YlbO protein is a Myb-like DNA binding domain-containing protein that is expressed under the control of SigE. Here, we analyzed gene expression and protein composition in ylbO-negative cells. SDS-PAGE analysis revealed that the protein profile of ylbO- negative spores differed from that of wild-type. Specifically, the expression of coat proteins CgeA, CotG, and CotY, which are controlled by SigK and GerE, was reduced in ylbO -negative cells. Northern blot analysis revealed that YlbO regulated the transcription of cgeA, cotG, and cotY. These results suggest that YlbO regulates the expression of some coat proteins during sporulation in B. subtilis directly or indirectly.

Genome-wide analysis of temporally regulated and compartment-specific gene expression in sporulating cells of Bacillus subtilis

Temporal and compartment-specific control of gene expression during sporulation inBacillus subtilisis governed by a cascade of four RNA polymerase subunits.σFin the prespore andσEin the mother cell control early stages of development, and are replaced at later stages byσGandσK, respectively. Ultimately, a comprehensive description of the molecular mechanisms underlying spore morphogenesis requires the knowledge of all the intervening genes and their assignment to specific regulons. Here, in an extension of earlier work, DNA macroarrays have been used, and members of the four compartment-specific sporulation regulons have been identified. Genes were identified and grouped based on: i) their temporal expression profile and ii) the use of mutants for each of the four sigma factors and abofAallele, which allowsσKactivation in the absence ofσG. As a further test, artificial production of active alleles of the sigma factors in non-sporulating cells was employed. A total of 439 genes were found, including previously characterized genes whose transcription is induced during sporulation: 55 in theσFregulon, 154σE-governed genes, 113σG-dependent genes, and 132 genes underσKcontrol. The results strengthen the view that the activities ofσF,σE,σGandσKare largely compartmentalized, both temporally as well as spatially, and that the major vegetative sigma factor (σA) is active throughout sporulation. The results provide a dynamic picture of the changes in the overall pattern of gene expression in the two compartments of the sporulating cell, and offer insight into the roles of the prespore and the mother cell at different times of spore morphogenesis.

Systematic analysis of SigD-regulated genes in Bacillus subtilis by DNA microarray and Northern blotting analyses

The SigD-regulated genes in Bacillus subtilis were systematically analyzed by comparing the pattern of transcripts derived from wild-type cells with those from sigD mutant cells using DNA microarray technology. One hundred and fifty-eight genes were found to be SigD-dependent candidates, 46 of which being known SigD-regulated genes. Northern blot analysis revealed that 18 of the remaining genes were SigD-dependent. The SigD consensus sequence was newly identified in the upstream regions of nine operons (11 genes): ybdO, yfmT-yfmS, hemAT, yjcP-yjcQ, yjfB, ylqB, yoaH, yscB and yxkC, and the other seven genes were assumed to be indirectly affected by a SigD mutation. Furthermore, yviE-yviF are likely to be SigD-dependent genes, because three independent sets of array data for yviE and yviF indicated they are SigD-dependent, and these genes are neighbors of flgL and hag transcribed by SigD RNA polymerase.

Ruthenium red staining for ultrastructural visualization of a glycoprotein layer surrounding the spore of Bacillus anthracis and Bacillus subtilis

Ruthenium red is a polycationic stain used to visualize acid polysaccharides on the outer surface of cells. Ruthenium red staining followed by electron microscopic analysis was used to demonstrate the presence of an external glycoprotein layer surrounding the spore of both Bacillus anthracis and Bacillus subtilis. This layer is less apparent with traditional staining methods used for electron microscopy. Renografin gradients were used to purify B. subtilis spores. These purified spores displayed greatly enhanced staining with ruthenium red, indicating nonspecific binding of renografin, which has a major carbohydrate constituent, methylglucamine. For B. anthracis, staining with ruthenium red was sufficiently intense that it was not significantly enhanced by renografin purification. In addition to demonstrating a previously undiscovered layer surrounding the spores of B. subtilis, the results help explain a long-standing controversy as to ultrastructural differences among these genetically closely related organisms. Ruthenium red staining provides an important addition to the identification of surface glycoproteins in studies to define similarities and differences in the exosporium layers of Bacillus species.

Novel Oligosaccharide Side Chains of the Collagen-like Region of BclA, the Major Glycoprotein of the Bacillus anthracis Exosporium

Spores of Bacillus anthracis, the causative agent of anthrax, are enclosed by a prominent loose fitting layer called the exosporium. The exosporium consists of a basal layer and an external hairlike nap. The filaments of the nap are composed of a highly immunogenic glycoprotein called BclA, which has a long, central collagen-like region with multiple XXG repeats. Most of the triplet repeats are PTG, and nearly all of the triplet repeats contain a threonine residue, providing multiple potential sites for O-glycosylation. In this study, we demonstrated that two O-linked oligosaccharides, a 715-Da tetrasaccharide and a 324-Da disaccharide, are released from spore- and exosporium-associated BclA by hydrazinolysis. Each oligosaccharide is probably attached to BclA through a GalNAc linker, which was lost during oligosaccharide release. We found that multiple copies of the tetrasaccharide are linked to the collagen-like region of BclA, whereas the disaccharide may be attached outside of this region. Using NMR, mass spectrometry, and other analytical techniques, we determined that the structure of the tetrasaccharide is 2-O-methyl-4-(3-hydroxy-3-methylbutamido)-4,6-dideoxy-beta-d-glucopyranosyl-(1-->3)-alpha-l-rhamnopyranosyl-(1-->3)-alpha-l-rhamnopyranosyl-(1-->2)-l-rhamnopyranose. The previously undescribed nonreducing terminal sugar (i.e. 2-O-methyl-4-(3-hydroxy-3-methylbutamido)-4,6-dideoxy-d-glucose) was given the trivial name anthrose. Anthrose was not found in spores of either Bacillus cereus or Bacillus thuringiensis, two species that are the most phylogenetically similar to B. anthracis. Thus, anthrose may be useful for species-specific detection of B. anthracis spores or as a new target for therapeutic intervention.

Functional relationship between SpoVIF and GerE in gene regulation during sporulation of Bacillus subtilis

The sporulation-specific SpoVIF (YjcC) protein of Bacillus subtilis is essential for the development of heat-resistant spores. The GerE protein, the smallest member of the LuxR-FixJ family, contains a helix-turn-helix (HTH) motif and is involved in the expression of various sporulation-specific genes. In this study, the gene expression and protein composition of sporulating spoVIF-negative cells were analysed. CgeA, CotG and CotS, which are GerE-dependent coat proteins, were not expressed in the spoVIF-negative cells. Northern blotting showed that SpoVIF regulated the transcription of cgeA, cotG and cotS in a manner similar to that of GerE. In spoVIF-negative cells, gerE mRNA was transcribed normally, but immunoblot analysis using anti-GerE antiserum showed that the quantity of GerE protein was considerably less than that in wild-type controls. Using GFP (green fluorescent protein) fusion proteins, the localization of SpoVIF and GerE was observed by fluorescence microscopy. SpoVIF-GFP was detectable in the mother cell compartment, as was GerE-GFP. These results suggest that SpoVIF directly or indirectly controls the function of the GerE protein, and that SpoVIF is required for gene regulation during the latter stages of sporulation.

Peptide ligands that bind selectively to spores of Bacillus subtilis and closely related species

As part of an effort to develop detectors for selected species of bacterial spores, we screened phage display peptide libraries for 7- and 12-mer peptides that bind tightly to spores of Bacillus subtilis. All of the peptides isolated contained the sequence Asn-His-Phe-Leu at the amino terminus and exhibited clear preferences for other amino acids, especially Pro, at positions 5 to 7. We demonstrated that the sequence Asn-His-Phe-Leu-Pro (but not Asn-His-Phe-Leu) was sufficient for tight spore binding. We observed equal 7-mer peptide binding to spores of B. subtilis and its most closely related species, Bacillus amyloliquefaciens, and slightly weaker binding to spores of the closely related species Bacillus globigii. These three species comprise one branch on the Bacillus phylogenetic tree. We did not detect peptide binding to spores of several Bacillus species located on adjacent and nearby branches of the phylogenetic tree nor to vegetative cells of B. subtilis. The sequence Asn-His-Phe-Leu-Pro was used to identify B. subtilis proteins that may employ this peptide for docking to the outer surface of the forespore during spore coat assembly and/or maturation. One such protein, SpsC, appears to be involved in the synthesis of polysaccharide on the spore coat. SpsC contains the Asn-His-Phe-Leu-Pro sequence at positions 6 to 10, and the first five residues of SpsC apparently must be removed to allow spore binding. Finally, we discuss the use of peptide ligands for bacterial detection and the use of short peptide sequences for targeting proteins during spore formation.

Morphogenesis of bacillus spore surfaces

Spores produced by bacilli are encased in a proteinaceous multilayered coat and, in some species (including Bacillus anthracis), further surrounded by a glycoprotein-containing exosporium. To characterize bacillus spore surface morphology and to identify proteins that direct formation of coat surface features, we used atomic-force microscopy (AFM) to image the surfaces of wild-type and mutant spores of Bacillus subtilis, as well as the spore surfaces of Bacillus cereus 569 and the Sterne strain of Bacillus anthracis. This analysis revealed that the coat surfaces in these strains are populated by a series of bumps ranging between 7 and 40 nm in diameter, depending on the species. Furthermore, a series of ridges encircled the spore, most of which were oriented along the long axis of the spore. The structures of these ridges differ sufficiently between species to permit species-specific identification. We propose that ridges are formed early in spore formation, when the spore volume likely decreases, and that when the spore swells during germination the ridges unfold. AFM analysis of a set of B. subtilis coat protein gene mutants revealed three coat proteins with roles in coat surface morphology: CotA, CotB, and CotE. Our data indicate novel roles for CotA and CotB in ridge pattern formation. Taken together, these results are consistent with the view that the coat is not inert. Rather, the coat is a dynamic structure that accommodates changes in spore volume.

Carbohydrates and glycoproteins of Bacillus anthracis and related bacilli: targets for biodetection

The spore is the form released in a bioterrorism attack. There is a real need for definition of new targets for Bacillus anthracis that might be incorporated into emerging biodetection technologies. Particularly of interest are macromolecules found in B. anthracis that are (1) spore-specific, (2) readily accessible on the spore surface and (3) distinct from those present in related organisms. One of the few biochemical methods to identify the spores of B. anthracis is based on the presence of rhamnose and 3-O-methyl rhamnose as determined by gas chromatography-mass spectrometry. Related organisms additionally contain 2-O-methyl rhamnose and fucose. Carbohydrates and glycoproteins of the B. cereus group of organisms and the related B. subilis group are reviewed here. It is hypothesized that the spore-specific carbohydrate is a component of the newly described glycoprotein of the exosporium of B. anthracis. Further work to define the protein and carbohydrate components of the glycoprotein of B. anthracis could be highly useful in developing new technologies for rapid biodetection.

Crystal structure of GerE, the ultimate transcriptional regulator of spore formation in Bacillus subtilis

The small, DNA-binding protein GerE regulates gene transcription in the terminally differentiated mother-cell compartment during late stages of sporulation in Bacillus subtilis. This versatile transcription factor shares sequence homology with the LuxR/FixJ/UhpA family of activators and modulates the expression of a number of genes, in particular those encoding the components of the coat that surrounds the mature spore. GerE orchestrates the final stages of coat deposition and maturation that lead to a spore with remarkable resistance properties but that must be responsive to low levels of germination signals. As this germination process is largely passive and can occur in the absence of de novo protein synthesis, the correct assembly of germination machinery, including germinant receptors and energy storage compounds, is crucial to the survival of the cell. The crystal structure of GerE has been solved at 2.05 A resolution using multi-wavelength anomalous dispersion techniques and reveals the nature of the GerE dimer. Each monomer comprises four alpha-helices, of which the central pair forms a helix-turn-helix DNA-binding motif. Implications for DNA-binding and the structural organisation of the LuxR/FixJ/UhpA family of transcription activator domains are discussed.

Expression of the ftsY gene, encoding a homologue of the alpha subunit of mammalian signal recognition particle receptor, is controlled by different promoters in vegetative and sporulating cells of Bacillus subtilis

Bacillus subtilis FtsY (Srb) is a homologue of the alpha subunit of the receptor for mammalian signal-recognition particle (SRP) and is essential for protein secretion and vegetative cell growth. The ftsY gene is expressed during both the exponential phase and sporulation. In vegetative cells, ftsY is transcribed with two upstream genes, rncS and smc, that are under the control of the major transcription factor sigma(A). During sporulation, Northern hybridization detected ftsY mRNA in wild-type cells, but not in sporulating cells of sigma(K) and gerE mutants. Therefore, ftsY is solely expressed during sporulation from a sigma(K)- and GerE-controlled promoter that is located immediately upstream of ftsY inside the smc gene. To examine the role of FtsY during sporulation, the B. subtilis strain ISR39 was constructed, a ftsY conditional mutant in which ftsY expression can be shut off during spore formation but not during the vegetative state. Electron microscopy showed that the outer coat of ISR39 spores was not completely assembled and immunoelectron microscopy localized FtsY to the inner and outer coats of wild-type spores.

Combined action of two transcription factors regulates genes encoding spore coat proteins of Bacillus subtilis

During sporulation of Bacillus subtilis, spore coat proteins encoded by cot genes are expressed in the mother cell and deposited on the forespore. Transcription of the cotB, cotC, and cotX genes by final sigma(K) RNA polymerase is activated by a small, DNA-binding protein called GerE. The promoter region of each of these genes has two GerE binding sites. 5' deletions that eliminated the more upstream GerE site decreased expression of lacZ fused to cotB and cotX by approximately 80% and 60%, respectively but had no effect on cotC-lacZ expression. The cotC-lacZ fusion was expressed later during sporulation than the other two fusions. Primer extension analysis confirmed that cotB mRNA increases first during sporulation, followed by cotX and cotC mRNAs over a 2-h period. In vitro transcription experiments suggest that the differential pattern of cot gene expression results from the combined action of GerE and another transcription factor, SpoIIID. A low concentration of GerE activated cotB transcription by final sigma(K) RNA polymerase, whereas a higher concentration was needed to activate transcription of cotX or cotC. SpoIIID at low concentration repressed cotC transcription, whereas a higher concentration only partially repressed cotX transcription and had little effect on cotB transcription. DNase I footprinting showed that SpoIIID binds strongly to two sites in the cotC promoter region, binds weakly to one site in the cotX promoter, and does not bind specifically to cotB. We propose that late in sporulation the rising level of GerE and the falling level of SpoIIID, together with the position and affinity of binding sites for these transcription factors in cot gene promoters, dictates the timing and level of spore coat protein synthesis, ensuring optimal assembly of the protein shell on the forespore surface.

Structure and assembly of the bacterial endospore coat

Many biological processes are mediated through the action of multiprotein complexes, often assembled at specific cellular locations. Bacterial endospores for example, are encased in a proteinaceous coat, which confers resistance to lysozyme and harsh chemicals and influences the spore response to germinants. In Bacillus subtilis, the coat is composed of more than 20 polypeptides, organized into three main layers: an amorphous undercoat; a lamellar, lightly staining inner structure; and closely apposed to it, a striated electron-dense outer coat. Synthesis of the coat proteins is temporally and spatially governed by a cascade of four mother cell-specific transcription factors. However, the order of assembly and final destination of the coat structural components may rely mainly on specific protein-protein interactions, as well as on the action of accessory morphogenetic proteins. Proteolytic events, protein-protein crosslinking, and protein glycosylation also play a role in the assembly process. These modifications are carried out by enzymes that may themselves be targeted to the coat layers. Coat genes have been identified by reverse genetics or, more recently, by screens for mother cell-specific promoters or for peptide sequences able to interact with certain bait proteins. A role for a given locus in coat assembly is established by a combination of regulatory, functional, morphological, and topological criteria. Because of the amenability of B. subtilis to genetic analysis (now facilitated by the knowledge of its genome sequence), coat formation has become an attractive model for the assembly of complex macromolecular structures during development.

A region of sigmaK involved in promoter activation by GerE in Bacillus subtilis

During endospore formation in Bacillus subtilis, the DNA binding protein GerE stimulates transcription from several promoters that are used by RNA polymerase containing sigmaK. GerE binds to a site on one of these promoters, cotX, that overlaps its -35 region. We tested the model that GerE interacts with sigmaK at the cotX promoter by seeking amino acid substitutions in sigmaK that interfered with GerE-dependent activation of the cotX promoter but which did not affect utilization of the sigmaK-dependent, GerE-independent promoter gerE. We identified two amino acid substitutions in sigmaK, E216K and H225Y, that decrease cotX promoter utilization but do not affect gerE promoter activity. Alanine substitutions at these positions had similar effects. We also examined the effects of the E216A and H225Y substitutions in sigmaK on transcription in vitro. We found that these substitutions specifically reduced utilization of the cotX promoter. These and other results suggest that the amino acid residues at positions 216 and 225 are required for GerE-dependent cotX promoter activity, that the histidine at position 225 of sigmaK may interact with GerE at the cotX promoter, and that this interaction may facilitate the initial binding of sigmaK RNA polymerase to the cotX promoter. We also found that the alanine substitutions at positions 216 and 225 of sigmaK had no effect on utilization of the GerE-dependent promoter cotD, which contains GerE binding sites that do not overlap with its -35 region.

Structure of the nucleotide-diphospho-sugar transferase, SpsA from Bacillus subtilis, in native and nucleotide-complexed forms

The enzymatic formation of glycosidic bonds may be catalyzed by the transfer of the glycosyl moiety from an activated nucleotide-diphospho-sugar donor to a specific acceptor. SpsA is a glycosyltransferase implicated in the synthesis of the spore coat of Bacillus subtilis, whose homologues include cellulose synthase and many lipopolysaccharide and bacterial O-antigen synthases. The three-dimensional crystal structure of SpsA has been determined by conventional MIR techniques at a resolution of 1.5 A. It is a two-domain protein with a nucleotide-binding domain together with an acceptor binding domain which features a disordered loop spanning the active site. The structures of SpsA in complex with both Mg-UDP and Mn-UDP have also been determined at 2.0 and 1.7 A, respectively. These complexes, together with the sequence conservation, begin to shed light on the mechanism of this ubiquitous family of inverting glycosyltransferases.

Negative regulation by the Bacillus subtilis GerE protein

GerE is a transcription factor produced in the mother cell compartment of sporulating Bacillus subtilis. It is a critical regulator of cot genes encoding proteins that form the spore coat late in development. Most cot genes, and the gerE gene, are transcribed by sigmaK RNA polymerase. Previously, it was shown that the GerE protein inhibits transcription in vitro of the sigK gene encoding sigmaK. Here, we show that GerE binds near the sigK transcriptional start site, to act as a repressor. A sigK-lacZ fusion containing the GerE-binding site in the promoter region was expressed at a 2-fold lower level during sporulation of wild-type cells than gerE mutant cells. Likewise, the level of SigK protein (i. e. pro-sigmaK and sigmaK) was lower in sporulating wild-type cells than in a gerE mutant. These results demonstrate that sigmaK-dependent transcription of gerE initiates a negative feedback loop in which GerE acts as a repressor to limit production of sigmaK. In addition, GerE directly represses transcription of particular cot genes. We show that GerE binds to two sites that span the -35 region of the cotD promoter. A low level of GerE activated transcription of cotD by sigmaK RNA polymerase in vitro, but a higher level of GerE repressed cotD transcription. The upstream GerE-binding site was required for activation but not for repression. These results suggest that a rising level of GerE in sporulating cells may first activate cotD transcription from the upstream site then repress transcription as the downstream site becomes occupied. Negative regulation by GerE, in addition to its positive effects on transcription, presumably ensures that sigmaK and spore coat proteins are synthesized at optimal levels to produce a germination-competent spore.

Bacillus subtilis spore coat

In response to starvation, bacilli and clostridia undergo a specialized program of development that results in the production of a highly resistant dormant cell type known as the spore. A proteinacious shell, called the coat, encases the spore and plays a major role in spore survival. The coat is composed of over 25 polypeptide species, organized into several morphologically distinct layers. The mechanisms that guide coat assembly have been largely unknown until recently. We now know that proper formation of the coat relies on the genetic program that guides the synthesis of spore components during development as well as on morphogenetic proteins dedicated to coat assembly. Over 20 structural and morphogenetic genes have been cloned. In this review, we consider the contributions of the known coat and morphogenetic proteins to coat function and assembly. We present a model that describes how morphogenetic proteins direct coat assembly to the specific subcellular site of the nascent spore surface and how they establish the coat layers. We also discuss the importance of posttranslational processing of coat proteins in coat morphogenesis. Finally, we review some of the major outstanding questions in the field.

Cell wall teichoic acid glycosylation in Listeria monocytogenes serotype 4b requires gtcA, a novel, serogroup-specific gene

We have identified a novel gene, gtcA, involved in the decoration of cell wall teichoic acid of Listeria monocytogenes serotype 4b with galactose and glucose. Insertional inactivation of gtcA brought about loss of reactivity with the serotype 4b-specific monoclonal antibody c74.22 and was accompanied by a complete lack of galactose and a marked reduction in the amounts of glucose on teichoic acid. Interestingly, the composition of membrane-associated lipoteichoic acid was not affected. Complementation of the mutants with the cloned gtcA in trans restored galactose and glucose on teichoic acid to wild-type levels. The complemented strains also recovered reactivity with c74.22. Within L. monocytogenes, sequences homologous to gtcA were found in all serogroup 4 isolates but not in strains of any other serotypes. In serotype 4b, gtcA appears to be the first member of a bicistronic operon which includes a gene with homology to Bacillus subtilis rpmE, encoding ribosomal protein L31. In contrast to gtcA, the latter gene appears conserved among all screened serotypes of L. monocytogenes.

New small, acid-soluble proteins unique to spores of Bacillus subtilis: identification of the coding genes and regulation and function of two of these genes

Eleven small, acid-soluble proteins (SASP) which are present in spores but not in growing cells of Bacillus subtilis were identified by sequence analysis of proteins separated by acrylamide gel electrophoresis of acid extracts from spores which lack the three major SASP (alpha, beta, and gamma). Six of these proteins are encoded by open reading frames identified previously or by analysis of the complete sequence of the B. subtilis genome, including two minor alpha/beta-type SASP (SspC and SspD) and a putative spore coat protein (CotK). Five proteins are encoded by short open reading frames that were not identified as coding regions in the analysis of the complete B. subtilis genomic sequence. Studies of the regulation of two of the latter genes, termed sspG and sspJ, showed that both are expressed only in sporulation. The sspG gene is transcribed in the mother cell compartment by RNA polymerase with the mother cell-specific sigma factor for RNA polymerase, sigmaK, and is cotranscribed with a downstream gene, yurS; sspG transcription also requires the DNA binding protein GerE. In contrast, sspJ is transcribed in the forespore compartment by RNA polymerase with the forespore-specific sigmaG and appears to give a monocistronic transcript. A mutation eliminating SspG had no effect on sporulation or spore properties, while loss of SspJ caused a slight decrease in the rate of spore outgrowth in an otherwise wild-type background.

Isolation, characterization, molecular gene cloning, and sequencing of a novel phytase from Bacillus subtilis

The Bacillus subtilis strain VTT E-68013 was chosen for purification and characterization of its excreted phytase. Purified enzyme had maximal phytase activity at pH 7 and 55 degrees C. Isolated enzyme required calcium for its activity and/or stability and was readily inhibited by EDTA. The enzyme proved to be highly specific since, of the substrates tested, only phytate, ADP, and ATP were hydrolyzed (100, 75, and 50% of the relative activity, respectively). The phytase gene (phyC) was cloned from the B. subtilis VTT E-68013 genomic library. The deduced amino acid sequence (383 residues) showed no homology to the sequences of other phytases nor to those of any known phosphatases. PhyC did not have the conserved RHGXRXP sequence found in the active site of known phytases, and therefore PhyC appears not to be a member of the phytase subfamily of histidine acid phosphatases but a novel enzyme having phytase activity. Due to its pH profile and optimum, it could be an interesting candidate for feed applications.

A spore coat protein, CotS, of Bacillus subtilis is synthesized under the regulation of sigmaK and GerE during development and is located in the inner coat layer of spores

The spore coat of Bacillus subtilis has a unique morphology and consists of polypeptides of different sizes, whose synthesis and assembly are precisely regulated by a cascade of transcription factors and regulatory proteins. We examined the factors that regulate cotS gene expression and CotS assembly into the coat layer of B. subtilis by Northern blot and Western blot analysis. Transcription of cotS mRNA was not detected in sporulating cells of sigmaK and gerE mutants by Northern blot analysis. By Western blot analysis using anti-CotS antibody, CotS was first detected in protein samples solubilized from wild-type cells at 5 h after the start of sporulation. CotS was not detected in the vegetative cells and spores of a gerE mutant or in the spores of mutants deficient in sigmaE, sigmaF, sigmaG, or sigmaK. CotS was detected in the sporangium but not in the spores of a cotE mutant. The sequence of the promoter region of cotS was similar to the consensus sequences for binding of sigmaK and GerE. These results demonstrate that sigmaK and GerE are required for cotS expression and that CotE is essential for the assembly of CotS in the coat. Immunoelectron microscopic observation using anti-CotS antibody revealed that CotS is located within the spore coat, in particular in the inner coats of dormant spores.

Cloning of the thermostable phytase gene (phy) from Bacillus sp. DS11 and its overexpression in Escherichia coli

Phytase hydrolyzes phytate to release inorganic phosphate, which would decrease the addition of phosphorus to feedstuffs for monogastric animals and thus reduce environmental pollution. The gene encoding phytase from Bacillus sp. DS11 was cloned in Escherichia coli and its sequence determined. A 560-bp DNA fragment was used as a probe to screen the genomic library. It was obtained through PCR of Bacillus sp. DS11 chromosomal DNA and two oligonucleotide primers based on N-terminal amino acid sequences of the purified protein and the cyanogen bromide-cleaved 21-kDa fragment. The phy cloned was encoded by a 2.2-kb fragment. This gene comprises 1152 nucleotides and encodes a polypeptide of 383 amino acids with a deduced molecular mass of 41,808 Da. Phytase was produced to 20% content of total soluble proteins in E. coli BL21 (DE3) using the pET22b(+) vector with the inducible T7 promoter. This is the first nucleic sequence report on phytase from a bacterial strain.

Bacillus popilliae cry18Aa operon is transcribed by sigmaE and sigmaK forms of RNA polymerase from a single initiation site

Bacillus popilliae is an obligate pathogen for larvae of the insect family Scarabaeidae (Coleoptera). It forms parasporal crystals upon sporulation. The gene cry18Aa coding for the parasporal crystal protein and an upstream open reading frame, orf1, were previously isolated from B.popilliae. Here we report an analysis of cry18Aa transcription in Bacillus thuringiensis. The only transcriptional start site of cry18Aa was found 29 bp upstream of the open reading frame orf1, suggesting that orf1 and cry18Aa are transcribed as an operon. lacZ fusion to the cry18Aa promoter was used to follow the time-course of cry18Aa transcription in wild type B.thuringiensis and in various B.thuringiensis sporulation-deficient mutants (spo0A, sigE or sigK). In wild type B.thuringiensis, the cry18Aa promoter was activated 2 h after the end of exponential growth and the expression lasted to the late sporulation phase. The results of promoter activity in Spo+or Spo-backgrounds together with the results of primer extension experiments suggest that the transcription from this promoter can be driven by both sigmaE and sigmaK types of RNA polymerase at a single start site. The promoter region of cry18Aa operon fits the consensus sequences of both sigmaE and sigmaK dependent promoters of Bacillus.

Bipolar localization of the replication origin regions of chromosomes in vegetative and sporulating cells of B. subtilis

To investigate chromosome segregation in B. subtilis, we introduced tandem copies of the lactose operon operator into the chromosome near the replication origin or terminus. We then visualized the position of the operator cassettes with green fluorescent protein fused to the Lac1 repressor. In sporulating bacteria, which undergo asymmetric cell division, origins localized near each pole of the cell whereas termini were restricted to the middle. In growing cells, which undergo binary fission, origins were observed at various positions but preferentially toward the poles early in the cell cycle. In contrast, termini showed little preference for the poles. These results indicate the existence of a mitotic-like apparatus that is responsible for moving the origin regions of newly formed chromosomes toward opposite ends of the cell.

CotM of Bacillus subtilis, a member of the alpha-crystallin family of stress proteins, is induced during development and participates in spore outer coat formation

We cloned and characterized a gene, cotM, that resides in the 173 degrees region of the Bacillus subtilis chromosome and is involved in spore outer coat assembly. We found that expression of the cotM gene is induced during development under sigma K control and is negatively regulated by the GerE transcription factor. Disruption of the cotM gene resulted in spores with an abnormal pattern of coat proteins. Electron microscopy revealed that the outer coat in cotM mutant spores had lost its multilayered type of organization, presenting a diffuse appearance. In particular, significant amounts of material were absent from the outer coat layers, which in some areas had a lamellar structure more typical of the inner coat. Occasionally, a pattern of closely spaced ridges protruding from its surface was observed. No deficiency associated with the inner coat or any other spore structure was found. CotM is related to the alpha-crystallin family of low-molecular-weight heat shock proteins, members of which can be substrates for transglutaminase-mediated protein cross-linking. CotM was not detected among the extractable spore coat proteins. These observations are consistent with a model according to which CotM is part of a cross-linked insoluble skeleton that surrounds the spore, serves as a matrix for the assembly of additional outer coat material, and confers structural stability to the final structure.

cse15, cse60, and csk22 are new members of mother-cell-specific sporulation regulons in Bacillus subtilis

We report on the characterization of three new transcription units expressed during sporulation in Bacillus subtilis. Two of the units, cse15 and cse60, were mapped at about 123 degrees and 62 degrees on the genetic map, respectively. Their transcription commenced around h 2 of sporulation and showed an absolute requirement for sigmaE. Maximal expression of both cse15 and cse60 further depended on the DNA-binding protein SpoIIID. Primer extension results revealed -10 and -35 sequences upstream of the cse15 and cse60 coding sequences very similar to those utilized by sigmaE-containing RNA polymerase. Alignment of these and other regulatory regions led to a revised consensus sequence for sigmaE-dependent promoters. A third transcriptional unit, designated csk22, was localized at approximately 173 degrees on the chromosome. Transcription of csk22 was activated at h 4 of sporulation, required the late mother-cell regulator sigmaK, and was repressed by the GerE protein. Sequences in the csk22 promoter region were similar to those of other sigmaK-dependent promoters. The cse60 locus was deduced to encode an acidic product of only 60 residues. A 37.6-kDa protein apparently encoded by cse15 was weakly related to the heavy chain of myosins, as well as to other myosin-like proteins, and is predicted to contain a central, 100 residue-long coiled-coil domain. Finally, csk22 is inferred to encode a 18.2-kDa hydrophobic product with five possible membrane-spanning helices, which could function as a transporter.

Molecular genetics of sporulation in Bacillus subtilis

The process of sporulation in the bacterium Bacillus subtilis proceeds through a well-defined series of morphological stages that involve the conversion of a growing cell into a two-cell-chamber sporangium within which a spore is produced. Over 125 genes are involved in this process, the transcription of which is temporally and spatially controlled by four DNA-binding proteins and five RNA polymerase sigma factors. Through a combination of genetic, biochemical, and cell biological approaches, regulatory networks have been elucidated that explicitly link the activation of these sigma factors to landmark events in the course of morphogenesis and to each other through pathways of intercellular communication. Signals targeting proteins to specific subcellular localizations and governing the assembly of macromolecular structures have been uncovered but their nature remains to be determined.