Bacterial SLH domain proteins are non-covalently anchored to the cell surface via a conserved mechanism involving wall polysaccharide pyruvylation

Comparative genome analysis ofBacillus cereusgroup genomes withBacillus subtilis

Genome features of the Bacillus cereus group genomes (representative strains of Bacillus cereus, Bacillus anthracis and Bacillus thuringiensis sub spp. israelensis) were analyzed and compared with the Bacillus subtilis genome. A core set of 1381 protein families among the four Bacillus genomes, with an additional set of 933 families common to the B. cereus group, was identified. Differences in signal transduction pathways, membrane transporters, cell surface structures, cell wall, and S-layer proteins suggesting differences in their phenotype were identified. The B. cereus group has signal transduction systems including a tyrosine kinase related to two-component system histidine kinases from B. subtilis. A model for regulation of the stress responsive sigma factor sigmaB in the B. cereus group different from the well studied regulation in B. subtilis has been proposed. Despite a high degree of chromosomal synteny among these genomes, significant differences in cell wall and spore coat proteins that contribute to the survival and adaptation in specific hosts has been identified.

The structure of secondary cell wall polymers: how Gram-positive bacteria stick their cell walls together

The cell wall of Gram-positive bacteria has been a subject of detailed chemical study over the past five decades. Outside the cytoplasmic membrane of these organisms the fundamental polymer is peptidoglycan (PG), which is responsible for the maintenance of cell shape and osmotic stability. In addition, typical essential cell wall polymers such as teichoic or teichuronic acids are linked to some of the peptidoglycan chains. In this review these compounds are considered as 'classical' cell wall polymers. In the course of recent investigations of bacterial cell surface layers (S-layers) a different class of 'non-classical' secondary cell wall polymers (SCWPs) has been identified, which is involved in anchoring of S-layers to the bacterial cell surface. Comparative analyses have shown considerable differences in chemical composition, overall structure and charge behaviour of these SCWPs. This review discusses the progress that has been made in understanding the structural principles of SCWPs, which may have useful applications in S-layer-based 'supramolecular construction kits' in nanobiotechnology.

Bacillus anthracis CapD, belonging to the γ-glutamyltranspeptidase family, is required for the covalent anchoring of capsule to peptidoglycan

Several examples of bacterial surface-structure anchoring have been described, but they do not include polyglutamate capsule. Bacillus anthracis capsule, which is composed only of poly-gamma- d-glutamate, is one of the two major virulence factors of the bacterium. We analysed its anchoring. We report that the polyglutamate is anchored directly to the peptidoglycan and that the bond is covalent. We constructed a capD mutant strain, capD being the fourth gene of the capsule biosynthetic operon. The mutant bacilli are surrounded by polyglutamate material that is not covalently anchored. Thus, CapD is required for the covalent anchoring of polyglutamate to the peptidoglycan. Sequence similarities suggest that CapD is a gamma-glutamyltranspeptidase. Furthermore, CapD is cleaved at the gamma-glutamyltranspeptidase consensus cleavage site, and the two subunits remain associated, as necessary for gamma-glutamyltranspeptidase activity. Other Gram-positive gamma-glutamyltranspeptidases are secreted, but CapD is located at the Bacillus surface, associated both with the membrane and the peptidoglycan. Polyglutamate is hydrolysed by CapD indicating that it is a CapD substrate. We suggest that CapD catalyses the capsule anchoring reaction. Interestingly, the CapD(-) strain is far less virulent than the parental strain.

Genetic analysis of Bacillus anthracis Sap S-layer protein crystallization domain

Bacillus anthracis, the aetiological agent of anthrax, synthesizes two surface-layer (S-layer) proteins. S-layers are two-dimensional crystalline arrays that completely cover bacteria. In rich medium, the B. anthracis S-layer consists of Sap during the exponential growth phase. Sap is a modular protein composed of an SLH (S-layer homology)-anchoring domain followed by a putative crystallization domain (Sapc). A projection map of the two-dimensional Sap array has been established on deflated bacteria. In this work, the authors used two approaches to investigate whether Sapc is the crystallization domain. The purified Sapc polypeptide (604 aa) was sufficient to form a crystalline structure, as illustrated by electron microscopy. Consistent with this result, the entire Sapc domain promoted auto-interaction in a bacterial two-hybrid screen developed for the present study. The screen was derived from a system that takes advantage of the Bordetella pertussis cyclase subdomain structure to enable one to identify peptides that interact. A screening strategy was then employed to study Sapc subdomains that mediate interaction. A random library, derived from the Sapc domain, was constructed and screened. The selected polypeptides interacting with the complete Sapc were all larger (155 aa and above) than the mean size of the randomly cloned peptides (approx. 60 residues). This result suggests that, in contrast with observations for other interactions studied with this two-hybrid system, large fragments were required to ensure efficient interaction. It was noteworthy that only one polypeptide, which spanned aa 148–358, was able to interact with less than the complete Sapc, in fact, with itself.

The genome of S-PM2, a "photosynthetic" T4-type bacteriophage that infects marine Synechococcus strains

Bacteriophage S-PM2 infects several strains of the abundant and ecologically important marine cyanobacterium Synechococcus. A large lytic phage with an isometric icosahedral head, S-PM2 has a contractile tail and by this criterion is classified as a myovirus (1). The linear, circularly permuted, 196,280-bp double-stranded DNA genome of S-PM2 contains 37.8% G+C residues. It encodes 239 open reading frames (ORFs) and 25 tRNAs. Of these ORFs, 19 appear to encode proteins associated with the cell envelope, including a putative S-layer-associated protein. Twenty additional S-PM2 ORFs have homologues in the genomes of their cyanobacterial hosts. There is a group I self-splicing intron within the gene encoding the D1 protein. A total of 40 ORFs, organized into discrete clusters, encode homologues of T4 proteins involved in virion morphogenesis, nucleotide metabolism, gene regulation, and DNA replication and repair. The S-PM2 genome encodes a few surprisingly large (e.g., 3,779 amino acids) ORFs of unknown function. Our analysis of the S-PM2 genome suggests that many of the unknown S-PM2 functions may be involved in the adaptation of the metabolism of the host cell to the requirements of phage infection. This hypothesis originates from the identification of multiple phage-mediated modifications of the host's photosynthetic apparatus that appear to be essential for maintaining energy production during the lytic cycle.

The three S-layer-like homology motifs of the S-layer protein SbpA of Bacillus sphaericus CCM 2177 are not sufficient for binding to the pyruvylated secondary cell wall polymer

The S-layer protein SbpA of Bacillus sphaericus CCM 2177 recognizes a pyruvylated secondary cell wall polymer (SCWP) as anchoring structure to the peptidoglycan-containing layer. Data analysis from surface plasmon resonance (SPR) spectroscopy revealed the existence of three different binding sites with high, medium and low affinity for rSbpA on SCWP immobilized to the sensor chip. The shortest C-terminal truncation with specific affinity to SCWP was rSbpA(31-318). Surprisingly, rSbpA(31-202) comprising the three S-layer-like homology (SLH) motifs did not bind at all. Analysis of the SbpA sequence revealed a 58-amino-acid-long SLH-like motif starting 11 amino acids after the third SLH motif. The importance of this motif for reconstituting the functional SCWP-binding domain was further demonstrated by construction of a chimaeric protein consisting of the SLH domain of SbsB, the S-layer protein of Geobacillus stearothermophilus PV72/p2 and the C-terminal part of SbpA. In contrast to SbsB or its SLH domain which did not recognize SCWP of B. sphaericus CCM 2177 as binding site, the chimaeric protein showed specific affinity. Deletion of 213 C-terminal amino acids of SbpA had no impact on the square (p4) lattice structure, whereas deletion of 350 amino acids was linked to a change in lattice type from square to oblique (p1).

Hydrophilic domains of scaffolding protein CbpA promote glycosyl hydrolase activity and localization of cellulosomes to the cell surface of Clostridium cellulovorans

CbpA, the scaffolding protein of Clostridium cellulovorans cellulosomes, possesses one family 3 cellulose binding domain, nine cohesin domains, and four hydrophilic domains (HLDs). Among the three types of domains, the function of the HLDs is still unknown. We proposed previously that the HLDs of CbpA play a role in attaching the cellulosome to the cell surface, since they showed some homology to the surface layer homology domains of EngE. Several recombinant proteins with HLDs (rHLDs) and recombinant EngE (rEngE) were examined to determine their binding to the C. cellulovorans cell wall fraction. Tandemly linked rHLDs showed higher affinity for the cell wall than individual rHLDs showed. EngE was shown to have a higher affinity for cell walls than rHLDs have. C. cellulovorans native cellulosomes were found to have higher affinity for cell walls than rHLDs have. When immunoblot analysis was carried out with the native cellulosome fraction bound to cell wall fragments, the presence of EngE was also confirmed, suggesting that the mechanism anchoring CbpA to the C. cellulovorans cell surface was mediated through EngE and that the HLDs play a secondary role in the attachment of the cellulosome to the cell surface. During a study of the role of HLDs on cellulose degradation, the mini-cellulosome complexes with HLDs degraded cellulose more efficiently than complexes without HLDs degraded cellulose. The rHLDs also showed binding affinity for crystalline cellulose and carboxymethyl cellulose. These results suggest that the CbpA HLDs play a major role and a minor role in C. cellulovorans cellulosomes. The primary role increases cellulose degradation activity by binding the cellulosome complex to the cellulose substrate; secondarily, HLDs aid the binding of the CbpA/cellulosome to the C. cellulovorans cell surface.

The structure of the cell wall peptidoglycan of Bacillus cereus RSVF1, a strain closely related to Bacillus anthracis

The peptidoglycan of Bacillus cereus RSVF1, a close relative of Bacillus anthracis, has several distinguishing features: the overwhelming majority of cross-linked muropeptides are dimers, higher oligomers are only present in minute quantities; and virtually all muropeptides lack the N-acetyl group from glucosamine residues, thus explaining resistance of the cell walls to lysozyme.

A novel Acetivibrio cellulolyticus anchoring scaffoldin that bears divergent cohesins

Sequencing of a cellulosome-integrating gene cluster in Acetivibrio cellulolyticus was completed. The cluster contains four tandem scaffoldin genes (scaA, scaB, scaC, and scaD) bounded upstream and downstream, respectively, by a presumed cellobiose phosphorylase and a nucleotide methylase. The sequences and properties of scaA, scaB, and scaC were reported previously, and those of scaD are reported here. The scaD gene encodes an 852-residue polypeptide that includes a signal peptide, three cohesins, and a C-terminal S-layer homology (SLH) module. The calculated molecular weight of the mature ScaD is 88,960; a 67-residue linker segment separates cohesins 1 and 2, and two approximately 30-residue linkers separate cohesin 2 from 3 and cohesin 3 from the SLH module. The presence of an SLH module in ScaD indicates its role as an anchoring protein. The first two ScaD cohesins can be classified as type II, similar to the four cohesins of ScaB. Surprisingly, the third ScaD cohesin belongs to the type I cohesins, like the seven ScaA cohesins. ScaD is the first scaffoldin to be described that contains divergent types of cohesins as integral parts of the polypeptide chain. The recognition properties among selected recombinant cohesins and dockerins from the different scaffoldins of the gene cluster were investigated by affinity blotting. The results indicated that the divergent types of ScaD cohesins also differ in their preference of dockerins. ScaD thus plays a dual role, both as a primary scaffoldin, capable of direct incorporation of a single dockerin-borne enzyme, and as a secondary scaffoldin that anchors the major primary scaffoldin, ScaA and its complement of enzymes to the cell surface.

Five Genes Involved in Biosynthesis of the Pyruvylated Galβ1,3-Epitope in Schizosaccharomyces pombe N-Linked Glycans

The N-linked galactomannans of Schizosaccharomyces pombe have pyruvylated Galbeta1,3-(PvGal) caps on a portion of the Galalpha1,2-residues in their outer chains (Gemmill, T. R., and Trimble, R. B. (1998) Glycobiology 8, 1087-1095). PvGal biosynthesis was investigated by ethyl methanesulfonate mutagenesis of S. pombe, followed by the isolation of cells devoid of negatively charged N-glycans by Q-Sepharose exclusion and failure to bind human serum amyloid P component, which acts as a lectin for terminal PvGal residues. Mutant glycans were characterized by lectin binding, saccharide composition, exoglycosidase sensitivity, and NMR spectroscopy. Restoration of the cell surface negative charge by complementation with an S. pombe genomic library led to the identification of five genes involved in PvGal biosynthesis, which we designated pvg1-pvg5. Pvg1p may be a pyruvyltransferase, since NMR of pvg1(-) mutant N-glycans revealed the absence of only the pyruvyl moiety. Pvg2p-Pvg5p are crucial for attachment of the Galbeta1,3-residue that becomes pyruvylated. Pvg3p is predicted to be a member of the beta1,3-galactosyltransferase family, and Pvg3p-green fluorescent protein labeling was consistent with Golgi localization. Predicted Pvg1p and Pvg3p functions imply that Galbeta1,3-is added to the galactomannans and is then pyruvylated in situ, rather than by an en bloc addition of PvGalbeta1,3-caps to the outer chain. Pvg4p-green fluorescent protein targeted to the nucleus, and its sequence contains a MADS-box DNA-binding and dimerization domain; however, it does not appear to solely control transcription of the other identified genes. Pvg2p and/or Pvg5p may contribute to an enzyme complex. Whereas a functional role for the PvGal epitope in S. pombe remains unclear, it is nonessential for either cell growth or mating under laboratory conditions.

In vivo Bacillus anthracis gene expression requires PagR as an intermediate effector of the AtxA signalling cascade

Transcription of the major Bacillus anthracis virulence genes is triggered by CO2, a signal mimicking the host environment. A 182-kb plasmid, pXO1, carries the anthrax toxin genes and the genes responsible for their regulation of transcription, namely atxA and, pagR, the second gene of the pag operon. AtxA has major effects on the physiology of B. anthracis. It coordinates the transcription activation of the toxin genes with that of the capsule biosynthetic enzyme operon, located on the second virulence plasmid, pXO2. In rich medium, B. anthracis synthesises alternatively two S-layer proteins (Sap and EA1). An exponential phase "Sap-layer" is subsequently replaced by a stationary phase "EA1-layer". S-layer gene transcription is controlled by alternative sigma factors and by Sap acting as a transcriptional repressor of eag. Furthermore, in vitro in presence of CO2 and in vivo, AtxA is part of the sap and eag regulatory network. Only eag is significantly expressed in these conditions and this is due to AtxA activating eag and repressing sap transcription. PagR, and not AtxA itself, is the direct effector of this regulation by binding to sap and eag promoter regions. Therefore, PagR mediates the effect of AtxA on eag and sap and is the most downstream element of a signalling cascade initiated by AtxA. Taken together, these results indicate that the B. anthracis transcriptional regulator AtxA is controlling the synthesis of the three toxin components and of the surface elements (capsule and S-layer). Thus, AtxA is a master regulator that coordinates the response to host signals by orchestrating positive and negative controls over genes located on all genetic elements.

Evidence that the algI/algJ gene cassette, required for O acetylation of Pseudomonas aeruginosa alginate, evolved by lateral gene transfer

Pseudomonas aeruginosa strains, isolated from chronically infected patients with cystic fibrosis, produce the O-acetylated extracellular polysaccharide, alginate, giving these strains a mucoid phenotype. O acetylation of alginate plays an important role in the ability of mucoid P. aeruginosa to form biofilms and to resist complement-mediated phagocytosis. The O-acetylation process is complex, requiring a protein with seven transmembrane domains (AlgI), a type II membrane protein (AlgJ), and a periplasmic protein (AlgF). The cellular localization of these proteins suggests a model wherein alginate is modified at the polymer level after the transport of O-acetyl groups to the periplasm. Here, we demonstrate that this mechanism for polysaccharide esterification may be common among bacteria, since AlgI homologs linked to type II membrane proteins are found in a variety of gram-positive and gram-negative bacteria. In some cases, genes for these homologs have been incorporated into polysaccharide biosynthetic operons other than for alginate biosynthesis. The phylogenies of AlgI do not correlate with the phylogeny of the host bacteria, based on 16S rRNA analysis. The algI homologs and the gene for their adjacent type II membrane protein present a mosaic pattern of gene arrangement, suggesting that individual components of the multigene cassette, as well as the entire cassette, evolved by lateral gene transfer. AlgJ and the other type II membrane proteins, although more diverged than AlgI, contain conserved motifs, including a motif surrounding a highly conserved histidine residue, which is required for alginate O-acetylation activity by AlgJ. The AlgI homologs also contain an ordered series of motifs that included conserved amino acid residues in the cytoplasmic domain CD-4; the transmembrane domains TM-C, TM-D, and TM-E; and the periplasmic domain PD-3. Site-directed mutagenesis studies were used to identify amino acids important for alginate O-acetylation activity, including those likely required for (i) the interaction of AlgI with the O-acetyl precursor in the cytoplasm, (ii) the export of the O-acetyl group across the cytoplasmic membrane, and (iii) the transfer of the O-acetyl group to a periplasmic protein or to alginate. These results indicate that AlgI belongs to a family of membrane proteins required for modification of polysaccharides and that a mechanism requiring an AlgI homolog and a type II membrane protein has evolved by lateral gene transfer for the esterification of many bacterial extracellular polysaccharides.

Construction of a functional S-layer fusion protein comprising an immunoglobulin G-binding domain for development of specific adsorbents for extracorporeal blood purification

The chimeric gene encoding a C-terminally-truncated form of the S-layer protein SbpA from Bacillus sphaericus CCM 2177 and two copies of the Fc-binding Z-domain was constructed, cloned, and heterologously expressed in Escherichia coli HMS174(DE3). The Z-domain is a synthetic analogue of the B-domain of protein A, capable of binding the Fc part of immunoglobulin G (IgG). The S-layer fusion protein rSbpA(31-1068)/ZZ retained the specific properties of the S-layer protein moiety to self-assemble in suspension and to recrystallize on supports precoated with secondary cell wall polymer (SCWP), which is the natural anchoring molecule for the S-layer protein in the bacterial cell wall. Due to the construction principle of the S-layer fusion protein, the ZZ-domains remained exposed on the outermost surface of the protein lattice. The binding capacity of the native or cross-linked monolayer for human IgG was determined by surface plasmon resonance measurements. For batch adsorption experiments, 3-microm-diameter, biocompatible cellulose-based, SCWP-coated microbeads were used for recrystallization of the S-layer fusion protein. In the case of the native monolayer, the binding capacity for human IgG was 5.1 ng/mm(2), whereas after cross-linking with dimethyl pimelimidate, 4.4 ng of IgG/mm(2) was bound. This corresponded to 78 and 65% of the theoretical saturation capacity of a planar surface for IgGs aligned in the upright position, respectively. Compared to commercial particles used as immunoadsorbents to remove autoantibodies from sera of patients suffering from an autoimmune disease, the IgG binding capacity of the S-layer fusion protein-coated microbeads was at least 20 times higher. For that reason, this novel type of microbeads should find application in the microsphere-based detoxification system.

An S-Layer Heavy Chain Camel Antibody Fusion Protein for Generation of a Nanopatterned Sensing Layer To Detect the Prostate-Specific Antigen by Surface Plasmon Resonance Technology

The bacterial cell surface layer (S-layer) protein of Bacillus sphaericus CCM 2177 assembles into a square lattice structure and recognizes a distinct type of secondary cell wall polymer (SCWP) as the proper anchoring structure in the rigid cell wall layer. For generating a nanopatterned sensing layer with high density and well defined distance of the ligand on the outermost surface, an S-layer fusion protein incorporating the sequence of a variable domain of a heavy chain camel antibody directed against prostate-specific antigen (PSA) was constructed, produced, and recrystallized on gold chips precoated with thiolated SCWP. The S-layer protein moiety consisted of the N-terminal part which specifically recognized the SCWP as binding site and the self-assembly domain. The PSA-specific variable domain of the camel heavy chain antibody was selected by several rounds of panning from a phage display library of an immunized dromedary, and was produced by heterologous expression in Escherichia coli. For construction of the S-layer fusion protein, the 3'-end of the sequence encoding the C-terminally truncated form rSbpA(31)(-)(1068) was fused via a short linker to the 5'-end of the sequence encoding cAb-PSA-N7. The S-layer fusion protein had retained the ability to self-assemble into the square lattice structure. According to the selected fusion site in the SbpA sequence, the cAb-PSA-N7 moiety remained located on the outer surface of the protein lattice. After recrystallization of the S-layer fusion protein on gold chips precoated with thiolated SCWP, the monomolecular protein lattice was exploited as sensing layer in surface plasmon resonance biochips to detect PSA.

Binding to pyruvylated compounds as an ancestral mechanism to anchor the outer envelope in primitive bacteria

Electron microscopy of isolated cell walls of the ancient bacterium Thermus thermophilus revealed that most of the peptidoglycan (PG) surface, apart from the septal region, was shielded against specific alphaPG antibodies. On the other hand, an antiserum raised against S-layer-attached cell wall fragments (alphaSAC) bound to most of the surface except for the septal regions. Treatments with alpha-amylase and pronase E made the entire cell wall surface uniformly accessible to alphaPG and severely decreased the binding of alphaSAC. We concluded that a layer of strongly bound secondary cell wall polymers (SCWPs) covers most of the cell wall surface in this ancient bacterium. A preliminary analysis revealed that such SCWPs constitute 14% of the cell wall and are essentially composed of sugars. Enzyme treatments of the cell walls revealed that SCWP was required in vitro for the binding of the S-layer protein through the S-layer homology (SLH) motif. The csaB gene was necessary for the attachment of the S-layer-outer membrane (OM) complex to the cell wall in growing cells of T. thermophilus. In vitro experiments confirmed that cell walls from a csaB mutant bound to the S-layer with a much lower affinity ( approximately 1/10) than that of the wild type. CsaB was found to be required for pyruvylation of components of the SCWP and for immunodetection with alpha-SAC antiserum. Therefore, the S-layer-OM complex of T. thermophilus binds to the cell wall through the SLH motif of the S-layer protein via a strong interaction with a highly immunogenic pyruvylated component of the SCWP. Immuno-cross-reactive compounds were detected with alphaSAC on cell walls of other Thermus spp. and in the phylogenetically related microorganism Deinococcus radiodurans. These results imply that the interaction between the SLH motif and pyruvylated components of the cell wall arose early during bacterial evolution as an ancestral mechanism for anchoring proteins and outer membranes to the cell walls of primitive bacteria.

Interaction of the crystalline bacterial cell surface layer protein SbsB and the secondary cell wall polymer of Geobacillus stearothermophilus PV72 assessed by real-time surface plasmon resonance biosensor technology

The interaction between S-layer protein SbsB and the secondary cell wall polymer (SCWP) of Geobacillus stearothermophilus PV72/p2 was investigated by real-time surface plasmon resonance biosensor technology. The SCWP is an acidic polysaccharide that contains N-acetylglucosamine, N-acetylmannosamine, and pyruvic acid. For interaction studies, recombinant SbsB (rSbsB) and two truncated forms consisting of either the S-layer-like homology (SLH) domain (3SLH) or the residual part of SbsB were used. Independent of the setup, the data showed that the SLH domain was exclusively responsible for SCWP binding. The interaction was found to be highly specific, since neither the peptidoglycan nor SCWPs from other organisms nor other polysaccharides were recognized. Data analysis from that setup in which 3SLH was immobilized on a sensor chip and SCWP represented the soluble analyte was done in accordance with a model that describes binding of a bivalent analyte to a fixed ligand in terms of an overall affinity for all binding sites. The measured data revealed the presence of at least two binding sites on a single SCWP molecule with a distance of about 14 nm and an overall Kd of 7.7 x 10(-7) M. Analysis of data from the inverted setup in which the SCWP was immobilized on a sensor chip was done in accordance with an extension of the heterogeneous-ligand model, which indicated the existence of three binding sites with low (Kd = 2.6 x 10(-5) M), medium (Kd = 6.1 x 10(-8) M), and high (Kd = 6.7 x 10(-11) M) affinities. Since in this setup 3SLH was the soluble analyte and the presence of small amounts of oligomers in even monomeric protein solutions cannot be excluded, the high-affinity binding site may result from avidity effects caused by binding of at least dimeric 3SLH. Solution competition assays performed with both setups confirmed the specificity of the protein-carbohydrate interaction investigated.

Biophysical characterization of the entire bacterial surface layer protein SbsB and its two distinct functional domains

The crystalline bacterial cell surface layer (S-layer) protein SbsB of Geobacillus stearothermophilus PV72/p2 was dissected into an N-terminal part defined by the three consecutive S-layer homologous motifs and the remaining large C-terminal part. Both parts of the mature protein were produced as separate recombinant proteins (rSbsB(1-178) and rSbsB(177-889)) and compared with the full-length form rSbsB(1-889) (rSbsB). Evidence for functional and structural integrity of the two truncated forms was provided by optical spectroscopic methods and electron microscopy. In particular, binding of the secondary cell wall polymer revealed a high affinity dissociation constant of 3 nm and could be assigned solely to the soluble rSbsB(1-178), whereas rSbsB(177-889) self-assembled into the same lattice as the full-length protein. Furthermore, thermal as well as guanidinium hydrochloride induced equilibrium unfolding profiles monitored by intrinsic fluorescence, and circular dichroism spectroscopy allowed characterization of rSbsB(1-178) as an alpha-helical protein with a single cooperative unfolding transition yielding a DeltaG value of 26.5 kJ mol(-1). The C-terminal rSbsB(177-889) could be characterized as a beta-sheet protein with typical multidomain unfolding, which is partially less stable as stand-alone protein. In general, the truncated forms showed identical properties compared with the full-length rSbsB with respect to structure and function. Consequently, rSbsB is characterized by its two functionally and structurally separated parts, the specific secondary cell wall polymer binding rSbsB(1-178) and the larger rSbsB(177-889) responsible for formation of the crystalline array.

Genome-based bioinformatic selection of chromosomal Bacillus anthracis putative vaccine candidates coupled with proteomic identification of surface-associated antigens

Bacillus anthracis (Ames strain) chromosome-derived open reading frames (ORFs), predicted to code for surface exposed or virulence related proteins, were selected as B. anthracis-specific vaccine candidates by a multistep computational screen of the entire draft chromosome sequence (February 2001 version, 460 contigs, The Institute for Genomic Research, Rockville, Md.). The selection procedure combined preliminary annotation (sequence similarity searches and domain assignments), prediction of cellular localization, taxonomical and functional screen and additional filtering criteria (size, number of paralogs). The reductive strategy, combined with manual curation, resulted in selection of 240 candidate ORFs encoding proteins with putative known function, as well as 280 proteins of unknown function. Proteomic analysis of two-dimensional gels of a B. anthracis membrane fraction, verified the expression of some gene products. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry analyses allowed identification of 38 spots cross-reacting with sera from B. anthracis immunized animals. These spots were found to represent eight in vivo immunogens, comprising of EA1, Sap, and 6 proteins whose expression and immunogenicity was not reported before. Five of these 8 immunogens were preselected by the bioinformatic analysis (EA1, Sap, 2 novel SLH proteins and peroxiredoxin/AhpC), as vaccine candidates. This study demonstrates that a combination of the bioinformatic and proteomic strategies may be useful in promoting the development of next generation anthrax vaccine.

Global effects of virulence gene regulators in a Bacillus anthracis strain with both virulence plasmids

Control of anthrax toxin and capsule synthesis, the two major virulence factors of Bacillus anthracis, has been associated with two regulatory genes, atxA and acpA, located on virulence plasmids pXO1 and pXO2, respectively. We used transcriptional profiling to determine whether atxA and/or acpA control genes other than those already described and to investigate functional similarities of the regulators. Transcription was assessed in a pXO1(+) pXO2(+) parent strain and in isogenic mutants in which one or both regulatory genes were deleted. We determined that in addition to the toxin and capsule genes, atxA controls expression of numerous other genes on both plasmids and the chromosome. Generally, plasmid-encoded genes were more highly regulated than chromosomal genes, and both positive and negative effects were observed. Certain atxA-regulated genes were affected synergistically in an atxA acpA mutant. Yet overall, acpA appears to be a minor regulator with fewer targets than atxA. In contrast to previous reports of acpA function in attenuated strains, acpA had a minimal influence on capsule gene transcription and capsule synthesis in a genetically complete strain. Surprisingly, acpA expression was positively affected by atxA, although atxA-activated capsule gene transcription is not acpA dependent. The newly discovered atxA-regulated targets include genes predicted to encode secreted proteins and proteins with roles in transcriptional regulation and signaling. Regulation of chromosomal genes by atxA is particularly intriguing, given that many of the target genes have homologues in other Bacillus species that lack atxA homologues. Given the global effect of atxA on gene expression in B. anthracis, previous assumptions regarding reduced virulence of strains harboring single plasmids must be reassessed and the potential roles of newly identified atxA-regulated genes should be investigated.

Generation of a functional monomolecular protein lattice consisting of an s-layer fusion protein comprising the variable domain of a camel heavy chain antibody

Crystalline bacterial cell surface layer (S-layer) proteins are composed of a single protein or glycoprotein species. Isolated S-layer subunits frequently recrystallize into monomolecular protein lattices on various types of solid supports. For generating a functional protein lattice, a chimeric protein was constructed, which comprised the secondary cell wall polymer-binding region and the self-assembly domain of the S-layer protein SbpA from Bacillus sphaericus CCM 2177, and a single variable region of a heavy chain camel antibody (cAb-Lys3) recognizing lysozyme as antigen. For construction of the S-layer fusion protein, the 3'-end of the sequence encoding the C-terminally truncated form rSbpA(31)(-)(1068) was fused via a short linker to the 5'-end of the sequence encoding cAb-Lys3. The functionality of the fused cAb-Lys3 in the S-layer fusion protein was proved by surface plasmon resonance measurements. Dot blot assays revealed that the accessibility of the fused functional sequence for the antigen was independent of the use of soluble or assembled S-layer fusion protein. Recrystallization of the S-layer fusion protein into the square lattice structure was observed on peptidoglycan-containing sacculi of B. sphaericus CCM 2177, on polystyrene or on gold chips precoated with thiolated secondary cell wall polymer, which is the natural anchoring molecule for the S-layer protein in the bacterial cell wall. Thereby, the fused cAb-Lys3 remained located on the outer S-layer surface and accessible for lysozyme binding. Together with solid supports precoated with secondary cell wall polymers, S-layer fusion proteins comprising rSbpA(31)(-)(1068) and cAbs directed against various antigens shall be exploited for building up monomolecular functional protein lattices as required for applications in nanobiotechnology.

Structural and functional analysis of the S-layer protein crystallisation domain of Lactobacillus acidophilus ATCC 4356: evidence for protein-protein interaction of two subdomains

The structure of the crystallisation domain, SAN, of the S(A)-protein of Lactobacillus acidophilus ATCC 4356 was analysed by insertion and deletion mutagenesis, and by proteolytic treatment. Mutant S(A)-protein synthesised in Escherichia coli with 7-13 amino acid insertions near the N terminus or within regions of sequence variation in SAN (amino acid position 7, 45, 114, 125, 193), or in the cell wall-binding domain (position 345) could form crystalline sheets, whereas insertions in conserved regions or in regions with predicted secondary structure elements (positions 30, 67, 88 and 156) destroyed this capacity. FACscan analysis of L.acidophilus synthesising three crystallising and one non-crystallising S(A)-protein c-myc (19 amino acid residues) insertion mutant was performed with c-myc antibodies. Fluorescence was most pronounced for insertions at positions 125 and 156, less for position 45 and severely reduced for position 7. By cytometric flow sorting a transformant harbouring the mutant S(A)-protein gene (position 125) was isolated that showed an increased fluorescense signal. Immunofluorescence microscopy suggested that the transformant synthesized mutant S(A)-protein only. PCR analysis of the transformant grown in the absence of selection pressure indicated that the mutant allele was stably integrated in the chromosome. Proteolytic treatment of S(A)-protein indicated that only sites near the middle of SAN are susceptible, although potential cleavage sites are present through the entire molecule. Expression in E.coli of DNA sequences encoding the two halves of SAN yielded peptides that could oligomerize. Our results indicate that SAN consists of a approximately 12kDa N and a approximately 18kDa C-terminal subdomain linked by a surface exposed loop. The capacity of S(A)-protein of L.acidophilus to present epitopes, up to approximately 19 amino acid residues in length, at the bacterial surface in a genetically stable form, makes the system, in principle, suitable for application as an oral delivery vehicle.

Bacillus anthracis cell envelope components

Bacillus anthracis is a Gram-positive bacterium harboring a complex parietal architecture. The cytoplasmic membrane is surrounded by a thick peptidoglycan of the A1 gamma type. Only one associated polymer, a polysaccharide composed of galactose, N-acetylglucosamine, and N-acetylmannosamine, is covalently linked to the peptidoglycan. Outside the cell wall is an S-layer. Two proteins can each compose the S-layer. They are noncovalently anchored to the cell wall polysaccharide by their SLH N-terminal domain. The poly-gamma-D-glutamate capsule, which covers the S-layer, has an antiphagocytic role and its synthesis is dependent on environmental factors mimicking the mammalian host, such as bicarbonate and a temperature of 37 degrees C.

The first biantennary bacterial secondary cell wall polymer and its influence on S-layer glycoprotein assembly

The cell surface of Aneurinibacillus thermoaerophilus DSM 10155 is covered with a square surface (S)-layer glycoprotein lattice. This S-layer glycoprotein, which was extracted with aqueous buffers after a freeze-thaw cycle of the bacterial cells, is the only completely water-soluble S-layer glycoprotein to be reported to date. The purified S-layer glycoprotein preparation had an overall carbohydrate content of 19%. Detailed chemical investigations indicated that the S-layer O-glycans of previously established structure accounted for 13% of total glycosylation. The remainder could be attributed to a peptidoglycan-associated secondary cell wall polymer. Structure analysis was performed using purified secondary cell wall polymer-peptidoglycan complexes. NMR spectroscopy revealed the first biantennary secondary cell wall polymer from the domain Bacteria, with the structure alpha-L-Glc p NAc-(1-->3)-beta-L-Man p NAc-(1-->4)-beta-L-Gal p NAc-(1-->3)-alpha-L-Glc p NAc-(1-->3)-beta-L-Man p NAc-(1-->4)-beta-L-Gal p NAc-(1-->3)-alpha-L-Glc p NAc-(1-->4)-[alpha-L-Glc p NAc-(1-->3)-beta-L-Man p NAc-(1-->4)-beta-L-Gal p NAc-(1-->3)-alpha-L-Glc p NAc-(1-->3)-beta-L-Man p NAc-(1-->4)-beta-L-Gal p NAc-(1-->3)-alpha-L-Glc p NAc-(1-->3)]-beta-L-Man p NAc-(1-->3)-alpha-L-Glc p NAc-(1-->3)-beta-L-Man p NAc-(1-->3)-alpha-L-Glc p NAc-(1-->3)-alpha-L-Glc p NAc-(1-->O)-PO(2)(-)-O-PO(2)(-)-(O-->6)-MurNAc- (where MurNAc is N -acetylmuramic acid). The neutral polysaccharide is linked via a pyrophosphate bond to the C-6 atom of every fourth N -acetylmuramic acid residue, in average, of the A1gamma-type peptidoglycan. In vivo, the biantennary polymer anchored the S-layer glycoprotein very effectively to the cell wall, probably due to the doubling of motifs for a proposed lectin-like binding between the polymer and the N-terminus of the S-layer protein. When the cellular support was removed during S-layer glycoprotein isolation, the co-purified polymer mediated the solubility of the S-layer glycoprotein in vitro. Initial crystallization experiments performed with the soluble S-layer glycoprotein revealed that the assembly property could be restored upon dissociation of the polymer by the addition of poly(ethylene glycols). The formed two-dimensional crystalline S-layer self-assembly products exhibited the same lattice symmetry as observed on intact bacterial cells.

Structural analysis and evidence for dynamic emergence of Bacillus anthracis S-layer networks

Surface layers (S-layers), which form the outermost layers of many Bacteria and Archaea, consist of protein molecules arranged in two-dimensional crystalline arrays. Bacillus anthracis, a gram-positive, spore-forming bacterium, responsible for anthrax, synthesizes two abundant surface proteins: Sap and EA1. Regulatory studies showed that EA1 and Sap appear sequentially at the surface of the parental strain. Sap and EA1 can form arrays. The structural parameters of S-layers from mutant strains (EA1(-) and Sap(-)) were determined by computer image processing of electron micrographs of negatively stained regular S-layer fragments or deflated whole bacteria. Sap and EA1 projection maps were calculated on a p1 symmetry basis. The unit cell parameters of EA1 were a = 69 A, b = 83 A, and gamma = 106 degrees, while those of Sap were a = 184 A, b = 81 A, and gamma = 84 degrees. Freeze-etching experiments and the analysis of the peripheral regions of the cell suggested that the two S-layers have different settings. We characterized the settings of each network at different growth phases. Our data indicated that the scattered emergence of EA1 destabilizes the Sap S-layer.

Search for potential vaccine candidate open reading frames in the Bacillus anthracis virulence plasmid pXO1: in silico and in vitro screening

A genomic analysis of the Bacillus anthracis virulence plasmid pXO1, aimed at identifying potential vaccine candidates and virulence-related genes, was carried out. The 143 previously defined open reading frames (ORFs) (R. T. Okinaka, K. Cloud, O. Hampton, A. R. Hoffmaster, K. K. Hill, P. Keim, T. M. Koehler, G. Lamke, S. Kumano, J. Mahillon, D. Manter, Y. Martinez, D. Ricke, R. Svensson, and P. J. Jackson, J. Bacteriol. 181:6509-6515, 1999) were subjected to extensive sequence similarity searches (with the nonredundant and unfinished microbial genome databases), as well as motif, cellular location, and domain analyses. A comparative genomics analysis was conducted with the related genomes of Bacillus subtilis, Bacillus halodurans, and Bacillus cereus and the pBtoxis plasmid of Bacillus thuringiensis var. israeliensis. As a result, the percentage of ORFs with clues about their functions increased from approximately 30% (as previously reported) to more than 60%. The bioinformatics analysis permitted identification of novel genes with putative relevance for pathogenesis and virulence. Based on our analyses, 11 putative proteins were chosen as targets for functional genomics studies. A rapid and efficient functional screening method was developed, in which PCR-amplified full-length linear DNA products of the selected ORFs were transcribed and directly translated in vitro and their immunogenicities were assessed on the basis of their reactivities with hyperimmune anti-B. anthracis antisera. Of the 11 ORFs selected for analysis, 9 were successfully expressed as full-length polypeptides, and 3 of these were found to be antigenic and to have immunogenic potential. The latter ORFs are currently being evaluated to determine their vaccine potential.

Archaeal surface layer proteins contain beta propeller, PKD, and beta helix domains and are related to metazoan cell surface proteins

The surface layer of archaeobacteria protects cells from extreme environments and, in Methanosarcina, may regulate cell adhesion. We identify three domain types that account for the complete architecture of numerous Methanosarcina surface layer proteins (SLPs). We solve the crystal structure for two of these domains, which correspond to the two N-terminal domains of an M. mazei SLP. One domain displays a unique, highly symmetrical, seven-bladed beta propeller fold, and the other belongs to the polycystic kidney disease (PKD) superfamily fold. The third domain is predicted to adopt a beta helix fold. These domains have homologs in metazoan cell surface proteins, suggesting remarkable relationships between domains in archaeal SLPs and metazoan cell surface proteins.

One repeat of the cell wall binding domain is sufficient for anchoring the Lactobacillus acidophilus surface layer protein

The N-terminal repeat (SAC1) of the S-protein of Lactobacillus acidophilus bound efficiently and specifically to cell wall fragments (CWFs) when fused to green fluorescent protein, whereas the C-terminal repeat (SAC2) did not. Treatment of CWFs with hydrofluoric acid, but not phenol, prevented binding. Apparently, SAC1 is necessary and sufficient for cell wall binding. Our data suggest that SAC anchors the S-protein to a cell wall teichoic acid.

Molecular characterization of the S-layer gene, sbpA, of Bacillus sphaericus CCM 2177 and production of a functional S-layer fusion protein with the ability to recrystallize in a defined orientation while presenting the fused allergen

The nucleotide sequence encoding the crystalline bacterial cell surface (S-layer) protein SbpA of Bacillus sphaericus CCM 2177 was determined by a PCR-based technique using four overlapping fragments. The entire sbpA sequence indicated one open reading frame of 3,804 bp encoding a protein of 1,268 amino acids with a theoretical molecular mass of 132,062 Da and a calculated isoelectric point of 4.69. The N-terminal part of SbpA, which is involved in anchoring the S-layer subunits via a distinct type of secondary cell wall polymer to the rigid cell wall layer, comprises three S-layer-homologous motifs. For screening of amino acid positions located on the outer surface of the square S-layer lattice, the sequence encoding Strep-tag I, showing affinity to streptavidin, was linked to the 5' end of the sequence encoding the recombinant S-layer protein (rSbpA) or a C-terminally truncated form (rSbpA(31-1068)). The deletion of 200 C-terminal amino acids did not interfere with the self-assembly properties of the S-layer protein but significantly increased the accessibility of Strep-tag I. Thus, the sequence encoding the major birch pollen allergen (Bet v1) was fused via a short linker to the sequence encoding the C-terminally truncated form rSpbA(31-1068). Labeling of the square S-layer lattice formed by recrystallization of rSbpA(31-1068)/Bet v1 on peptidoglycan-containing sacculi with a Bet v1-specific monoclonal mouse antibody demonstrated the functionality of the fused protein sequence and its location on the outer surface of the S-layer lattice. The specific interactions between the N-terminal part of SbpA and the secondary cell wall polymer will be exploited for an oriented binding of the S-layer fusion protein on solid supports to generate regularly structured functional protein lattices.

Display of proteins on bacteria

Display of heterologous proteins on the surface of microorganisms, enabled by means of recombinant DNA technology, has become an increasingly used strategy in various applications in microbiology, biotechnology and vaccinology. Gram-negative, Gram-positive bacteria, viruses and phages are all being investigated in such applications. This review will focus on the bacterial display systems and applications. Live bacterial vaccine delivery vehicles are being developed through the surface display of foreign antigens on the bacterial surfaces. In this field, 'second generation' vaccine delivery vehicles are at present being generated by the addition of mucosal targeting signals, through co-display of adhesins, in order to achieve targeting of the live bacteria to immunoreactive sites to thereby increase immune responses. Engineered bacteria are further being evaluated as novel microbial biocatalysts with heterologous enzymes immobilized as surface exposed on the bacterial cell surface. A discussion has started whether bacteria can find use as new types of whole-cell diagnostic devices since single-chain antibodies and other type of tailor-made binding proteins can be displayed on bacteria. Bacteria with increased binding capacity for certain metal ions can be created and potential environmental or biosensor applications for such recombinant bacteria as biosorbents are being discussed. Certain bacteria have also been employed for display of various poly-peptide libraries for use as devices in in vitro selection applications. Through various selection principles, individual clones with desired properties can be selected from such libraries. This article explains the basic principles of the different bacterial display systems, and discusses current uses and possible future trends of these emerging technologies.

Developmental switch of S-layer protein synthesis in Bacillus anthracis

Adjustment of the synthesis of abundant protein to the requirements of the cell involves processes critical to the minimization of energy expenditure. The regulation of S-layer genes might be a good model for such processes because expression must be controlled, such that the encoded proteins exactly cover the surface of the bacterium. Bacillus anthracis has two S-layer genes, sap and eag, encoding the S-layer proteins Sap and EA1 respectively. We report that the production and surface localization of Sap and EA1 are under developmental control, suggesting that an exponential phase 'Sap layer' is subsequently replaced by a stationary phase 'EA1 layer'. This switch is controlled at the transcriptional level: sap is most certainly transcribed by RNA polymerase containing sigmaA, whereas eag expression depends on sigmaH. More importantly, Sap is required for the temporal control of eag, and EA1 is involved in strict feedback regulation of eag. This control may be direct because both S-layer proteins bind, in vitro, the eag promoter, specifically suggesting that they might act as transcriptional repressors.

CASH--a beta-helix domain widespread among carbohydrate-binding proteins

In this article, we describe a novel, widespread domain (CASH) that is shared by many carbohydrate-binding proteins and sugar hydrolases. This domain occurs in more than 1000 proteins distributed among all three kingdoms of life. The CASH domain is characterized by internal repetitions of glycines and hydrophobic residues that correspond to the repetitive units of a predicted or observed right-handed beta-helix structure of the pectate lyase superfamily.

Anthrax spores make an essential contribution to vaccine efficacy

Anthrax is caused by Bacillus anthracis, a gram-positive spore-forming bacterium. Septicemia and toxemia rapidly lead to death in infected mammal hosts. Currently used acellular vaccines against anthrax consist of protective antigen (PA), one of the anthrax toxin components. However, in experimental animals such vaccines are less protective than live attenuated strains. Here we demonstrate that the addition of formaldehyde-inactivated spores (FIS) of B. anthracis to PA elicits total protection against challenge with virulent B. anthracis strains in mice and guinea pigs. The toxin-neutralizing activities of sera from mice immunized with PA alone or PA plus FIS were similar, suggesting that the protection conferred by PA plus FIS was not only a consequence of the humoral response to PA. A PA-deficient challenge strain was constructed, and its virulence was due solely to its multiplication. Immunization with FIS alone was sufficient to protect mice partially, and guinea pigs totally, against infection with this strain. This suggests that spore antigens contribute to protection. Guinea pigs and mice had very different susceptibilities to infection with the nontoxigenic strain, highlighting the importance of verifying the pertinence of animal models for evaluating anthrax vaccines.

Anthrax

Bacillus anthracis was shown to be the etiological agent of anthrax by R. Koch and L. Pasteur at the end of the nineteenth century. The concepts on which medical microbiology are based arose from their work on this bacterium. The link between plasmids and major virulence factors of B. anthracis was not discovered until the 1980s. The three toxin components are organized in two A-B type toxins, and the bacilli are covered by an antiphagocytic polyglutamic capsule. Structure-function analysis of the toxins indicated that the common B-domain binds to a ubiquitous cell receptor and forms a heptamer after proteolytic activation. One enzyme moiety is an adenylate cyclase and the other is a Zn(2+) metalloprotease, which is able to cleave MAPKKs. The capsule covers an S-layer sequentially composed of two distinct proteins. Knowledge of the toxins facilitates the design of safer veterinary vaccines. Spore-structure analysis could contribute to the improvement of human nonliving vaccines. The phylogeny of B. anthracis within the Bacillus cereus group is also reviewed.

Plasmid-encoded autolysin in Bacillus anthracis: modular structure and catalytic properties

A Bacillus anthracis virulence plasmid-encoded peptidoglycan hydrolase (AmiA) with sequence similarity to N-acetylmuramoyl-L-alanine amidases hydrolyzes peptidoglycan independently of cell wall binding. Residues H341, E355, H415, and E486 are absolutely required for catalysis. Many AmiA paralogs are fused to different sorting signals, suggesting that these modular proteins result from domain shuffling.

The incompatibility between the PlcR- and AtxA-controlled regulons may have selected a nonsense mutation in Bacillus anthracis

Bacillus anthracis, Bacillus thuringiensis and Bacillus cereus are members of the Bacillus cereus group. These bacteria express virulence in diverse ways in mammals and insects. The pathogenic properties of B. cereus and B. thuringiensis in mammals results largely from the secretion of non-specific toxins, including haemolysins, the production of which depends upon a pleiotropic activator PlcR. In B. anthracis, PlcR is inactive because of a nonsense mutation in the plcR gene. This suggests that the phenotypic differences between B. anthracis on the one hand and B. thuringiensis and B. cereus on the other could result at least partly from loss of the PlcR regulon. We expressed a functional PlcR in B. anthracis. This resulted in the transcriptional activation of genes weakly expressed in the absence of PlcR. The transcriptional activation correlated with the induction of enzymatic activities and toxins including haemolysins. The toxicity of a B. anthracis PlcR+ strain was assayed in the mouse subcutaneous and nasal models of infection. It was no greater than that of the parental strain, suggesting that the PlcR regulon has no influence on B. anthracis virulence. The PlcR regulon had dramatic effects on the sporulation of a B. anthracis strain containing the virulence plasmid pXO1. This resulted from incompatible interactions with the major AtxA-controlled virulence regulon. We propose that the PlcR-controlled regulon in B. anthracis has been counterselected on account of its disadvantageous effects.

Synergy between the N- and C-terminal domains of InlB for efficient invasion of non-phagocytic cells by Listeria monocytogenes

InlB is a Listeria monocytogenes protein promoting entry in non-phagocytic cells, and has been shown recently to activate the hepatocyte growth factor receptor (HGFR or Met). The N-terminal domain of InlB (LRRs) binds and activates Met, whereas the C-terminal domain of InlB (GW modules) mediates loose attachment of InlB to the listerial surface. As HGF activation of Met is tightly controlled by glycosaminoglycans (GAGs), we tested if GAGs also modulate the Met-InlB interactions. We show that InlB-dependent invasion of non-phagocytic cells decreases up to 10 times in the absence of GAGs, and that soluble heparin releases InlB from the bacterial surface and promotes its clustering. Furthermore, we demonstrate that InlB binds cellular GAGs by its GW modules, and that this interaction is required for efficient InlB-mediated invasion. Therefore, GW modules have an unsuspected dual function: they attach InlB to the bacterial surface and enhance entry triggered by the LRRs domain. Our results thus provide the first evidence for a synergy between two host factor-binding domains of a bacterial invasion protein, and reinforce similarities between InlB and mammalian growth factors.

Distribution of S-layers on the surface of Bacillus cereus strains: phylogenetic origin and ecological pressure

Bacillus anthracis, Bacillus cereus and Bacillus thuringiensis have been described as members of the Bacillus cereus group but are, in fact, one species. B. anthracis is a mammal pathogen, B. thuringiensis an entomopathogen and B. cereus a ubiquitous soil bacterium and an occasional human pathogen. In two clinical isolates of B. cereus, in some B. thuringiensis strains and in B. anthracis, an S-layer has been described. We investigated how the S-layer is distributed in B. cereus, and whether phylogeny or ecology could explain its presence on the surface of some but not all strains. We first developed a simple biochemical assay to test for the presence of the S-layer. We then used the assay with 51 strains of known genetic relationship: 26 genetically diverse B. cereus and 25 non-B. anthracis of the B. anthracis cluster. When present, the genetic organization of the S-layer locus was analysed further. It was identical in B. cereus and B. anthracis. Nineteen strains harboured an S-layer, 16 of which belonged to the B. anthracis cluster. All 19 were B. cereus clinical isolates or B. thuringiensis, except for one soil and one dairy strain. These findings suggest a common phylogenetic origin for the S-layer at the surface of B. cereus strains and, presumably, ecological pressure on its maintenance.

A general strategy for identification of S-layer genes in the Bacillus cereus group: molecular characterization of such a gene in Bacillus thuringiensis subsp. galleriae NRRL 4045

Despite its possible role in virulence, there has been little molecular characterization of members of the S-layer protein family of the Bacillus cereus group. It is hypothesized that the components of the S-layers are likely to display similar anchoring structures in Bacillus thuringiensis and Bacillus anthracis. Based on inferred sequence similarities, a DNA fragment encoding the cell-wall-anchoring domain of an S-layer component of BAC: thuringiensis subsp. galleriae NRRL 4045 was isolated. The complete gene was identified and sequenced. An ORF of 2463 nt was identified, which was predicted to encode a protein of 821 aa, SlpA. The mature SlpA protein (792 residues) carries three S-layer homology (SLH) motifs towards its amino terminus, each about 50 aa long. These motifs were sufficient to bind Bac. thuringiensis and Bac. anthracis cell walls in vitro by interacting with peptidoglycan-associated polymers, confirming a common wall-anchoring mechanism. The slpA null-allele mutant was constructed and shown to possess no other abundant surface protein. It is proposed that the method described in this paper could be applied to the identification and deletion of any Bac. cereus S-layer gene and is of great value for the molecular and functional characterization of these genes.

The multidomain xylanase Xyn10B as a cellulose-binding protein in Clostridium stercorarium

The cells of Clostridium stercorarium F-9 grown on cellobiose bound to insoluble cellulose allomorphs such as phosphoric acid-swollen cellulose (ASC). Treatment of the cells with 3 M guanidine hydrochloride extracted surface-layer proteins from the cells and abolished the affinity of the cells for ASC. SDS-polyacrylamide gel electrophoresis, zymogram, and immunological analyses indicated that one of the major surface layer proteins was Xyn10B, which is a modular xylanase comprising two family 22 carbohydrate-binding modules (CBMs), a family 10 catalytic domain of glycosyl hydrolases, a family 9 CBM, and two S-layer homologous (SLH) domains. The C. stercorarium F-9 cells treated with guanidine hydrochloride coprecipitated with ASC upon the addition of a derivative of Xyn10B containing both a CBM and SLH domain in addition to a catalytic domain, but not a derivative without Xyn10B-SLH domains, suggesting that Xyn10B functioned as a cellulose-binding protein in C. stercorarium F-9.

Multiple regulatory mechanisms act on the 5' untranslated region of the S-layer gene from Thermus thermophilus HB8

The role of the 5' untranslated region (5'UTR) of the S-layer gene from Thermus thermophilus was analyzed through the isolation of Delta 5'UTR mutants. In these mutants the half-life of splA mRNA was strongly reduced and slpA transcription was no longer subjected to growth phase-dependent repression. Overproduction and detachment of the external envelopes of the mutants were observed in stationary phase.