The SLH-domain protein BslO is a determinant of Bacillus anthracis chain length

Overlapping and Distinct Functions of the Paralogous PagR Regulators of Bacillus anthracis

The Bacillus anthracis pagA gene, encoding the protective antigen component of anthrax toxin, is part of a bicistronic operon on pXO1 that codes for its own repressor, PagR1. In addition to the pagAR1 operon, PagR1 regulates sap and eag, two chromosome genes encoding components of the surface layer, a mounting structure for surface proteins involved in virulence. Genomic studies have revealed a PagR1 paralog, PagR2, encoded by a gene on pXO2. The amino acid sequences of the paralogues are 71% identical and show similarity to the ArsR family of transcription regulators. We determined that the expression of either rPagR1 or rPagR2 in a ΔpagR1 pXO1⁺/pXO2^- (PagR1-PagR2) background repressed the expression of pagA, sap, eag, and a newly discovered target, atxA, encoding virulence activator AtxA. Despite the redundancy in PagR1 and PagR2 function, we determined that purified rPagR1 bound DNA corresponding to the control regions of all four target genes and existed as a dimer in cell lysates, whereas rPagR2 exhibited weak binding to the DNA of the pagA and atxA promoters, did not bind sap or eag promoter DNA, and did not appear as a dimer in cell lysates. A single amino acid change in PagR2, S81Y, designed to match the native Y81 of PagR1, allowed for DNA-binding to the sap and eag promoters. Moreover, the S81Y mutation allowed for the detection of PagR2 homomultimers in coaffinity purification experiments. Our results expand our knowledge of the roles of the paralogues in B. anthracis gene expression and provide a potential mechanistic basis for differences in the functions of these repressors. IMPORTANCE The protective antigen component of the anthrax toxin is essential for the delivery of the enzymatic components of the toxin into host target cells. The toxin genes and other virulence genes of B. anthracis are regulated by multiple trans-acting regulators that respond to a variety of host-related signals. PagR1, one such trans-acting regulator, connects the regulation of plasmid-encoded and chromosome-encoded virulence genes by controlling both protective antigen and surface layer protein expression. Whether PagR2, a paralog of PagR1, also functions as a trans-acting regulator was unknown. This work advances our knowledge of the complex model of virulence regulation in B. anthracis and furthers our understanding of the intriguing evolution of this pathogen.

Dynamic Profile of S-Layer Proteins Controls Surface Properties of Emetic Bacillus cereus AH187 Strain

Many prokaryotes are covered by a two-dimensional array of proteinaceous subunits. This surface layers (S-layer) is incompletely characterized for many microorganisms. Here, we studied Bacillus cereus AH187. A genome analysis identified two genes encoding the S-layer proteins SL2 and EA1, which we experimentally confirmed to encode the two protein components of the S-layer covering the surface of B. cereus. Shotgun proteomics analysis indicated that SL2 is the major component of the B. cereus S-layer at the beginning of exponential growth, whereas EA1 becomes more abundant than SL2 during later stages of stationary growth. Microscopy analysis revealed the spatial organization of SL2 and EA1 at the surface of B. cereus to depend on their temporal-dynamics during growth. Our results also show that a mutant strain lacking functional SL2 and EA1 proteins has distinct surface properties compared to its parental strain, in terms of stiffness and hydrophilicity during the stationary growth phase. Surface properties, self-aggregation capacity, and bacterial adhesion were observed to correlate. We conclude that the dynamics of SL2 and EA1 expression is a key determinant of the surface properties of B. cereus AH187, and that the S-layer could contribute to B. cereus survival in starvation conditions.

Bacillus anthracis chain length, a virulence determinant, is regulated by membrane localized serine/threonine protein kinase PrkC

Anthrax is a zoonotic disease caused by Bacillus anthracis, a spore-forming pathogen that displays a chaining phenotype. It has been reported that the chaining phenotype acts as a virulence factor in B. anthracis In this study, we identify a serine/threonine protein kinase of B. anthracis, PrkC, the only kinase localized at the bacteria-host interface, as a determinant of B. anthracis chain length. In vitro, prkC disruption strain (BAS ΔprkC) grew as shorter chains throughout the bacterial growth cycle. A comparative analysis between the parent strain and BAS ΔprkC indicated that the levels of proteins, BslO and Sap, associated with the regulation of the bacterial chain length, were upregulated in BAS ΔprkC BslO is a septal murein hydrolase that catalyzes daughter cell separation and Sap is an S-layer structural protein required for the septal localization of BslO. PrkC disruption also has a significant effect on bacterial growth, cell wall thickness, and septa formation. Upregulation of ftsZ in BAS ΔprkC was also observed. Altogether, our results indicate that PrkC is required for maintaining optimum growth, cell wall homeostasis and most importantly - for the maintenance of the chaining phenotype.IMPORTANCEChaining phenotype acts as a virulence factor in Bacillus anthracis This is the first study that identifies a 'signal transduction protein' with an ability to regulate the chaining phenotype in Bacillus anthracis We show that the disruption of the lone surface-localized serine/threonine protein kinase, PrkC, leads to the shortening of the bacterial chains. We report upregulation of the de-chaining proteins in the PrkC disruption strain. Apart from this, we also report for the first time that PrkC disruption results in an attenuated cell growth, a decrease in the cell wall thickness and aberrant cell septa formation during the logarithmic phase of growth - a growth phase where PrkC is expressed maximally.

The Bacillus anthracis Cell Envelope: Composition, Physiological Role, and Clinical Relevance

Anthrax is a highly resilient and deadly disease caused by the spore-forming bacterial pathogen Bacillus anthracis. The bacterium presents a complex and dynamic composition of its cell envelope, which changes in response to developmental and environmental conditions and host-dependent signals. Because of their easy to access extracellular locations, B. anthracis cell envelope components represent interesting targets for the identification and development of novel therapeutic and vaccine strategies. This review will focus on the novel insights regarding the composition, physiological role, and clinical relevance of B. anthracis cell envelope components.

Distinct Pathways Carry Out α and β Galactosylation of Secondary Cell Wall Polysaccharide in Bacillus anthracis

Bacillus anthracis, the causative agent of anthrax disease, elaborates a secondary cell wall polysaccharide (SCWP) that is required for the retention of surface layer (S-layer) and S-layer homology (SLH) domain proteins. Genetic disruption of the SCWP biosynthetic pathway impairs growth and cell division. B. anthracis SCWP is comprised of trisaccharide repeats composed of one ManNAc and two GlcNAc residues with O-3-α-Gal and O-4-β-Gal substitutions. UDP-Gal, synthesized by GalE1, is the substrate of galactosyltransferases that modify the SCWP repeat. Here, we show that the gtsE gene, which encodes a predicted glycosyltransferase with a GT-A fold, is required for O-4-β-Gal modification of trisaccharide repeats. We identify a DXD motif critical for GtsE activity. Three distinct genes, gtsA, gtsB, and gtsC, are required for O-3-α-Gal modification of trisaccharide repeats. Based on the similarity with other three-component glycosyltransferase systems, we propose that GtsA transfers Gal from cytosolic UDP-Gal to undecaprenyl phosphate (C55-P), GtsB flips the C55-P-Gal intermediate to the trans side of the membrane, and GtsC transfers Gal onto trisaccharide repeats. The deletion of galE1 does not affect growth in vitro, suggesting that galactosyl modifications are dispensable for the function of SCWP. The deletion of gtsA, gtsB, or gtsC leads to a loss of viability, yet gtsA and gtsC can be deleted in strains lacking galE1 or gtsE We propose that the loss of viability is caused by the accumulation of undecaprenol-bound precursors and present an updated model for SCWP assembly in B. anthracis to account for the galactosylation of repeat units.IMPORTANCE Peptidoglycan is a conserved extracellular macromolecule that protects bacterial cells from turgor pressure. Peptidoglycan of Gram-positive bacteria serves as a scaffold for the attachment of polymers that provide defined bacterial interactions with their environment. One such polymer, B. anthracis SCWP, is pyruvylated at its distal end to serve as a receptor for secreted proteins bearing the S-layer homology domain. Repeat units of SCWP carry three galactoses in B. anthracis Glycosylation is a recurring theme in nature and often represents a means to mask or alter conserved molecular signatures from intruders such as bacteriophages. Several glycosyltransferase families have been described based on bioinformatics prediction, but few have been studied. Here, we describe the glycosyltransferases that mediate the galactosylation of B. anthracis SCWP.

The Bacillus cereus Group: Bacillus Species with Pathogenic Potential

The Bacillus cereus group includes several Bacillus species with closely related phylogeny. The most well-studied members of the group, B. anthracis, B. cereus, and B. thuringiensis, are known for their pathogenic potential. Here, we present the historical rationale for speciation and discuss shared and unique features of these bacteria. Aspects of cell morphology and physiology, and genome sequence similarity and gene synteny support close evolutionary relationships for these three species. For many strains, distinct differences in virulence factor synthesis provide facile means for species assignment. B. anthracis is the causative agent of anthrax. Some B. cereus strains are commonly recognized as food poisoning agents, but strains can also cause localized wound and eye infections as well as systemic disease. Certain B. thuringiensis strains are entomopathogens and have been commercialized for use as biopesticides, while some strains have been reported to cause infection in immunocompromised individuals. In this article we compare and contrast B. anthracis, B. cereus, and B. thuringiensis, including ecology, cell structure and development, virulence attributes, gene regulation and genetic exchange systems, and experimental models of disease.

4,6- O-Pyruvyl Ketal Modified N-Acetylmannosamine of the Secondary Cell Wall Polysaccharide of Bacillus anthracis Is the Anchoring Residue for Its Surface Layer Proteins

The secondary cell wall polysaccharide (SCWP) of Bacillus anthracis plays a key role in the organization of the cell envelope of vegetative cells and is intimately involved in host-guest interactions. Genetic studies have indicated that it anchors S-layer and S-layer-associated proteins, which are involved in multiple vital biological functions, to the cell surface of B. anthracis. Phenotypic observations indicate that specific functional groups of the terminal unit of SCWP, including 4,6- O-pyruvyl ketal and acetyl esters, are important for binding of these proteins. These observations are based on genetic manipulations and have not been corroborated by direct binding studies. To address this issue, a synthetic strategy was developed that could provide a range of pyruvylated oligosaccharides derived from B. anthracis SCWP bearing base-labile acetyl esters and free amino groups. The resulting oligosaccharides were used in binding studies with a panel of S-layer and S-layer-associated proteins, which identified structural features of SCWP important for binding. A single pyruvylated ManNAc monosaccharide exhibited strong binding to all proteins, making it a promising structure for S-layer protein manipulation. The acetyl esters and free amine of SCWP did not significantly impact binding, and this observation is contrary to a proposed model in which SCWP acetylation is a prerequisite for association of some but not all S-layer and S-layer-associated proteins.

The S-layer protein of a Clostridium difficile SLCT-11 strain displays a complex glycan required for normal cell growth and morphology

Clostridium difficile is a bacterial pathogen that causes major health challenges worldwide. It has a well-characterized surface (S)-layer, a para-crystalline proteinaceous layer surrounding the cell wall. In many bacterial and archaeal species, the S-layer is glycosylated, but no such modifications have been demonstrated in C. difficile. Here, we show that a C. difficile strain of S-layer cassette type 11, Ox247, has a complex glycan attached via an O-linkage to Thr-38 of the S-layer low-molecular-weight subunit. Using MS and NMR, we fully characterized this glycan. We present evidence that it is composed of three domains: (i) a core peptide-linked tetrasaccharide with the sequence -4-α-Rha-3-α-Rha-3-α-Rha-3-β-Gal-peptide; (ii) a repeating pentasaccharide with the sequence -4-β-Rha-4-α-Glc-3-β-Rha-4-(α-Rib-3-)β-Rha-; and (iii) a nonreducing end-terminal 2,3 cyclophosphoryl-rhamnose attached to a ribose-branched sub-terminal rhamnose residue. The Ox247 genome contains a 24-kb locus containing genes for synthesis and protein attachment of this glycan. Mutations in genes within this locus altered or completely abrogated formation of this glycan, and their phenotypes suggested that this S-layer modification may affect sporulation, cell length, and biofilm formation of C. difficile In summary, our findings indicate that the S-layer protein of SLCT-11 strains displays a complex glycan and suggest that this glycan is required for C. difficile sporulation and control of cell shape, a discovery with implications for the development of antimicrobials targeting the S-layer.

Assembly and Function of the Bacillus anthracis S-Layer

Bacillus anthracis, the anthrax agent, is a member of the Bacillus cereus sensu lato group, which includes invasive pathogens of mammals or insects as well as nonpathogenic environmental strains. The genes for anthrax pathogenesis are located on two large virulence plasmids. Similar virulence plasmids have been acquired by other B. cereus strains and enable the pathogenesis of anthrax-like diseases. Among the virulence factors of B. anthracis is the S-layer-associated protein BslA, which endows bacilli with invasive attributes for mammalian hosts. BslA surface display and function are dependent on the bacterial S-layer, whose constituents assemble by binding to the secondary cell wall polysaccharide (SCWP) via S-layer homology (SLH) domains. B. anthracis and other pathogenic B. cereus isolates harbor genes for the secretion of S-layer proteins, for S-layer assembly, and for synthesis of the SCWP. We review here recent insights into the assembly and function of the S-layer and the SCWP.

The Surface Layer Homology Domain-Containing Proteins of Alkaliphilic Bacillus pseudofirmus OF4 Play an Important Role in Alkaline Adaptation via Peptidoglycan Synthesis

It is well known that the Na⁺ cycle and the cell wall are essential for alkaline adaptation of Na⁺-dependent alkaliphilic Bacillus species. In Bacillus pseudofirmus OF4, surface layer protein A (SlpA), the most abundant protein in the surface layer (S-layer) of the cell wall, is involved in alkaline adaptation, especially under low Na⁺ concentrations. The presence of a large number of genes that encode S-layer homology (SLH) domain-containing proteins has been suggested from the genome sequence of B. pseudofirmus OF4. However, other than SlpA, the functions of SLH domain-containing proteins are not well known. Therefore, a deletion mutant of the csaB gene, required for the retention of SLH domain-containing proteins on the cell wall, was constructed to investigate its physiological properties. The csaB mutant strain of B. pseudofirmus OF4 had a chained morphology and alkaline sensitivity even under a 230 mM Na⁺ concentration at which there is no growth difference between the parental strain and the slpA mutant strain. Ultra-thin section transmission electron microscopy showed that a csaB mutant strain lacked an S-layer part, and its peptidoglycan (PG) layer was disturbed. The slpA mutant strain also lacked an S-layer part, although its PG layer was not disturbed. These results suggested that the surface layer homology domain-containing proteins of B. pseudofirmus OF4 play an important role in alkaline adaptation via peptidoglycan synthesis.

Galactosylation of the Secondary Cell Wall Polysaccharide of Bacillus anthracis and Its Contribution to Anthrax Pathogenesis

Bacillus anthracis, the causative agent of anthrax disease, elaborates a secondary cell wall polysaccharide (SCWP) that is essential for bacterial growth and cell division. B. anthracis SCWP is comprised of trisaccharide repeats with the structure, [→4)-β-ManNAc-(1→4)-β-GlcNAc(O3-α-Gal)-(1→6)-α-GlcNAc(O3-α-Gal, O4-β-Gal)-(1→]6-12 The genes whose products promote the galactosylation of B. anthracis SCWP are not yet known. We show here that the expression of galE1, encoding a UDP-glucose 4-epimerase necessary for the synthesis of UDP-galactose, is required for B. anthracis SCWP galactosylation. The galE1 mutant assembles surface (S) layer and S layer-associated proteins that associate with ketal-pyruvylated SCWP via their S layer homology domains similarly to wild-type B. anthracis, but the mutant displays a defect in γ-phage murein hydrolase binding to SCWP. Furthermore, deletion of galE1 diminishes the capsulation of B. anthracis with poly-d-γ-glutamic acid (PDGA) and causes a reduction in bacterial virulence. These data suggest that SCWP galactosylation is required for the physiologic assembly of the B. anthracis cell wall envelope and for the pathogenesis of anthrax disease.IMPORTANCE Unlike virulent Bacillus anthracis isolates, B. anthracis strain CDC684 synthesizes secondary cell wall polysaccharide (SCWP) trisaccharide repeats without galactosyl modification, exhibits diminished growth in vitro in broth cultures, and is severely attenuated in an animal model of anthrax. To examine whether SCWP galactosylation is a requirement for anthrax disease, we generated variants of B. anthracis strains Sterne 34F2 and Ames lacking UDP-glucose 4-epimerase by mutating the genes galE1 and galE2 We identified galE1 as necessary for SCWP galactosylation. Deletion of galE1 decreased the poly-d-γ-glutamic acid (PDGA) capsulation of the vegetative form of B. anthracis and increased the bacterial inoculum required to produce lethal disease in mice, indicating that SCWP galactosylation is indeed a determinant of anthrax disease.

The CodY-dependent clhAB2 operon is involved in cell shape, chaining and autolysis in Bacillus cereus ATCC 14579

The Gram-positive pathogen Bacillus cereus is able to grow in chains of rod-shaped cells, but the regulation of chaining remains largely unknown. Here, we observe that glucose-grown cells of B. cereus ATCC 14579 form longer chains than those grown in the absence of glucose during the late exponential and transition growth phases, and identify that the clhAB₂ operon is required for this chain lengthening phenotype. The clhAB₂ operon is specific to the B. cereus group (i.e., B. thuringiensis, B. anthracis and B. cereus) and encodes two membrane proteins of unknown function, which are homologous to the Staphylococcus aureus CidA and CidB proteins involved in cell death control within glucose-grown cells. A deletion mutant (ΔclhAB₂) was constructed and our quantitative image analyses show that ΔclhAB₂ cells formed abnormal short chains regardless of the presence of glucose. We also found that glucose-grown cells of ΔclhAB₂ were significantly wider than wild-type cells (1.47 μm ±CI_95% 0.04 vs 1.19 μm ±CI_95% 0.03, respectively), suggesting an alteration of the bacterial cell wall. Remarkably, ΔclhAB₂ cells showed accelerated autolysis under autolysis-inducing conditions, compared to wild-type cells. Overall, our data suggest that the B. cereus clhAB₂ operon modulates peptidoglycan hydrolase activity, which is required for proper cell shape and chain length during cell growth, and down-regulates autolysin activity. Lastly, we studied the transcription of clhAB₂ using a lacZ transcriptional reporter in wild-type, ccpA and codY deletion-mutant strains. We found that the global transcriptional regulatory protein CodY is required for the basal level of clhAB₂ expression under all conditions tested, including the transition growth phase while CcpA, the major global carbon regulator, is needed for the high-level expression of clhAB₂ in glucose-grown cells.

Identification of the chain-dispersing peptidoglycan hydrolase LytB of Streptococcus gordonii

Bacterial cell division ends with the separation of the daughter cells, a process that requires peptidoglycan hydrolases (PGHs). Bacteria lacking cell separating PGHs are impaired in cell separation with the formation of long chains or clusters. We identified a gene in Streptococcus gordonii encoding for a putative glucosaminidase (lytB). The lytB isogenic mutant grew in long bacterial chains and resulted in impaired biofilm formation. Purified recombinant LytB showed a murolytic activity on Micrococcus lysodeikticus cell suspension and was able to disperse the long chains of the mutant, restoring the wild type diplococci/short chain phenotype. LytB protein was localized only in culture supernatant cell fraction of S. gordonii, and co-cultures of wild type and lytB mutant showed a significant reduction of bacterial chain length, indicating that LytB is a secreted enzyme. Our results demonstrate that LytB is a secreted peptidoglycan hydrolase required for S. gordonii cell separation.

Genes Required for Bacillus anthracis Secondary Cell Wall Polysaccharide Synthesis

The secondary cell wall polysaccharide (SCWP) is thought to be essential for vegetative growth and surface (S)-layer assembly in Bacillus anthracis; however, the genetic determinants for the assembly of its trisaccharide repeat structure are not known. Here, we report that WpaA (BAS0847) and WpaB (BAS5274) share features with membrane proteins involved in the assembly of O-antigen lipopolysaccharide in Gram-negative bacteria and propose that WpaA and WpaB contribute to the assembly of the SCWP in B. anthracis Vegetative forms of the B. anthracis wpaA mutant displayed increased lengths of cell chains, a cell separation defect that was attributed to mislocalization of the S-layer-associated murein hydrolases BslO, BslS, and BslT. The wpaB mutant was defective in vegetative replication during early logarithmic growth and formed smaller colonies. Deletion of both genes, wpaA and wpaB, did not yield viable bacilli, and when depleted of both wpaA and wpaB, B. anthracis could not maintain cell shape, support vegetative growth, or assemble SCWP. We propose that WpaA and WpaB fulfill overlapping glycosyltransferase functions of either polymerizing repeat units or transferring SCWP polymers to linkage units prior to LCP-mediated anchoring of the polysaccharide to peptidoglycan.

IMPORTANCE

The secondary cell wall polysaccharide (SCWP) is essential for Bacillus anthracis growth, cell shape, and division. SCWP is comprised of trisaccharide repeats (→4)-β-ManNAc-(1→4)-β-GlcNAc-(1→6)-α-GlcNAc-(1→) with α-Gal and β-Gal substitutions; however, the genetic determinants and enzymes for SCWP synthesis are not known. Here, we identify WpaA and WpaB and report that depletion of these factors affects vegetative growth, cell shape, and S-layer assembly. We hypothesize that WpaA and WpaB are involved in the assembly of SCWP prior to transfer of this polymer onto peptidoglycan.

Multidomain, Surface Layer-associated Glycoside Hydrolases Contribute to Plant Polysaccharide Degradation by Caldicellulosiruptor Species

The genome of the extremely thermophilic bacterium Caldicellulosiruptor kronotskyensisencodes 19 surface layer (S-layer) homology (SLH) domain-containing proteins, the most in any Caldicellulosiruptorspecies genome sequenced to date. These SLH proteins include five glycoside hydrolases (GHs) and one polysaccharide lyase, the genes for which were transcribed at high levels during growth on plant biomass. The largest GH identified so far in this genus, Calkro_0111 (2,435 amino acids), is completely unique toC. kronotskyensisand contains SLH domains. Calkro_0111 was produced recombinantly inEscherichia colias two pieces, containing the GH16 and GH55 domains, respectively, as well as putative binding and spacer domains. These displayed endo- and exoglucanase activity on the β-1,3-1,6-glucan laminarin. A series of additional truncation mutants of Calkro_0111 revealed the essential architectural features required for catalytic function. Calkro_0402, another of the SLH domain GHs inC. kronotskyensis, when produced inE. coli, was active on a variety of xylans and β-glucans. Unlike Calkro_0111, Calkro_0402 is highly conserved in the genus Caldicellulosiruptorand among other biomass-degrading Firmicutes but missing from Caldicellulosiruptor bescii As such, the gene encoding Calkro_0402 was inserted into the C. besciigenome, creating a mutant strain with its S-layer extensively decorated with Calkro_0402. This strain consequently degraded xylans more extensively than wild-typeC. bescii The results here provide new insights into the architecture and role of SLH domain GHs and demonstrate that hemicellulose degradation can be enhanced through non-native SLH domain GHs engineered into the genomes of Caldicellulosiruptorspecies.

Bacillus anthracis tagO Is Required for Vegetative Growth and Secondary Cell Wall Polysaccharide Synthesis

Bacillus anthracis elaborates a linear secondary cell wall polysaccharide (SCWP) that retains surface (S)-layer and associated proteins via their S-layer homology (SLH) domains. The SCWP is comprised of trisaccharide repeats [→4)-β-ManNAc-(1→4)-β-GlcNAc-(1→6)-α-GlcNAc-(1→] and tethered via acid-labile phosphodiester bonds to peptidoglycan. Earlier work identified UDP-GlcNAc 2-epimerases GneY (BAS5048) and GneZ (BAS5117), which act as catalysts of ManNAc synthesis, as well as a polysaccharide deacetylase (BAS5051), as factors contributing to SCWP synthesis. Here, we show that tagO (BAS5050), which encodes a UDP-N-acetylglucosamine:undecaprenyl-P N-acetylglucosaminyl 1-P transferase, the enzyme that initiates the synthesis of murein linkage units, is required for B. anthracis SCWP synthesis and S-layer assembly. Similar to gneY-gneZ mutants, B. anthracis strains lacking tagO cannot maintain cell shape or support vegetative growth. In contrast, mutations in BAS5051 do not affect B. anthracis cell shape, vegetative growth, SCWP synthesis, or S-layer assembly. These data suggest that TagO-mediated murein linkage unit assembly supports SCWP synthesis and attachment to the peptidoglycan via acid-labile phosphodiester bonds. Further, B. anthracis variants unable to synthesize SCWP trisaccharide repeats cannot sustain cell shape and vegetative growth.

IMPORTANCE

Bacillus anthracis elaborates an SCWP to support vegetative growth and envelope assembly. Here, we show that some, but not all, SCWP synthesis is dependent on tagO-derived murein linkage units and subsequent attachment of SCWP to peptidoglycan. The data implicate secondary polymer modifications of peptidoglycan and subcellular distributions as a key feature of the cell cycle in Gram-positive bacteria and establish foundations for work on the molecular functions of the SCWP and on inhibitors with antibiotic attributes.

Bacillus anthracis SlaQ Promotes S-Layer Protein Assembly

Bacillus anthracis vegetative forms assemble an S-layer comprised of two S-layer proteins, Sap and EA1. A hallmark of S-layer proteins are their C-terminal crystallization domains, which assemble into a crystalline lattice once these polypeptides are deposited on the bacterial surface via association between their N-terminal S-layer homology domains and the secondary cell wall polysaccharide. Here we show that slaQ, encoding a small cytoplasmic protein conserved among pathogenic bacilli elaborating S-layers, is required for the efficient secretion and assembly of Sap and EA1. S-layer protein precursors cosediment with SlaQ, and SlaQ appears to facilitate Sap assembly. Purified SlaQ polymerizes and when mixed with purified Sap promotes the in vitro formation of tubular S-layer structures. A model is discussed whereby SlaQ, in conjunction with S-layer secretion factors SecA2 and SlaP, promotes localized secretion and S-layer assembly in B. anthracis.

IMPORTANCE

S-layer proteins are endowed with the propensity for self-assembly into crystalline arrays. Factors promoting S-layer protein assembly have heretofore not been reported. We identified Bacillus anthracis SlaQ, a small cytoplasmic protein that facilitates S-layer protein assembly in vivo and in vitro.

Bacillus anthracis lcp Genes Support Vegetative Growth, Envelope Assembly, and Spore Formation

Bacillus anthracis, a spore-forming pathogen, replicates as chains of vegetative cells by regulating the separation of septal peptidoglycan. Surface (S)-layer proteins and B. anthracis S-layer-associated proteins (BSLs) function as chain length determinants and are assembled in the envelope by binding to the secondary cell wall polysaccharide (SCWP). B. anthracis expresses six different genes encoding LytR-CpsA-Psr (LCP) enzymes (lcpB1 to -4, lcpC, and lcpD), which when expressed in Staphylococcus aureus promote attachment of wall teichoic acid to peptidoglycan. Mutations in B. anthracis lcpB3 and lcpD cause aberrations in cell size and chain length that can be explained as discrete defects in SCWP assembly; however, the function of the other lcp genes is not known. By deleting combinations of lcp genes from the B. anthracis genome, we generated variants with single lcp genes. B. anthracis expressing lcpB3 alone displayed physiological cell size, vegetative growth, spore formation, and S-layer assembly. Strains expressing lcpB1 or lcpB4 displayed defects in cell size and shape, S-layer assembly, and spore formation yet sustained vegetative growth. In contrast, the lcpB2 strain was unable to grow unless the gene was expressed from a multicopy plasmid (lcpB2(++)), and variants expressing lcpC or lcpD displayed severe defects in growth and cell shape. The lcpB2(++), lcpC, or lcpD strains supported neither S-layer assembly nor spore formation. We propose a model whereby LCP enzymes fulfill partially overlapping functions in transferring SCWP molecules to discrete sites within the bacterial envelope.

IMPORTANCE

Products of genes essential for bacterial envelope assembly represent targets for antibiotic development. The LytR-CpsA-Psr (LCP) enzymes tether bactoprenol-linked intermediates of secondary cell wall polymers to the C6 hydroxyl of N-acetylmuramic acid in peptidoglycan; however, the role of LCPs as a target for antibiotic therapy is not defined. We show here that LCP enzymes are essential for the cell cycle, vegetative growth, and spore formation of Bacillus anthracis, the causative agent of anthrax disease. Furthermore, we assign functions for each of the six LCP enzymes, including cell size and shape, vegetative growth and sporulation, and S-layer and S-layer-associated protein assembly.

Structure, bioactivity, and resistance mechanism of streptomonomicin, an unusual lasso Peptide from an understudied halophilic actinomycete

Natural products are the most historically significant source of compounds for drug development. However, unacceptably high rates of compound rediscovery associated with large-scale screening of common microbial producers have resulted in the abandonment of many natural product drug discovery efforts, despite the increasing prevalence of clinically problematic antibiotic resistance. Screening of underexplored taxa represents one strategy to avoid rediscovery. Herein we report the discovery, isolation, and structural elucidation of streptomonomicin (STM), an antibiotic lasso peptide from Streptomonospora alba, and report the genome for its producing organism. STM-resistant clones of Bacillus anthracis harbor mutations to walR, the gene encoding a response regulator for the only known widely distributed and essential two-component signal transduction system in Firmicutes. To the best of our knowledge, Streptomonospora had been hitherto biosynthetically and genetically uncharacterized, with STM being the first reported compound from the genus. Our results demonstrate that understudied microbes remain fruitful reservoirs for the rapid discovery of novel, bioactive natural products.

LytR-CpsA-Psr enzymes as determinants of Bacillus anthracis secondary cell wall polysaccharide assembly

Bacillus anthracis, the causative agent of anthrax, replicates as chains of vegetative cells by regulating the separation of septal peptidoglycan. Surface (S)-layer proteins and associated proteins (BSLs) function as chain length determinants and bind to the secondary cell wall polysaccharide (SCWP). In this study, we identified the B. anthracis lcpD mutant, which displays increased chain length and S-layer assembly defects due to diminished SCWP attachment to peptidoglycan. In contrast, the B. anthracis lcpB3 variant displayed reduced cell size and chain length, which could be attributed to increased deposition of BSLs. In other bacteria, LytR-CpsA-Psr (LCP) proteins attach wall teichoic acid (WTA) and polysaccharide capsule to peptidoglycan. B. anthracis does not synthesize these polymers, yet its genome encodes six LCP homologues, which, when expressed in S. aureus, promote WTA attachment. We propose a model whereby B. anthracis LCPs promote attachment of SCWP precursors to discrete locations in the peptidoglycan, enabling BSL assembly and regulated separation of septal peptidoglycan.

GneZ, a UDP-GlcNAc 2-epimerase, is required for S-layer assembly and vegetative growth of Bacillus anthracis

Bacillus anthracis, the causative agent of anthrax, forms an S-layer atop its peptidoglycan envelope and displays S-layer proteins and Bacillus S-layer-associated (BSL) proteins with specific functions to support cell separation of vegetative bacilli and growth in infected mammalian hosts. S-layer and BSL proteins bind via the S-layer homology (SLH) domain to the pyruvylated secondary cell wall polysaccharide (SCWP) with the repeat structure [→4)-β-ManNAc-(1→4)-β-GlcNAc-(1→6)-α-GlcNAc-(1→]n, where α-GlcNAc and β-GlcNAc are substituted with two and one galactosyl residues, respectively. B. anthracis gneY (BAS5048) and gneZ (BAS5117) encode nearly identical UDP-GlcNAc 2-epimerase enzymes that catalyze the reversible conversion of UDP-GlcNAc and UDP-ManNAc. UDP-GlcNAc 2-epimerase enzymes have been shown to be required for the attachment of the phage lysin PlyG with the bacterial envelope and for bacterial growth. Here, we asked whether gneY and gneZ are required for the synthesis of the pyruvylated SCWP and for S-layer assembly. We show that gneZ, but not gneY, is required for B. anthracis vegetative growth, rod cell shape, S-layer assembly, and synthesis of pyruvylated SCWP. Nevertheless, inducible expression of gneY alleviated all the defects associated with the gneZ mutant. In contrast to vegetative growth, neither germination of B. anthracis spores nor the formation of spores in mother cells required UDP-GlcNAc 2-epimerase activity.

Biogenesis and functions of bacterial S-layers

The outer surface of many archaea and bacteria is coated with a proteinaceous surface layer (known as an S-layer), which is formed by the self-assembly of monomeric proteins into a regularly spaced, two-dimensional array. Bacteria possess dedicated pathways for the secretion and anchoring of the S-layer to the cell wall, and some Gram-positive species have large S-layer-associated gene families. S-layers have important roles in growth and survival, and their many functions include the maintenance of cell integrity, enzyme display and, in pathogens and commensals, interaction with the host and its immune system. In this Review, we discuss our current knowledge of S-layer and related proteins, including their structures, mechanisms of secretion and anchoring and their diverse functions.

Bacillus anthracis acetyltransferases PatA1 and PatA2 modify the secondary cell wall polysaccharide and affect the assembly of S-layer proteins

The envelope of Bacillus anthracis encompasses a proteinaceous S-layer with two S-layer proteins (Sap and EA1). Protein assembly in the envelope of B. anthracis requires S-layer homology domains (SLH) within S-layer proteins and S-layer-associated proteins (BSLs), which associate with the secondary cell wall polysaccharide (SCWP), an acetylated carbohydrate that is tethered to peptidoglycan. Here, we investigated the contributions of two putative acetyltransferases, PatA1 and PatA2, on SCWP acetylation and S-layer assembly. We show that mutations in patA1 and patA2 affect the chain lengths of B. anthracis vegetative forms and perturb the deposition of the BslO murein hydrolase at cell division septa. The patA1 and patA2 mutants are defective for the assembly of EA1 in the envelope but retain the ability of S-layer formation with Sap. SCWP isolated from the patA1 patA2 mutant lacked acetyl moieties identified in wild-type polysaccharide and failed to associate with the SLH domains of EA1. A model is discussed whereby patA1- and patA2-mediated acetylation of SCWP enables the deposition of EA1 as well as BslO near the septal region of the B. anthracis envelope.

Bacillus cereus G9241 S-layer assembly contributes to the pathogenesis of anthrax-like disease in mice

Bacillus cereus G9241, the causative agent of anthrax-like disease, harbors virulence plasmids encoding anthrax toxins as well as hyaluronic acid (HA) and B. cereus exopolysaccharide (BPS) capsules. B. cereus G9241 also harbors S-layer genes, including homologs of Bacillus anthracis surface array protein (Sap), extractable antigen 1 (EA1), and the S-layer-associated proteins (BSLs). In B. anthracis, S-layer proteins and BSLs attach via their S-layer homology domains (SLH) to the secondary cell wall polysaccharide (SCWP) in a manner requiring csaB, a predicted ketalpyruvate transferase. Here we used a genetic approach to analyze B. cereus G9241 S-layer assembly and function. Variants lacking the csaB gene synthesized SCWP but failed to retain Sap, EA1, and BSLs in the bacterial envelope. The B. cereus G9241 csaB mutant assembled capsular polysaccharides but displayed an increase in chain length relative to the wild-type strain. This phenotype is likely due to its inability to deposit BslO murein hydrolase at divisional septa. During growth under capsule-inducing conditions, B. cereus G9241 assembled BSLs (BslA and BslO) and the Sap S-layer protein, but not EA1, in the envelope. Finally, csaB-mediated assembly of S-layer proteins and BSLs in B. cereus G9241 contributes to the pathogenesis of anthrax-like disease in mice.

Role of the Group B antigen of Streptococcus agalactiae: a peptidoglycan-anchored polysaccharide involved in cell wall biogenesis

Streptococcus agalactiae (Group B streptococcus, GBS) is a leading cause of infections in neonates and an emerging pathogen in adults. The Lancefield Group B carbohydrate (GBC) is a peptidoglycan-anchored antigen that defines this species as a Group B Streptococcus. Despite earlier immunological and biochemical characterizations, the function of this abundant glycopolymer has never been addressed experimentally. Here, we inactivated the gene gbcO encoding a putative UDP-N-acetylglucosamine-1-phosphate:lipid phosphate transferase thought to catalyze the first step of GBC synthesis. Indeed, the gbcO mutant was unable to synthesize the GBC polymer, and displayed an important growth defect in vitro. Electron microscopy study of the GBC-depleted strain of S. agalactiae revealed a series of growth-related abnormalities: random placement of septa, defective cell division and separation processes, and aberrant cell morphology. Furthermore, vancomycin labeling and peptidoglycan structure analysis demonstrated that, in the absence of GBC, cells failed to initiate normal PG synthesis and cannot complete polymerization of the murein sacculus. Finally, the subcellular localization of the PG hydrolase PcsB, which has a critical role in cell division of streptococci, was altered in the gbcO mutant. Collectively, these findings show that GBC is an essential component of the cell wall of S. agalactiae whose function is reminiscent of that of conventional wall teichoic acids found in Staphylococcus aureus or Bacillus subtilis. Furthermore, our findings raise the possibility that GBC-like molecules play a major role in the growth of most if not all beta-hemolytic streptococci.

Synthesis of lipoteichoic acids in Bacillus anthracis

Lipoteichoic acid (LTA), a glycerol phosphate polymer, is a component of the envelope of Gram-positive bacteria that has hitherto not been identified in Bacillus anthracis, the causative agent of anthrax. LTA synthesis in Staphylococcus aureus and other microbes is catalyzed by the product of the ltaS gene, a membrane protein that polymerizes polyglycerol phosphate from phosphatidyl glycerol. Here we identified four ltaS homologues, designated ltaS1 to -4, in the genome of Bacillus anthracis. Polyglycerol phosphate-specific monoclonal antibodies were used to detect LTA in the envelope of B. anthracis strain Sterne (pXO1(+) pXO2(-)) vegetative forms. B. anthracis mutants lacking ltaS1, ltaS2, ltaS3, or ltaS4 did not display defects in growth or LTA synthesis. In contrast, B. anthracis strains lacking both ltaS1 and ltaS2 were unable to synthesize LTA and exhibited reduced viability, altered envelope morphology, aberrant separation of vegetative forms, and decreased sporulation efficiency. Expression of ltaS1 or ltaS2 alone in B. anthracis as well as in other microbes was sufficient for polyglycerol phosphate synthesis. Thus, similar to S. aureus, B. anthracis employs LtaS enzymes to synthesize LTA, an envelope component that promotes bacterial growth and cell division.

Secretion genes as determinants of Bacillus anthracis chain length

Bacillus anthracis grows in chains of rod-shaped cells, a trait that contributes to its escape from phagocytic clearance in host tissues. Using a genetic approach to search for determinants of B. anthracis chain length, we identified mutants with insertional lesions in secA2. All isolated secA2 mutants exhibited an exaggerated chain length, whereas the dimensions of individual cells were not changed. Complementation studies revealed that slaP (S-layer assembly protein), a gene immediately downstream of secA2 on the B. anthracis chromosome, is also a determinant of chain length. Both secA2 and slaP are required for the efficient secretion of Sap and EA1 (Eag), the two S-layer proteins of B. anthracis, but not for the secretion of S-layer-associated proteins or of other secreted products. S-layer assembly via secA2 and slaP contributes to the proper positioning of BslO, the S-layer-associated protein, and murein hydrolase, which cleaves septal peptidoglycan to separate chains of bacilli. SlaP was found to be both soluble in the bacterial cytoplasm and associated with the membrane. The purification of soluble SlaP from B. anthracis-cleared lysates did not reveal a specific ligand, and the membrane association of SlaP was not dependent on SecA2, Sap, or EA1. We propose that SecA2 and SlaP promote the efficient secretion of S-layer proteins by modifying the general secretory pathway of B. anthracis to transport large amounts of Sap and EA1.

Surface-layer (S-layer) proteins sap and EA1 govern the binding of the S-layer-associated protein BslO at the cell septa of Bacillus anthracis

The Gram-positive pathogen Bacillus anthracis contains 24 genes whose products harbor the structurally conserved surface-layer (S-layer) homology (SLH) domain. Proteins endowed with the SLH domain associate with the secondary cell wall polysaccharide (SCWP) following secretion. Two such proteins, Sap and EA1, have the unique ability to self-assemble into a paracrystalline layer on the surface of bacilli and form S layers. Other SLH domain proteins can also be found within the S layer and have been designated Bacillus S-layer-associated protein (BSLs). While both S-layer proteins and BSLs bind the same SCWP, their deposition on the cell surface is not random. For example, BslO is targeted to septal peptidoglycan zones, where it catalyzes the separation of daughter cells. Here we show that an insertional lesion in the sap structural gene results in elongated chains of bacilli, as observed with a bslO mutant. The chain length of the sap mutant can be reduced by the addition of purified BslO in the culture medium. This complementation in trans can be explained by an increased deposition of BslO onto the surface of sap mutant bacilli that extends beyond chain septa. Using fluorescence microscopy, we observed that the Sap S layer does not overlap the EA1 S layer and slowly yields to the EA1 S layer in a growth-phase-dependent manner. Although present all over bacilli, Sap S-layer patches are not observed at septa. Thus, we propose that the dynamic Sap/EA1 S-layer coverage of the envelope restricts the deposition of BslO to the SCWP at septal rings.

Protein secretion and surface display in Gram-positive bacteria

The cell wall peptidoglycan of Gram-positive bacteria functions as a surface organelle for the transport and assembly of proteins that interact with the environment, in particular, the tissues of an infected host. Signal peptide-bearing precursor proteins are secreted across the plasma membrane of Gram-positive bacteria. Some precursors carry C-terminal sorting signals with unique sequence motifs that are cleaved by sortase enzymes and linked to the cell wall peptidoglycan of vegetative forms or spores. The sorting signals of pilin precursors are cleaved by pilus-specific sortases, which generate covalent bonds between proteins leading to the assembly of fimbrial structures. Other precursors harbour surface (S)-layer homology domains (SLH), which fold into a three-pronged spindle structure and bind secondary cell wall polysaccharides, thereby associating with the surface of specific Gram-positive microbes. Type VII secretion is a non-canonical secretion pathway for WXG100 family proteins in mycobacteria. Gram-positive bacteria also secrete WXG100 proteins and carry unique genes that either contribute to discrete steps in secretion or represent distinctive substrates for protein transport reactions.