A lipopolysaccharide-binding domain of the Campylobacter fetus S-layer protein resides within the conserved N terminus of a family of silent and divergent homologs

Molecular Logic of Prokaryotic Surface Layer Structures

Most prokaryotic cells are encased in a surface layer (S-layer) consisting of a paracrystalline array of repeating lattice-forming proteins. S-layer proteins populate a vast and diverse sequence space, performing disparate functions in prokaryotic cells, including cellular defense, cell-shape maintenance, and regulation of import and export of materials. This article highlights recent advances in the understanding of S-layer structure and assembly, made possible by rapidly evolving structural and cell biology methods. We underscore shared assembly principles revealed by recent work and discuss a common molecular framework that may be used to understand the structural organization of S-layer proteins across bacteria and archaea.

Despite enormous sequence diversity in surface (S)-layer proteins, structural diversity is much lower than previously thought.

S-layer proteins have a bipartite arrangement with a lattice-forming and an anchoring segment.

Novel structural biology methods are revealing the architectures of S-layers in situ.

S-layer assembly across prokaryotes is tightly coupled to the cell cycle, including the cell division machinery.

Homologous Recombination between Genetically Divergent Campylobacter fetus Lineages Supports Host-Associated Speciation

Homologous recombination is a major driver of bacterial speciation. Genetic divergence and host association are important factors influencing homologous recombination. Here, we study these factors for Campylobacter fetus, which shows a distinct intraspecific host dichotomy. Campylobacter fetus subspecies fetus (Cff) and venerealis are associated with mammals, whereas C. fetus subsp. testudinum (Cft) is associated with reptiles. Recombination between these genetically divergent C. fetus lineages is extremely rare. Previously it was impossible to show whether this barrier to recombination was determined by the differential host preferences, by the genetic divergence between both lineages or by other factors influencing recombination, such as restriction-modification, CRISPR/Cas, and transformation systems. Fortuitously, a distinct C. fetus lineage (ST69) was found, which was highly related to mammal-associated C. fetus, yet isolated from a chelonian. The whole genome sequences of two C. fetus ST69 isolates were compared with those of mammal- and reptile-associated C. fetus strains for phylogenetic and recombination analysis. In total, 5.1–5.5% of the core genome of both ST69 isolates showed signs of recombination. Of the predicted recombination regions, 80.4% were most closely related to Cft, 14.3% to Cff, and 5.6% to C. iguaniorum. Recombination from C. fetus ST69 to Cft was also detected, but to a lesser extent and only in chelonian-associated Cft strains. This study shows that despite substantial genetic divergence no absolute barrier to homologous recombination exists between two distinct C. fetus lineages when occurring in the same host type, which provides valuable insights in bacterial speciation and evolution.

Comparative Genomics of Campylobacter fetus from Reptiles and Mammals Reveals Divergent Evolution in Host-Associated Lineages

Campylobacter fetus currently comprises three recognized subspecies, which display distinct host association. Campylobacter fetus subsp. fetus and C fetus subsp. venerealis are both associated with endothermic mammals, primarily ruminants, whereas C fetus subsp. testudinum is primarily associated with ectothermic reptiles. Both C. fetus subsp. testudinum and C. fetus subsp. fetus have been associated with severe infections, often with a systemic component, in immunocompromised humans. To study the genetic factors associated with the distinct host dichotomy in C. fetus, whole-genome sequencing and comparison of mammal- and reptile-associated C fetus was performed. The genomes of C fetus subsp. testudinum isolated from either reptiles or humans were compared with elucidate the genetic factors associated with pathogenicity in humans. Genomic comparisons showed conservation of gene content and organization among C fetus subspecies, but a clear distinction between mammal- and reptile-associated C fetus was observed. Several genomic regions appeared to be subspecies specific, including a putative tricarballylate catabolism pathway, exclusively present in C fetus subsp. testudinum strains. Within C fetus subsp. testudinum, sapA, sapB, and sapAB type strains were observed. The recombinant locus iamABC (mlaFED) was exclusively associated with invasive C fetus subsp. testudinum strains isolated from humans. A phylogenetic reconstruction was consistent with divergent evolution in host-associated strains and the existence of a barrier to lateral gene transfer between mammal- and reptile-associated C fetus Overall, this study shows that reptile-associated C fetus subsp. testudinum is genetically divergent from mammal-associated C fetus subspecies.

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.

A new addition to the cell plan of anammox bacteria: "Candidatus Kuenenia stuttgartiensis" has a protein surface layer as the outermost layer of the cell

Anammox bacteria perform anaerobic ammonium oxidation (anammox) and have a unique compartmentalized cell consisting of three membrane-bound compartments (from inside outwards): the anammoxosome, riboplasm, and paryphoplasm. The cell envelope of anammox bacteria has been proposed to deviate from typical bacterial cell envelopes by lacking both peptidoglycan and a typical outer membrane. However, the composition of the anammox cell envelope is presently unknown. Here, we investigated the outermost layer of the anammox cell and identified a proteinaceous surface layer (S-layer) (a crystalline array of protein subunits) as the outermost component of the cell envelope of the anammox bacterium "Candidatus Kuenenia stuttgartiensis." This is the first description of an S-layer in the phylum of the Planctomycetes and a new addition to the cell plan of anammox bacteria. This S-layer showed hexagonal symmetry with a unit cell consisting of six protein subunits. The enrichment of the S-layer from the cell led to a 160-kDa candidate protein, Kustd1514, which has no homology to any known protein. This protein is present in a glycosylated form. Antibodies were generated against the glycoprotein and used for immunogold localization. The antiserum localized Kustd1514 to the S-layer and thus verified that this protein forms the "Ca. Kuenenia stuttgartiensis" S-layer.

Identification of antigenic epitopes of the SapA protein of Campylobacter fetus using a phage display peptide library

In this study, we immunized mice with prokaryotically expressed recombinant surface layer protein, SapA, of Campylobacter fetus, generated hybridomas secreting mouse monoclonal antibodies (mAb) targeting SapA, and purified the mAb A2D5 from mouse ascites using saturated ammonium sulfate solution. The mAb A2D5, coated onto ELISA plates, was used to screen the phage random 12-peptide library through three rounds of panning. Following panning, 15 phage clones were randomly chosen and tested for reactivity with mAb A2D5 by indirect ELISA. Single-stranded DNA from positive clones was sequenced and compared with the sequence of SapA to predict the key epitope. ELISA and/or Western blot analyses further validated that synthetic peptides and recombinant peptide mimotopes all interact with mAb A2D5. Nine of ten positive phage clones identified by screening were sequenced successfully. Seven clones shared the same sequence HYDRHNYHWWHT; one had the sequence LSKNLPLTALGN; and the final one had the sequence SGMKEPELRSYS. These three sequences shared high homology with SapA J05577 in the region GNEKDFVTKIYSIALGNTSDVDGINYW, in which the underlined amino acids may serve as key residues in the epitope. ELISA and/or Western blot analyses showed that mAb A2D5 not only interacted with the four synthetic peptide mimotopes, but also with 14 prokaryotically expressed recombinant peptide mimotopes. The mimotopes identified in this study will aid future studies into the pathological processes and immune mechanisms of the SapA protein of C. fetus.

Nanobiotechnology with S-layer proteins as building blocks

One of the key challenges in nanobiotechnology is the utilization of self- assembly systems, wherein molecules spontaneously associate into reproducible aggregates and supramolecular structures. In this contribution, we describe the basic principles of crystalline bacterial surface layers (S-layers) and their use as patterning elements. The broad application potential of S-layers in nanobiotechnology is based on the specific intrinsic features of the monomolecular arrays composed of identical protein or glycoprotein subunits. Most important, physicochemical properties and functional groups on the protein lattice are arranged in well-defined positions and orientations. Many applications of S-layers depend on the capability of isolated subunits to recrystallize into monomolecular arrays in suspension or on suitable surfaces (e.g., polymers, metals, silicon wafers) or interfaces (e.g., lipid films, liposomes, emulsomes). S-layers also represent a unique structural basis and patterning element for generating more complex supramolecular structures involving all major classes of biological molecules (e.g., proteins, lipids, glycans, nucleic acids, or combinations of these). Thus, S-layers fulfill key requirements as building blocks for the production of new supramolecular materials and nanoscale devices as required in molecular nanotechnology, nanobiotechnology, biomimetics, and synthetic biology.

Development and evaluation of an indirect enzyme-linked immunosorbent assay for the detection of antibodies against Campylobacter fetus in cattle

Campylobacteriosis is a zoonosis that occurs worldwide. Infection with Campylobacter fetus (C. fetus) causes infertility and abortion in sheep and cattle. The current study focuses on the SapA gene of C. fetus that encodes surface array proteins and plays an important role in the virulence of C. fetus. The SapA-N (1398bp) and SapA-C (1422bp) fragments were amplified from the C. fetusSapA gene using polymerase chain reaction (PCR), and the corresponding recombinant proteins rSapA-N and rSapA-C were expressed in Escherichia. coli BL21 cells. Results of Western blotting and enzyme-linked immunosorbent assay (ELISA) showed that the immunological activity of rSapA-N was higher than that of rSapA-C (P<0.05). Therefore, rSapA-N was selected to establish an indirect ELISA for detecting antibodies against C. fetus. The diagnostic criteria were as follows: S/P0.45: positive; S/P<0.4: negative; 0.45>S/P0.4: suspected. The specificity and sensitivity of our method were 94.3% and 88.6%, respectively. Moreover, no cross-reactions were observed between rSapA-N and serum samples that were positive for other bovine bacterial pathogens diseases such as Mycobacterium avium subspecies paratuberculosis. One hundred and two serum samples from cows that had experienced abortion were tested. Four and 2 C. fetus-positive serum samples were found among the 70 bovine brucellosis-positive samples and the 32 infectious bovine rhinotracheitis (IBR)-positive samples, respectively. The findings suggest that the rSapA-N-based ELISA method has immense potential in future applications.

Are S-layers exoskeletons? The basic function of protein surface layers revisited

Surface protein or glycoprotein layers (S-layers) are common structures of the prokaryotic cell envelope. They are either associated with the peptidoglycan or outer membrane of bacteria, and constitute the only cell wall component of many archaea. Despite their occurrence in most of the phylogenetic branches of microorganisms, the functional significance of S-layers is assumed to be specific for genera or groups of organisms in the same environment rather than common to all prokaryotes. Functional aspects have usually been investigated with isolated S-layer sheets or proteins, which disregards the interactions between S-layers and the underlying cell envelope components. This study discusses the synergistic effects in cell envelope assemblies, the hypothetical role of S-layers for cell shape formation, and the existence of a common function in view of new insights.

S-layer anchoring and localization of an S-layer-associated protease in Caulobacter crescentus

The S-layer of the gram-negative bacterium Caulobacter crescentus is composed of a single protein, RsaA, that is secreted and assembled into a hexagonal crystalline array that covers the organism. Despite the widespread occurrence of comparable bacterial S-layers, little is known about S-layer attachment to cell surfaces, especially for gram-negative organisms. Having preliminary indications that the N terminus of RsaA anchors the monomer to the cell surface, we developed an assay to distinguish direct surface attachment from subunit-subunit interactions where small RsaA fragments are incubated with S-layer-negative cells to assess the ability of the fragments to reattach. In doing so, we found that the RsaA anchoring region lies in the first approximately 225 amino acids and that this RsaA anchoring region requires a smooth lipopolysaccharide species found in the outer membrane. By making mutations at six semirandom sites, we learned that relatively minor perturbations within the first approximately 225 amino acids of RsaA caused loss of anchoring. In other studies, we confirmed that only this N-terminal region has a direct role in S-layer anchoring. As a by-product of the anchoring studies, we discovered that Sap, the C. crescentus S-layer-associated protease, recognized a cleavage site in the truncated RsaA fragments that is not detected by Sap in full-length RsaA. This, in turn, led to the discovery that Sap was an extracellular membrane-bound protease, rather than intracellular, as previously proposed. Moreover, Sap was secreted to the cell surface primarily by the S-layer type I secretion apparatus.

Conservation and diversity of sap homologues and their organization among Campylobacter fetus isolates

Campylobacter fetus surface layer proteins (SLPs), encoded by sapA homologues, are important in virulence. In wild-type C. fetus strain 23D, all eight sapA homologues are located in the 54-kb sap island, and SLP expression reflects the position of a unique sapA promoter in relation to the sapA homologues. The extensive homologies in the sap island include both direct and inverted repeats, which allow DNA rearrangements, deletion, or duplication; these elements confer substantial potential for genomic plasticity. To better understand C. fetus sap island diversity and variation mechanisms, we investigated the organization and distribution of sapA homologues among 18 C. fetus strains of different subspecies, serotypes, and origins. For all type A strains, the boundaries of the sap island were relatively consistent. A 187-bp noncoding DNA insertion near the upstream boundary of the sap island was found in two of three reptile strains studied. The sapA homologue profiles were strain specific, and six new sapA homologues were recognized. Several homologues from reptile strains are remarkably conserved in relation to their corresponding mammalian homologues. In total, the observed differences suggest that the sap island has evolved differing genotypes that are plastic, perhaps enabling colonization of varied niches, in addition to antigenic variation.

Structure and genotypic plasticity of the Campylobacter fetus sap locus

The Campylobacter fetus surface layer proteins (SLPs), encoded by five to nine sapA homologues, are major virulence factors. To characterize the sapA homologues further, a 65.9 kb C. fetus genomic region encompassing the sap locus from wild-type strain 23D was completely sequenced and analysed; 44 predicted open reading frames (ORFs) were recognized. The 53.8 kb sap locus contained eight complete and one partial sapA homologues, varying from 2769 to 3879 bp, sharing conserved 553-2622 bp 5' regions, with partial sharing of 5' and 3' non-coding regions. All eight sapA homologues were expressed in Escherichia coli as antigenic proteins and reattached to the surface of SLP- strain 23B, indicating their conserved function. Analysis of the sap homologues indicated three phylogenetic groups. Promoter-specific polymerase chain reactions (PCRs) and sapA homologue-specific reverse transcription (RT)-PCRs showed that the unique sapA promoter can potentially express all eight sapA homologues. Reciprocal DNA recombination based on the 5' conserved regions can involve each of the eight sapA homologues, with frequencies from 10(-1) to 10(-3). Intragenic recombination between sapA7 and sapAp8, mediated by their conserved regions with a 10(-1)-10(-2) frequency, allows the formation of new sap homologues. As divergent SLP C-termini possess multiple antigenic sites, their reciprocal recombination behind the unique sap promoter leads to continuing antigenic variation.

Role of S-layer protein antigenic diversity in the immune responses of sheep experimentally challenged with Campylobacter fetus subsp. fetus

Surface layer proteins (SLPs) are essential for induction of abortion by Campylobacter fetus subsp. fetus in experimentally challenged ewes. These proteins are encoded by multiple sap genes and vary in size and antigenicity. The role of SLP antigenic variation during experimental ovine infection was investigated. Following subcutaneous challenge, the SLPs were highly antigenic, and antibodies were detected in serum, milk, bile, and urine. Fecal anti-SLP antibodies were detected only in animals challenged orally. Ewes challenged with wild-type strain 23D with variable SLPs developed detectable circulating anti-SLP immunoglobulin G (IgG) antibodies by 2 weeks postchallenge. In contrast, ewes challenged with mutants of 23D that had fixed expression of a single SLP developed antibodies within 1 week postchallenge, suggesting that antigenic variation in SLPs may delay the host antibody response. Although not statistically significant, the data from challenge experiments in which vaccinated ewes were used suggested that SLP-expressing vaccines could protect animals from abortion and that this effect was independent of the SLP expressed, indicating involvement of conserved epitopes in the SLP. The conserved 184-amino-acid N-terminal region of the SLP, identified from previously published sequences, was epitope mapped with rabbit anti-SLP antisera by using overlapping synthetic 20-mer peptides. Two putative epitopes were identified at amino acids 81 to 110 and 141 to 160. Amino acids 81 to 100 also bound serum IgG antibodies from experimentally challenged sheep. Conserved antigenic regions of the SLP that induce protective immune responses may enable development of synthetic vaccine candidates for C. fetus subsp. fetus-associated ovine abortion.

Campylobacter surface-layers (S-layers) and immune evasion

Many pathogenic bacteria have evolved mechanisms for evading host immune systems. One evasion mechanism is manifest by the surface layer (S-layer), a paracrystalline protein structure composed of S-layer proteins (SLPs). The S-layer, possessed by 2 Campylobacter species (C. fetus and C. rectus), is external to the bacterial outer membrane and can have multiple functions in immune avoidance. C. fetus is a pathogen of ungulates and immunocompromised humans, in whom it causes disseminated bloodstream disease. In C. fetus, the S-layer is required for dissemination and is involved in 2 mechanisms of evasion. First, the S-layer confers resistance to complement-mediated killing in non-immune serum by preventing the binding of complement factor C3b to the C. fetus cell surface. S-layer expressing C. fetus strains remain susceptible to complement-independent killing, utilizing opsonic antibodies directed against the S-layer. However, C. fetus has also evolved a mechanism for avoiding antibody-mediated killing by high-frequency antigenic variation of SLPs. Antigenic variation is accomplished by complex DNA inversion events involving a family of multiple SLP-encoding genes and a single SLP promoter. Inversion events result in the expression of antigenically variant S-layers, which require distinct antibody responses for killing. C. rectus is implicated in the pathogenesis of periodontal disease and also possesses an S-layer that appears to be involved in evading the human system. Although studied less extensively than its C. fetus counterpart, the C. rectus S-layer appears to confer resistance to complement-mediated killing and to cause the down-regulation of proinflammatory cytokines.

Domains in the S-layer protein CbsA of Lactobacillus crispatus involved in adherence to collagens, laminin and lipoteichoic acids and in self-assembly

The protein regions in the S-layer protein CbsA of Lactobacillus crispatus JCM 5810, needed for binding to collagens and laminin, anchoring to bacterial cell wall, as well as self-assembly, were mapped by deletion analysis of His-tagged peptides isolated from Escherichia coli and by heterologous expression on Lactobacillus casei. Mature CbsA is 410 amino acids long, and stepwise genetic truncation at both termini revealed that the region 32-271 carries the infor-mation for self-assembly of CbsA into a periodic structure. The lactobacillar S-layer proteins exhibit sequence variation in their assembly domain, but the border regions 30-34 and 269-274 in CbsA are conserved in valine-rich short sequences. Short deletions or substitutions at these regions affected the morphology of His-CbsA polymers, which varied from sheet-like to cylindrical tubular polymers, and further truncation beyond the DNA encoding residues 32 and 271 leads to a non-periodic aggregation. The self-assembly of the truncated peptides, as seen by electron microscopy, was correlated with their behaviour in a cross-linking study. The shorter peptides not forming a regular polymer were observed by the cross-linking study and mass spectrometry to form dimers, trimers and tetramers, whereas the other peptides were cross-linked to large multimers only. Binding of solubilized type I and IV collagens was observed with the His-CbsA peptides 1-274 and 31-287, but not with the smaller peptides regardless of their ability to form regular polymers. Strain JCM 5810 also adheres to immobilized laminin and, in order to analyse the possible laminin binding by CbsA, cbsA and its fragments were expressed on the surface of L. casei. Expression of the CbsA peptides 1-274, 1-287, 28-287 and 31-287 on L. casei conferred adhesiveness to both laminin and collagen immobilized on glass as well as to laminin- and collagen-containing regions in chicken colon and ileum. The C-terminal peptides 251-410 and 288-410 bound to L. crispatus JCM 5810 cells from which the S-layer had been depleted by chemical extraction, whereas no binding was seen with the His-CbsA peptides 1-250 or 1-269 or to cells with an intact S-layer. The His-CbsA peptides 251-410 and 288-410 bound to teichoic acids of several bacterial species. The results show that CbsA is an adhesive complex with an N-terminal assembly domain exhibiting affinity for pericellular tissue components and a cationic C-terminal domain binding to negatively charged cell wall components.

Campylobacter fetus uses multiple loci for DNA inversion within the 5' conserved regions of sap homologs

Campylobacter fetus cells possess multiple promoterless sap homologs, each capable of expressing a surface layer protein (SLP) by utilizing a unique promoter present on a 6.2-kb invertible element. Each sap homolog includes a 626-bp 5' conserved region (FCR) with 74 bp upstream and 552 bp within the open reading frame. After DNA inversion, the splice is seamless because the FCRs are identical. In mutant strain 23D:ACA2K101, in which sapA and sapA2 flanking the invertible element in opposite orientations were disrupted by promoterless chloramphenicol resistance (Cm(r)) and kanamycin resistance (Km(r)) cassettes, respectively, the frequency of DNA inversion is 100-fold lower than that of wild-type strain 23D. To define the roles of a 15-bp inverted repeat (IR) and a Chi-like site (CLS) in the FCR, we mutagenized each upstream of sapA2 in 23D:ACA2K101 by introducing NotI and KpnI sites to create strains 23D:ACA2K101N and 23D:ACA2K101K, respectively. Alternatively selecting colonies for Cm(r) or Km(r) showed that mutagenizing the IR or CLS had no apparent effect on the frequency of the DNA inversion. However, mapping the unique NotI or KpnI site in relation to the Cm(r) or Km(r) cassette in the cells that changed phenotype showed that splices occurred both upstream and downstream of the mutated sites. PCR and sequence analyses also showed that the splice could occur in the 425-bp portion of the FCR downstream of the cassettes. In total, these data indicate that C. fetus can use multiple sites within the FCR for its sap-related DNA inversion.

Evidence that the Campylobacter fetus sap locus is an ancient genomic constituent with origins before mammals and reptiles diverged

Campylobacter fetus bacteria, isolated from both mammals and reptiles, may be either subsp. fetus or subsp. venerealis and either serotype A or serotype B. Surface layer proteins, expressed and secreted by genes in the sap locus, play an important role in C. fetus virulence. To assess whether the sap locus represents a pathogenicity island and to gain further insights into C. fetus evolution, we examined several C. fetus genes in 18 isolates. All of the isolates had 5 to 9 sapA or sapB homologs. One strain (85-387) possessed both sapA and sapB homologs, suggesting a recombinational event in the sap locus between sapA and sapB strains. When we amplified and analyzed nucleotide sequences from portions of housekeeping gene recA (501 bp) and sapD (450 bp), a part of the 6-kb sap invertible element, the phylogenies of the genes were highly parallel. Among the 15 isolates from mammals, serotype A and serotype B strains generally had consistent positions. The fact that the serotype A C. fetus subsp. fetus and subsp. venerealis strains were on the same branch suggests that their differentiation occurred after the type A-type B split. Isolates from mammals and reptiles formed two distinct tight phylogenetic clusters that were well separated. Sequence analysis of 16S rRNA showed that the reptile strains form a distinct phylotype between mammalian C. fetus and Campylobacter hyointestinalis. The phylogenies and sequence results showing that sapD and recA have similar G + C contents and substitution rates suggest that the sap locus is not a pathogenicity island but rather is an ancient constituent of the C. fetus genome, integral to its biology.

Surface-accessible residues in the monomeric and assembled forms of a bacterial surface layer protein

The S-layer protein SbsB of the thermophilic, Gram-positive organism Bacillus stearothermophilus PV72/p2 forms a crystalline, porous array constituting the outermost component of the cell envelope. SbsB has a molecular mass of 98 kDa, and the corresponding S-layer exhibits an oblique lattice symmetry. To investigate the molecular structure and assembly of SbsB, we replaced 75 residues (mainly serine, threonine, and alanine), located throughout the primary sequence, with cysteine, which is not found in the wild-type protein. As determined by electron microscopy, 72 out of 75 mutants formed regularly-structured self-assembly products identical to wild-type, thereby proving that the replacement of most of the selected amino acids by cysteine does not dramatically alter the structure of the protein. The three defective mutants, which showed a greatly reduced ability to self-assemble, were, however, successfully incorporated into S-layers of wild-type protein. Monomeric SbsB mutants and SbsB mutants assembled into S-layers were subjected to a surface accessibility screen by targeted chemical modification with a 5-kDa hydrophilic cysteine-reactive polyethylene glycol conjugate. In the monomeric form of SbsB, 34 of the examined residues were not surface accessible, while 23 were classified as very accessible, and 18 were of intermediate surface accessibility. By contrast, in the assembled S-layers, 57 of the mutated residues were not accessible, six were very accessible, and 12 of intermediate accessibility. Together with other structural information, the results suggest a model for SbsB in which functional domains are segregated along the length of the polypeptide chain.

Campylobacter fetus sap inversion occurs in the absence of RecA function

Phase variation of Campylobacter fetus surface layer proteins (SLPs) occurs by inversion of a 6.2-kb DNA segment containing the unique sap promoter, permitting expression of a single SLP-encoding gene. Previous work has shown that the C. fetus sap inversion system is RecA dependent. When we challenged a pregnant ewe with a recA mutant of wild-type C. fetus (strain 97-211) that expressed the 97-kDa SLP, 15 of the 16 ovine-passaged isolates expressed the 97-kDa protein. However, one strain (97-209) expressed a 127-kDa SLP, suggesting that chromosomal rearrangement may have occurred to enable SLP switching. Lack of RecA function in strains 97-211 and 97-209 was confirmed by their sensitivity to the DNA-damaging agent methyl methanesulfonate. Southern hybridization and PCR of these strains indicated that the aphA insertion into recA was stably present. However, Southern hybridizations demonstrated that in strain 97-209 inversion had occurred in the sap locus. PCR data confirmed inversion of the 6.2-kb DNA element and indicated that in these recA mutants the sap inversion frequency is reduced by 2 to 3 log(10) units compared to that in the wild type. Thus, although the major sap inversion pathway in C. fetus is RecA dependent, alternative lower-frequency, RecA-independent inversion mechanisms exist.

S-layer-supported lipid membranes

Many prokaryotic organisms (archaea and bacteria) are covered by a regularly ordered surface layer (S-layer) as the outermost cell wall component. S-layers are built up of a single protein or glycoprotein species and represent the simplest biological membrane developed during evolution. Pores in S-layers are of regular size and morphology, and functional groups on the protein lattice are aligned in well-defined positions and orientations. Due to the high degree of structural regularity S-layers represent unique systems for studying the structure, morphogenesis, and function of layered supramolecular assemblies. Isolated S-layer subunits of numerous organisms are able to assemble into monomolecular arrays either in suspension, at air/water interfaces, on planar mono- and bilayer lipid films, on liposomes and on solid supports (e.g. silicon wafers). Detailed studies on composite S-layer/lipid structures have been performed with Langmuir films, freestanding bilayer lipid membranes, solid supported lipid membranes, and liposomes. Lipid molecules in planar films and liposomes interact via their head groups with defined domains on the S-layer lattice. Electrostatic interactions are the most prevalent forces. The hydrophobic chains of the lipid monolayers are almost unaffected by the attachment of the S-layer and no impact on the hydrophobic thickness of the membranes has been observed. Upon crystallization of a coherent S-layer lattice on planar and vesicular lipid membranes, an increase in molecular order is observed, which is reflected in a decrease of the membrane tension and an enhanced mobility of probe molecules within an S-layer-supported bilayer. Thus, the terminology 'semifluid membrane' has been introduced for describing S-layer-supported lipid membranes. The most important feature of composite S-layer/lipid membranes is an enhanced stability in comparison to unsupported membranes.

Roles of the surface layer proteins of Campylobacter fetus subsp. fetus in ovine abortion

The role of the surface (S)-layer proteins of Campylobacter fetus subsp. fetus has been investigated using an ovine model of abortion. Wild-type strain 23D induced abortion in up to 90% of pregnant ewes challenged subcutaneously. Isolates recovered from both dams and fetuses expressed S-layer proteins with variable molecular masses. The spontaneous S-layer-negative variant, strain 23B, neither colonized nor caused abortions in pregnant ewes. A series of isogenic sapA and recA mutants, derived from 23D, also were investigated in this model. A mutant (501 [sapA recA(+)]) caused abortion in one of five challenged animals and was recovered from the placenta of a second animal. Another mutant (502 [sapA recA]) with no S-layer protein expression caused no colonization or abortions in challenged animals but caused abortion when administered intraplacentally. Mutants 600(2) and 600(4), both recA, had fixed expression of 97- and 127-kDa S-layer proteins, respectively. Two of the six animals challenged with mutant 600(4) were colonized, but there were no abortions. As expected, all five strains recovered expressed a 127-kDa S-layer protein. In contrast, mutant 600(2) was recovered from the placentas of all five challenged animals and caused abortion in two. Unexpectedly, one of the 16 isolates expressed a 127-kDa rather than a 97-kDa S-layer protein. Thus, these studies indicate that S-layer proteins appear essential for colonization and/or translocation to the placenta but are not required to mediate fetal injury and that S-layer variation may occur in a recA strain.

Campylobacter fetus surface layer proteins are transported by a type I secretion system

The virulence of Campylobacter fetus, a bacterial pathogen of ungulates and humans, is mediated in part by the presence of a paracrystalline surface layer (S-layer) that confers serum resistance. The subunits of the S-layer are S-layer proteins (SLPs) that are secreted in the absence of an N-terminal signal sequence and attach to either type A or B C. fetus lipopolysaccharide in a serospecific manner. Antigenic variation of multiple SLPs (encoded by sapA homologs) of type A strain 23D occurs by inversion of a promoter-containing DNA element flanked by two sapA homologs. Cloning and sequencing of the entire 6.2-kb invertible region from C. fetus 23D revealed a probable 5.6-kb operon of four overlapping genes (sapCDEF, with sizes of 1,035, 1,752, 1,284, and 1,302 bp, respectively) transcribed in the opposite direction from sapA. The four genes also were present in the invertible region of type B strain 84-107 and were virtually identical to their counterparts in the type A strain. Although SapC had no database homologies, SapD, SapE, and SapF had predicted amino acid homologies with type I protein secretion systems (typified by Escherichia coli HlyBD/TolC or Erwinia chrysanthemi PrtDEF) that utilize C-terminal secretion signals to mediate the secretion of hemolysins, leukotoxins, or proteases from other bacterial species. Analysis of the C termini of four C. fetus SLPs revealed conserved structures that are potential secretion signals. A C. fetus sapD mutant neither produced nor secreted SLPs. E. coli expressing C. fetus sapA and sapCDEF secreted SapA, indicating that the sapCDEF genes are sufficient for SLP secretion. C. fetus SLPs therefore are transported to the cell surface by a type I secretion system.

The S-layer protein from Campylobacter rectus: sequence determination and function of the recombinant protein

The gene encoding the crystalline surface layer (S-layer) protein from Campylobacter rectus, designated slp, was sequenced and the recombinant gene product was expressed in Escherichia coli. The gene consisted of 4086 nucleotides encoding a protein with 1361 amino acids. The N-terminal amino acid sequence revealed that Slp did not contain a signal sequence, but that the initial methionine residue was processed. The deduced amino acid sequence displayed some common characteristic features of S-layer proteins previously reported. A homology search showed a high similarity to the Campylobacter fetus S-layer proteins, especially in their N-terminus. The C-terminal third of Slp exhibited homology with the RTX toxins from Gram-negative bacteria via the region including the glycine-rich repeats. The Slp protein had the same N-terminal sequence as a 104-kDa cytotoxin isolated from the culture supernatants of C. rectus. However, neither native nor recombinant Slp showed cytotoxicity against HL-60 cells or human peripheral white blood cells. These data support the idea that the N-terminus acts as an anchor to the cell surface components and that the C-terminus is involved in the assembly and/or transport of the protein.

Cloning of a sapB homologue (sapB2) encoding a putative 112-kDa Campylobacter fetus S-layer protein and its use for identification and molecular genotyping

A sap gene encoding a surface layer protein was isolated from a Campylobacter fetus ssp. fetus CIP 53.96T cosmid library. This sap gene, which shows significant homology with the sapB conserved region, was named sapB2. The complete ORF of 3339 nucleotides encodes a 1112-amino acid polypeptide with a calculated molecular mass of 112 kDa. High homology with the sapB gene was found in a region beginning 67 bp before the ORF and proceeding 546 bp into the ORF. Similarly, 98% homology with the sapA2 gene was observed in a 2038-bp region beginning 540 bp after the initiation codon. In the present study, we show that this sapB2 gene has two main interesting features: the 5' end of the region which presents high homology with the sapA2 homologue was found to be present in every C. fetus strain, and the fragment (IG01) comprising the region which presents homology with the sapB conserved region and the 5' end of the sapA2 homologue region, when used as a probe, can reveal genomic polymorphism among C. fetus strains. We exploited these features to develop a PCR assay for the specific detection of C. fetus and to set up a method for typing C. fetus isolates. The PCR assay was found to be species-specific. Oligonucleotide primers derived from the 5' end of sapA2 homologue region were used in a polymerase chain reaction test on genomic DNA extracted from 101 Campylobacter fetus, 18 Campylobacter non-fetus and seven non-Campylobacter strains. A 220-bp fragment was amplified only when C. fetus DNA was used as a target. In Southern blot analysis, the IG01 probe was found to hybridize only with DNA extracted from C. fetus strains. Moreover, IG01 hybridized with several fragments of HindIII-digested DNA, giving a specific pattern for each strain.

Influence of the secondary cell wall polymer on the reassembly, recrystallization, and stability properties of the S-layer protein from Bacillus stearothermophilus PV72/p2

The high-molecular-weight secondary cell wall polymer (SCWP) from Bacillus stearothermophilus PV72/p2 is mainly composed of N-acetylglucosamine (GlcNAc) and N-acetylmannosamine (ManNAc) and is involved in anchoring the S-layer protein via its N-terminal region to the rigid cell wall layer. In addition to this binding function, the SCWP was found to inhibit the formation of self-assembly products during dialysis of the guanidine hydrochloride (GHCl)-extracted S-layer protein. The degree of assembly (DA; percent assembled from total S-layer protein) that could be achieved strongly depended on the amount of SCWP added to the GHCl-extracted S-layer protein and decreased from 90 to 10% when the concentration of the SCWP was increased from 10 to 120 microg/mg of S-layer protein. The SCWP kept the S-layer protein in the water-soluble state and favored its recrystallization on solid supports such as poly-L-lysine-coated electron microscopy grids. Derived from the orientation of the base vectors of the oblique S-layer lattice, the subunits had bound with their charge-neutral outer face, leaving the N-terminal region with the polymer binding domain exposed to the ambient environment. From cell wall fragments about half of the S-layer protein could be extracted with 1 M GlcNAc, indicating that the linkage type between the S-layer protein and the SCWP could be related to that of the lectin-polysaccharide type. Interestingly, GlcNAc had an effect on the in vitro self-assembly and recrystallization properties of the S-layer protein that was similar to that of the isolated SCWP. The SCWP generally enhanced the stability of the S-layer protein against endoproteinase Glu-C attack and specifically protected a potential cleavage site in position 138 of the mature S-layer protein.

The Caulobacter crescentus paracrystalline S-layer protein is secreted by an ABC transporter (type I) secretion apparatus

Caulobacter crescentus is a gram-negative bacterium that produces a two-dimensional crystalline array on its surface composed of a single 98-kDa protein, RsaA. Secretion of RsaA to the cell surface relies on an uncleaved C-terminal secretion signal. In this report, we identify two genes encoding components of the RsaA secretion apparatus. These components are part of a type I secretion system involving an ABC transporter protein. These genes, lying immediately 3' of rsaA, were found by screening a Tn5 transposon library for the loss of RsaA transport and characterizing the transposon-interrupted genes. The two proteins presumably encoded by these genes were found to have significant sequence similarity to ABC transporter and membrane fusion proteins of other type I secretion systems. The greatest sequence similarity was found to the alkaline protease (AprA) transport system of Pseudomonas aeruginosa and the metalloprotease (PrtB) transport system of Erwinia chrysanthemi. The prtB and aprA genes were introduced into C. crescentus, and their products were secreted by the RsaA transport system. Further, defects in the S-layer protein transport system led to the loss of this heterologous secretion. This is the first report of an S-layer protein secreted by a type I secretion apparatus. Unlike other type I secretion systems, the RsaA transport system secretes large amounts of its substrate protein (it is estimated that RsaA accounts for 10 to 12% of the total cell protein). Such levels are expected for bacterial S-layer proteins but are higher than for any other known type I secretion system.

A new member of the S-layer protein family: characterization of the crs gene from Campylobacter rectus

Strains of the periodontal pathogen Campylobacter rectus express a 150- to 166-kDa protein on their cell surface. This protein forms a paracrystalline lattice, called the surface layer (S-layer), on the outer membrane of this gram-negative bacterium. To initiate a genetic analysis of the function of the S-layer in the pathogenesis of C. rectus, we have cloned and characterized its gene. The S-layer gene (crs) from C. rectus 314 encodes a cell surface protein which does not have a cleaved signal peptide at its amino terminus. Although the amino acid sequence deduced from the crs gene has 50% identity with the amino-terminal 30 amino acids of the four S-layer proteins from Campylobacter fetus, the similarity decreases to less than 16% over the rest of the protein. Thus, the crs gene from C. rectus encodes a novel S-layer protein whose precise role in pathogenesis may differ from that of S-layer proteins from other organisms. Southern and Northern blot analyses with probes from different segments of the crs gene indicate that the S-layer gene is a single-copy, monocistronic gene in C. rectus. RNA end mapping and sequence analyses were used to define the crs promoter; there is an exact match to the Escherichia coli -10 promoter consensus sequence but only a weak match to the -35 consensus element. Southern blots of DNA from another strain of C. rectus, ATCC 33238, demonstrated that the crs gene is also present in that strain but that there are numerous restriction fragment length polymorphisms in the second half of the gene. This finding suggests that the carboxy halves of the S-layer proteins from strains 314 and 33238 differ. It remains to be determined whether the diversities in sequence are reflected in functional or antigenic differences important for the pathogenesis of different C. rectus isolates.

Nested DNA inversion of Campylobacter fetus S-layer genes is recA dependent

Wild-type strains of Campylobacter fetus are covered by a monomolecular array of surface layer proteins (SLPs) critical for virulence. Each cell possesses eight SLP gene cassettes, tightly clustered in the genome, that encode SLPs of 97 to 149 kDa. Variation of SLP expression occurs by a mechanism of nested DNA rearrangement that involves the inversion of a 6.2-kb sapA promoter-containing element alone or together with one or more flanking SLP gene cassettes. The presence of extensive regions of identity flanking the 5' and 3' ends of each SLP gene cassette and of a Chi-like recognition sequence within the 5' region of identity suggests that rearrangement of SLP gene cassettes may occur by a generalized (RecA-dependent) homologous recombination pathway. To explore this possibility, we cloned C. fetus recA and created mutant strains by marker rescue, in which recA is disrupted in either S+ or S- strains. These mutants then were assessed for their abilities to alter SLP expression either in the presence or absence of a complementary shuttle plasmid harboring native recA. In contrast to all previously reported programmed DNA inversion systems, inversion in C. fetus is recA dependent.

Functions of S-layers

Although S-layers are being increasingly identified on Bacteria and Archaea, it is enigmatic that in most cases S-layer function continues to elude us. In a few instances, S-layers have been shown to be virulence factors on pathogens (e.g. Campylobacter fetus ssp. fetus and Aeromonas salmonicida), protective against Bdellovibrio, a depository for surface-exposed enzymes (e.g. Bacillus stearothermophilus), shape-determining agents (e.g. Thermoproteus tenax) and nucleation factors for fine-grain mineral development (e.g. Synechococcus GL 24). Yet, for the vast majority of S-layered bacteria, the natural function of these crystalline arrays continues to be evasive. The following review up-dates the functional basis of S-layers and describes such diverse topics as the effect of S-layers on the Gram stain, bacteriophage adsorption in lactobacilli, phagocytosis by human polymorphonuclear leukocytes, the adhesion of a high-molecular-mass amylase, outer membrane porosity, and the secretion of extracellular enzymes of Thermoanaerobacterium. In addition, the functional aspect of calcium on the Caulobacter S-layer is explained.

Molecular biology of S-layers

In this chapter we report on the molecular biology of crystalline surface layers of different bacterial groups. The limited information indicates that there are many variations on a common theme. Sequence variety, antigenic diversity, gene expression, rearrangements, influence of environmental factors and applied aspects are addressed. There is considerable variety in the S-layer composition, which was elucidated by sequence analysis of the corresponding genes. In Corynebacterium glutamicum one major cell wall protein is responsible for the formation of a highly ordered, hexagonal array. In contrast, two abundant surface proteins from the S-layer of Bacillus anthracis. Each protein possesses three S-layer homology motifs and one protein could be a virulence factor. The antigenic diversity and ABC transporters are important features, which have been studied in methanogenic archaea. The expression of the S-layer components is controlled by three genes in the case of Thermus thermophilus. One has repressor activity on the S-layer gene promoter, the second codes for the S-layer protein. The rearrangement by reciprocal recombination was investigated in Campylobacter fetus. 7-8 S-layer proteins with a high degree of homology at the 5' and 3' ends were found. Environmental changes influence the surface properties of Bacillus stearothermophilus. Depending on oxygen supply, this species produces different S-layer proteins. Finally, the molecular bases for some applications are discussed. Recombinant S-layer fusion proteins have been designed for biotechnology.

The O-specific polysaccharide chain of Campylobacter fetus serotype A lipopolysaccharide is a partially O-acetylated 1,3-linked alpha-D-mannan

A polysaccharide fraction liberated from Campylobacter fetus subsp. fetus serotype A lipopolysaccharide by mild acid hydrolysis followed by gel-permeation chromatography contained a partially O-acetylated D-mannan chain, as an O-specific polysaccharide, with a core oligosaccharide attached. The structure of the polysaccharide was studied by O-deacetylation, methylation, and 1H- and 13C-NMR spectroscopy, including computer-assisted analysis of the 13C-NMR spectrum. A structure of -->3)-alpha-D-Manp2Ac-(1--> was established as the structure of the O-specific polysaccharide, the degree of O-acetylation of the mannose residues at position 2 being estimated as 80-90%. As judged by the ratio of mannose to core constituents, the D-mannan chain consists on average of 10-12 monosaccharide units.

Molecular characterization of the Bacillus stearothermophilus PV72 S-layer gene sbsB induced by oxidative stress

S-layer protein variation from a hexagonally ordered (SbsA; 130 kDa) to a obliquely ordered (SbsB; 98 kDa) protein in Bacillus stearothermophilus PV72 is mediated by an increased oxygen supply. To elucidate the molecular basis of S-layer protein variation in B. stearothermophilus PV72, the sbsB gene, coding for the 98-kDa protein, was cloned by means of inverse PCR technology and sequenced. The sbsB coding region cloned in pUC18 was expressed in Escherichia coli, without its own regulatory upstream sequences but with its putative transcriptional terminator. The reading frame of sbsB (2,760 nucleotides) is predicted to encode a protein of 920 amino acids, including the signal sequence. Amino acid sequence comparison of SbsA and SbsB did not reveal any significant homology. The expression of sbsB in E. coli resulted in an accumulation of SbsB self-assembly products in the cytoplasm.

Nested DNA inversion as a paradigm of programmed gene rearrangement

Programmed gene rearrangements are employed by a variety of microorganisms, including viruses, prokaryotes, and simple eukaryotes, to control gene expression. In most instances in which organisms mediate host evasion by large families of homologous gene cassettes, the mechanism of variation is not thought to involve DNA inversion. Here we report that Campylobacter fetus, a pathogenic Gram-negative bacterium, reassorts a single promoter, controlling surface-layer protein expression, and one or more complete ORFs strictly by DNA inversion. Rearrangements were independent of the distance between sites of inversion. These rearrangements permit variation in protein expression from the large surface-layer protein gene family and suggest an expanding paradigm of programmed DNA rearrangements among microorganisms.

Purification and characterization of Campylobacter rectus surface layer proteins

Campylobacter rectus is a putative periodontopathogen which expresses a proteinaceous surface layer (S-layer) external to the outer membrane. S-layers are considered to play a protective role for the microorganism in hostile environments. The S-layer proteins from six different C. rectus strains (five human isolates and a nonhuman primate [NHP] isolate) were isolated, purified, and characterized. The S-layer proteins of these strains varied in molecular mass (ca. 150 to 166 kDa) as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. They all reacted with monospecific rabbit antiserum to the purified S-layer of C. rectus 314, but a quantitative enzyme-linked immunosorbent assay demonstrated a strong antigenic relationship among the five human strains, while the NHP strain, 6250, showed weaker reactivity. Amino acid composition analysis showed that the S-layers of four C. rectus strains contained large proportions of acidic amino acids (13 to 27%) and that >34% of the amino acid residues were hydrophobic. Amino acid sequence analysis of six S-layer proteins revealed that the first 15 amino-terminal amino acids were identical and showed seven residues of identity with the amino-terminal sequence of the Campylobacter fetus S-layer protein SapA1. CNBr peptide profiles of the S-layer proteins from C. rectus 314, ATCC 33238, and 6250 confirmed that the S-layer proteins from the human strains were similar to each other and somewhat different from that of the NHP isolate (strain 6250). However, the S-layer proteins from the two human isolates do show some structural heterogeneity. For example, there was a 17-kDa fragment unique to the C. rectus 314 S-layer. The amino-terminal sequence of this peptide had homology with the C. rectus 51-kDa porin and was composed of nearly 50% hydrophobic residues. Thus, the S-layer protein from C. rectus has structural heterogeneity among different human strains and immunoheterogeneity with the NHP strain.

Linker mutagenesis of the Caulobacter crescentus S-layer protein: toward a definition of an N-terminal anchoring region and a C-terminal secretion signal and the potential for heterologous protein secretion

Linker insertion mutagenesis was used to modify the paracrystalline surface layer (S-layer) protein (RsaA) of the gram-negative bacterium Caulobacter crescentus. Eleven unique BamHI linker insertions in the cloned rsaA gene were identified; at the protein level, these linker insertions introduced 4 to 6 amino acids at positions ranging from the extreme N terminus to the extreme C terminus of the 1,026-amino-acid RsaA protein. All linker-peptide insertions in the RsaA N terminus caused the secreted protein to be shed into the growth medium, suggesting that the RsaA N terminus is involved in cell surface anchoring. One linker-peptide insertion in the RsaA C terminus (amino acid 784) had no effect on S-layer biogenesis, while another (amino acid 907) disrupted secretion of the protein, suggesting that RsaA possesses a secretion signal lying C terminal to amino acid 784, near or including amino acid 907. Unlike extreme N- or C-terminal linker-peptide insertions, those more centrally located in the RsaA primary sequence had no apparent effect on S-layer biogenesis. By using a newly introduced linker-encoded restriction site, a 3' fragment of the rsaA gene encoding the last 242 C-terminal amino acids of the S-layer protein was expressed in C. crescentus from heterologous Escherichia coli lacZ transcription and translation initiation information. This C-terminal portion of RsaA was secreted into the growth medium, confirming the presence of a C-terminal secretion signal. The use of the RsaA C terminus for the secretion of heterologous proteins in C. crescentus was explored by fusing 109 amino acids of an envelope glycoprotein from infectious hematopoietic necrosis virus, a pathogen of salmonid fish, to the last 242 amino acids of the RsaA C terminus. The resulting hybrid protein was successfully secreted into the growth medium and accounted for 10% of total protein in a stationary-phase culture. Based on these results and features of the RsaA primary sequence, we propose that the C. crescentus S-layer protein is secreted by a type I secretion system, relying on a stable C-terminal secretion signal in a manner analogous to E. coli alpha-hemolysin, the first example of an S-layer protein secreted by such a pathway.

Dynamics in oxygen-induced changes in S-layer protein synthesis from Bacillus stearothermophilus PV72 and the S-layer-deficient variant T5 in continuous culture and studies of the cell wall composition

Stable synthesis of the hexagonally ordered (p6) S-layer protein from the wild-type strain of Bacillus stearothermophilus PV72 could be achieved in continuous culture on complex medium only under oxygen-limited conditions when glucose was used as the sole carbon source. Depending on the adaptation of the wild-type strain to low oxygen supply, the dynamics in oxygen-induced changes in S-layer protein synthesis was different when the rate of aeration was increased to a level that allowed dissimilation of amino acids. If oxygen supply was increased at the beginning of continuous culture, synthesis of the p6 S-layer protein from the wild-type strain (encoded by the sbsA gene) was immediately stopped and replaced by that of a new type of S-layer protein (encoded by the sbsB gene) which assembled into an oblique (p2) lattice. In cells adapted to a prolonged low oxygen supply, first, low-level p2 S-layer protein synthesis and second, synchronous synthesis of comparable amounts of both types of S-layer proteins could be induced by stepwise increasing the rate of aeration. The time course of changes in S-layer protein synthesis was followed up by immunogold labelling of whole cells. Synthesis of the p2 S-layer protein could also be induced in the p6-deficient variant T5. Hybridization data obtained by applying the radiolabelled N-terminal and C-terminal sbsA fragments and the N-terminal sbsB fragment to the genomic DNA of all the three organisms indicated that changes in S-layer protein synthesis were accompanied by chromosomal rearrangement. Chemical analysis of peptidoglycan-containing sacculi and extraction and recrystallization experiments revealed that at least for the wild-type strain, a cell wall polymer consisting of N-acetylglucosamine and glucose is responsible for binding of the p6 S-layer protein to the rigid cell wall layer.

Biotechnology and biomimetic with crystalline bacterial cell surface layers (S-layers)

Crystalline bacterial cell surface layers (S-layers) are the outermost cell envelope component of many eubacteria and archaeobacteria. S-layers are composed of a single protein or glycoprotein species and exhibit oblique, square or hexagonal lattice symmetry. Pores passing through these monomolecular arrays show identical size and morphology, and functional groups are aligned in well-defined positions and orientations. Due to these unique features, S-layers have broad application potential in biotechnology including functioning as biomimetic membranes. Presently, S-layers are used (i) for the production of isoporous ultrafiltration membranes with very well defined molecular sieving and adsorption properties, (ii) as matrices for the controlled immobilization of biologically active macromolecules (e.g., enzymes, antibodies, ligands) as required for biosensors, affinity membranes and affinity microparticles as well as for solid phase assays, (iii) as stabilizing structures for Langmuir-Blodgett films and liposomes and (iv) as carriers and adjuvants for weakly immunogenic antigens and haptens.

Generation of Campylobacter fetus S-layer protein diversity utilizes a single promoter on an invertible DNA segment

Wild-type strains of Campylobacter fetus contain a monomolecular array of surface layer proteins (SLPs) and vary the antigenicity of the predominant SLP expressed. Reciprocal recombination events among the eight genomic SLP gene cassettes, which encode 97- to 149 kDa SLPs, permit this variation. To explore whether SLP expression utilizes a single promoter, we created mutant bacterial strains using insertional mutagenesis by rescue of a marker from plasmids. Experimental analysis of the mutants created clearly indicates that SLP expression solely utilizes the single sapA promoter, and that for variation C. fetus uses a mechanism of DNA rearrangement involving inversion of a 6.2 kb segment of DNA containing this promoter. This DNA inversion positions the sapA promoter immediately upstream of one of two oppositely oriented SLP gene cassettes, leading to its expression. Additionally, a second mechanism of DNA rearrangement occurs to replace at least one of the two SLP gene cassettes bracketing the invertible element. As previously reported promoter inversions in prokaryotes, yeasts and viruses involve alternate expression of at most two structural genes, the ability of C. fetus to use this phenomenon to express one of multiple cassettes is novel.

Identification, cloning, and nucleotide sequence of a silent S-layer protein gene of Lactobacillus acidophilus ATCC 4356 which has extensive similarity with the S-layer protein gene of this species

The bacterial S-layer forms a regular structure, composed of a monolayer of one (glyco)protein, on the surfaces of many prokaryotic species. S-layers are reported to fulfil different functions, such as attachment structures for extracellular enzymes and major virulence determinants for pathogenic species. Lactobacillus acidophilus ATCC 4356, which originates from the human pharynx, possesses such an S-layer. No function has yet been assigned to the S-layer of this species. Besides the structural gene (slpA) for the S-layer protein (S-protein) which constitutes this S-layer, we have identified a silent gene (slpB), which is almost identical to slpA in two regions. From the deduced amino acid sequence, it appears that the mature SB-protein (44,884 Da) is 53% similar to the SA-protein (43,636 Da) in the N-terminal and middle parts of the proteins. The C-terminal parts of the two proteins are identical except for one amino acid residue. The physical properties of the deduced S-proteins are virtually the same. Northern (RNA) blot analysis shows that only the slpA gene is expressed in wild-type cells, in line with the results from sequencing and primer extension analyses, which reveal that only the slpA gene harbors a promoter, which is located immediately upstream of the region where the two genes are identical. The occurrence of in vivo chromosomal recombination between the two S-protein-encoding genes will be described elsewhere.

Southern blotting analyses of strains of Campylobacter fetus using the conserved region of sapA

Chromosomal DNA of 27 strains of Campylobacter fetus was analyzed by Southern blotting with a probe of the conserved region of sapA. The probe hybridized with 23 strains that produced type A lipopolysaccharide. These strains had more than six sapA homologs. In Southern blots of SalI-digested chromosomal DNA separated by pulsed-field gel electrophoresis, one fragment from 19 strains and two fragments from 4 strains hybridized. These data indicate that multiple sapA homologs are localized to a limited region on the chromosomal DNA of C. fetus and thus suggest the possibility of developing a typing system using this method.

Segmental conservation of sapA sequences in type B Campylobacter fetus cells

Campylobacter fetus cells may exist as either of two defined serogroups (type A or B) based on their lipopolysaccharide (LPS) composition. Wild-type strains contain surface array proteins (S-layer proteins) that have partial antigenic cross-reactivity but bind exclusively to LPS from homologous (type A or B) cells. Type A cells possess 8 homologs of sapA, which encodes a 97-kDa S-layer protein; the gene products of these homologs have a conserved N terminus of 184 amino acids. To further explore the structural relationships between the C. fetus S-layer proteins and their encoding genes, we sought to clone and express an S-layer protein from type B strain 84-91. The cloned type B gene (sapB) was similar in structure to the previously cloned type A gene (sapA) and encoded a full-length 936-amino acid (97-kDa) S-layer protein. Sequence analysis of sapB indicated that the conserved N-terminal encoding region in sapA was absent but that the remainder of the ORF (encoding 751 amino acids) was identical to that of sapA in spite of the nonconserved nature of this region among sapA homologs. Noncoding sequences both 300 base pairs 5' and 1000 base pairs 3' to the sapB and sapA ORFs, including the sapA promoter and transcriptional terminator sequences, were essentially identical. Southern analyses revealed that the sapB N-terminal encoding region was conserved in multiple copies in type B strains but was absent in type A strains. Recombinant sapA and sapB products bound to a substantially greater degree to cells of the homologous LPS type compared with the heterologous LPS type, indicating that the conserved sapA- and sapB-encoded N termini are critical for LPS binding specificity. The parallel genetic organization and identity at the nucleotide level in both coding and noncoding regions for sap homologs in types A and B cells indicates the necessity of both homolog conservation and high fidelity DNA replication in the biology of sap diversity.