Cloning of the major protein of the Caulobacter crescentus periodic surface layer: detection and characterization of the cloned peptide by protein expression assays

Recombination and gene loss occur simultaneously during bacterial horizontal gene transfer

Bacteria can acquire new genes by incorporating environmental DNA into their genomes, yet genome sizes stay relatively constant. In nature, gene acquisition is a rare event so it is difficult to observe. However, the Caulobacter crescentus CB2A genome contains 114 insertions of genetic material from the closely-related NA1000 strain, providing a unique opportunity to analyze the horizontal transfer of genetic material. Analyses of these insertions led to a new model that involves preferential recombination at non-homologous regions that are flanked by regions of homology and does not involve any mutational processes. The net result is the replacement of segments of the recipient genome instead of the simple addition of genetic material during horizontal gene transfer. Analyses of the genomes of closely related strains of other bacterial and archaea genera, suggested that horizontal gene transfer occurs preferentially in non-homologous regions in these organisms as well. Thus, it appears to be a general phenomenon that prokaryotic horizontal gene transfer occurs preferentially at sites where the incoming DNA contains a non-homologous region that is flanked by regions of homology. Therefore, gene replacement is a common phenomenon during horizontal gene transfer.

Generation of a Dual-Target, Safe, Inexpensive Microbicide that Protects Against HIV-1 and HSV-2 Disease

HSV-2 infection is a significant health problem and a major co-morbidity factor for HIV-1 acquisition, increasing risk of infection 2-4 fold. Condom based prevention strategies for HSV-2 and HIV-1 have not been effective at stopping the HIV-1 pandemic, indicating that alternative prevention strategies need to be investigated. We have previously developed an inexpensive HIV-1 specific microbicide that utilizes the S-layer mediated display capabilities of Caulobacter crescentus, and have shown that recombinant C. crescentus displaying HIV entry blocking proteins are able to provide significant protection from HIV-1 infection in vitro. Here we demonstrate that recombinant C. crescentus are safe for topical application and describe 5 new recombinant C. crescentus that provide protection from HIV-1 infection in vitro. Further, we demonstrate protection from disease following intravaginal infection with HSV-2 in a murine model using C. crescentus expressing the anti-viral lectins Cyanovirin-N and Griffithsin, as well as α-1-antitrypsin and indolicidin. Interestingly, C. crescentus alone significantly reduced HSV-2 replication in vaginal lavage fluid. Protection from HSV-2 disease was strongly associated with early cytokine production in the vaginal tract. Our data support the potential for a dual-target microbicide that can protect against both HIV-1 and HSV-2, which could have an enormous impact on public health.

Development of an HIV-1 Microbicide Based on Caulobacter crescentus: Blocking Infection by High-Density Display of Virus Entry Inhibitors

The HIV/AIDS pandemic remains an enormous global health concern. Despite effective prevention options, 2.6 million new infections occur annually, with women in developing countries accounting for more than half of these infections. New prevention strategies that can be used by women are urgently needed. Topical microbicides specific for HIV-1 represent a promising prevention strategy. Conceptually, using harmless bacteria to display peptides or proteins capable of blocking entry provides an inexpensive approach to microbicide development. To avoid the potential pitfalls of engineering commensal bacteria, our strategy is to genetically display infection inhibitors on a non-native bacterium and rely on topical application of stabilized bacteria before potential virus exposure. Due to the high density cell-surface display capabilities and the inherent low toxicity of the bacterium, the S-layer mediated protein display capabilities of the non-pathogenic bacterium Caulobacter crescentus has been exploited for this approach. We have demonstrated that C. crescentus displaying MIP1α or CD4 interfered with the virus entry pathway and provided significant protection from HIV-1 pseudovirus representing clade B in a standard single cycle infection assay. Here we have expanded our C. crescentus based microbicide approach with additional and diverse classes of natural and synthetic inhibitors of the HIV-1 entry pathway. All display constructs provided variable but significant protection from HIV-1 infection; some with protection as high as 70%. Further, we describe protection from infection with additional viral clades. These findings indicate the significant potential for engineering C. crescentus to be an effective and readily adaptable HIV-1 microbicide platform.

Enhanced neutralization of HIV by antibodies displayed on the S-layer of Caulobacter crescentus

Innovative methods of prevention are needed to stop the more than two million new HIV-1 infections annually, particularly in women. Local application of anti-HIV antibodies has been shown to be effective at preventing infection in nonhuman primates; however, the concentrations needed are cost prohibitive. Display of antibodies on a particulate platform will likely prolong effectiveness of these anti-HIV agents and lower the cost of goods. Here, we demonstrate that the bacterium Caulobacter crescentus and its highly expressed surface-layer (S-layer) protein can provide this antibody display platform. Caulobacters displaying protein G, alone or with CD4 codisplay, successfully captured HIV-1-specific antibodies and demonstrated functional neutralization. Compared to soluble antibodies, a neutralizing anti-HIV antibody displayed on Caulobacter was as effective or more effective at neutralizing diverse HIV-1 isolates. Moreover, when an antibody reactive with an epitope induced by CD4 binding (CD4i) was codisplayed with CD4, there was significant enhancement in HIV-1 neutralization. These results suggest that caulobacters displaying anti-HIV antibodies offer a distinct improvement in the use of antibodies as microbicides. Furthermore, these reagents can specifically evaluate anti-HIV antibodies in concert with other HIV-1 blocking agents to assess the most suitable tools for conversion to scFvs, allowing for direct display within the S-layer protein and further reducing cost of goods. In summary, C. crescentus, which can be easily produced and chemically stabilized at low cost, is well suited for engineering as an effective platform, offering an inexpensive way to produce and deliver HIV-1-specific microbicides.

A bacterial extracellular DNA inhibits settling of motile progeny cells within a biofilm

In natural systems, bacteria form complex, surface-attached communities known as biofilms. This lifestyle presents numerous advantages compared with unattached or planktonic life, such as exchange of nutrients, protection from environmental stresses and increased tolerance to biocides. Despite such benefits, dispersal also plays an important role in escaping deteriorating environments and in successfully colonizing favourable, unoccupied habitat patches. The α-proteobacterium Caulobacter crescentus produces a motile swarmer cell and a sessile stalked cell at each cell division. We show here that C. crescentus extracellular DNA (eDNA) inhibits the ability of its motile cell type to settle in a biofilm. eDNA binds to the polar holdfast, an adhesive structure required for permanent surface attachment and biofilm formation, thereby inhibiting cell attachment. Because stalked cells associate tightly with the biofilm through their holdfast, we hypothesize that this novel mechanism acts on swarmer cells born in a biofilm, where eDNA can accumulate to a sufficient concentration to inhibit their ability to settle. By targeting a specific cell type in a biofilm, this mechanism modulates biofilm development and promotes dispersal without causing a potentially undesirable dissolution of the existing biofilm.

Characterization of In Vivo Reporter Systems for Gene Expression and Biosensor Applications Based on luxAB Luciferase Genes

Advances in genetic engineering methods have allowed the development of an increasing number of practical and scientific applications for bioluminescence with lux genes cloned from a variety of organisms. Bioluminescence derived from the shortened lux operon (luxAB genes) is a complex process, and applications seem to be proliferating in advance of an understanding of the underlying biochemical processes. In this report, we describe a two-phase kinetic behavior of the light emission which must be properly taken into account in any quantitative measurements of the bioluminescence signal. By using strains of Escherichia coli and Caulobacter crescentus, this behavior was characterized and interpreted in terms of the biochemistry underlying the bacterial luciferase mechanism. We show that the intensity profile of each of the two phases of the luminescence signal is responsive (and exhibits different sensitivities) to the concentration of added decanal and other components of the assay mix, as well as to the order of mixing and incubation times. This study illustrates the importance of appropriate protocol design, and specific recommendations for using the luxAB system as a molecular reporter are presented, along with versatile assay protocols that yield meaningful and reproducible signals.

Mutations in the Lipopolysaccharide biosynthesis pathway interfere with crescentin-mediated cell curvature in Caulobacter crescentus

Bacterial cell morphogenesis requires coordination among multiple cellular systems, including the bacterial cytoskeleton and the cell wall. In the vibrioid bacterium Caulobacter crescentus, the intermediate filament-like protein crescentin forms a cell envelope-associated cytoskeletal structure that controls cell wall growth to generate cell curvature. We undertook a genetic screen to find other cellular components important for cell curvature. Here we report that deletion of a gene (wbqL) involved in the lipopolysaccharide (LPS) biosynthesis pathway abolishes cell curvature. Loss of WbqL function leads to the accumulation of an aberrant O-polysaccharide species and to the release of the S layer in the culture medium. Epistasis and microscopy experiments show that neither S-layer nor O-polysaccharide production is required for curved cell morphology per se but that production of the altered O-polysaccharide species abolishes cell curvature by apparently interfering with the ability of the crescentin structure to associate with the cell envelope. Our data suggest that perturbations in a cellular pathway that is itself fully dispensable for cell curvature can cause a disruption of cell morphogenesis, highlighting the delicate harmony among unrelated cellular systems. Using the wbqL mutant, we also show that the normal assembly and growth properties of the crescentin structure are independent of its association with the cell envelope. However, this envelope association is important for facilitating the local disruption of the stable crescentin structure at the division site during cytokinesis.

Identification of promoters for efficient gene expression in Magnetospirillum gryphiswaldense

To develop an expression system for the magnetotactic bacterium Magnetospirillum gryphiswaldense, we compared gene expression from the widely used Escherichia coli P(lac) promoter with that from known and predicted genuine M. gryphiswaldense promoters. With the use of green fluorescent protein as a reporter, the highest expression level was observed with the magnetosomal P(mamDC) promoter. We demonstrate that this promoter can be used for the expression of modified magnetosome proteins to generate "antibody-binding" magnetosomes.

Characterization of the Caulobacter crescentus holdfast polysaccharide biosynthesis pathway reveals significant redundancy in the initiating glycosyltransferase and polymerase steps

Caulobacter crescentus cells adhere to surfaces by using an extremely strong polar adhesin called the holdfast. The polysaccharide component of the holdfast is comprised in part of oligomers of N-acetylglucosamine. The genes involved in the export of the holdfast polysaccharide and the anchoring of the holdfast to the cell were previously discovered. In this study, we identified a cluster of polysaccharide biosynthesis genes (hfsEFGH) directly adjacent to the holdfast polysaccharide export genes. Sequence analysis indicated that these genes are involved in the biosynthesis of the minimum repeat unit of the holdfast polysaccharide. HfsE is predicted to be a UDP-sugar lipid-carrier transferase, the glycosyltransferase that catalyzes the first step in polysaccharide biosynthesis. HfsF is predicted to be a flippase, HfsG is a glycosyltransferase, and HfsH is similar to a polysaccharide (chitin) deacetylase. In-frame hfsG and hfsH deletion mutants resulted in severe deficiencies both in surface adhesion and in binding to the holdfast-specific lectin wheat germ agglutinin. In contrast, hfsE and hfsF mutants exhibited nearly wild-type levels of adhesion and holdfast synthesis. We identified three paralogs to hfsE, two of which are redundant to hfsE for holdfast synthesis. We also identified a redundant paralog to the hfsC gene, encoding the putative polysaccharide polymerase, and present evidence that the hfsE and hfsC paralogs, together with the hfs genes, are absolutely required for proper holdfast synthesis.

S-layer-mediated display of the immunoglobulin G-binding domain of streptococcal protein G on the surface of Caulobacter crescentus: development of an immunoactive reagent

The immunoglobulin G (IgG)-binding streptococcal protein G is often used for immunoprecipitation or immunoadsorption-based assays, as it exhibits binding to a broader spectrum of host species IgG and IgG subclasses than the alternative, Staphylococcus aureus protein A. Caulobacter crescentus produces a hexagonally arranged paracrystalline protein surface layer (S-layer) composed of a single secreted protein, RsaA, that is notably tolerant of heterologous peptide insertions while maintaining the surface-attached crystalline character. Here, a protein G IgG-binding domain, GB1, was expressed as an insertion into full-length RsaA on the cell surface to produce densely packed immunoreactive particles. GB1 insertions at five separate sites were expressed, and all bound rabbit and goat IgG, but expression levels were reduced compared to those of wild-type RsaA and poor binding to mouse IgG was noted. To remedy this, we used the 20-amino-acid Muc1 peptide derived from human mucins as a spacer, since insertions of multiple tandem repeats were well tolerated for RsaA secretion and assembly. This strategy worked remarkably well, and recombinant RsaA proteins, containing up to three GB1 domains, surrounded by Muc1 peptides, not only were secreted and assembled but did so at wild-type levels. The ability to bind IgG (including mouse IgG) increased as GB1 units were added, and those with three GB1 domains bound twice as much rabbit IgG per cell as S. aureus cells (Pansorbin). The ability of recombinant protein G-Caulobacter cells to function as immunoactive reagents was assessed in an immunoprecipitation assay using a FLAG-tagged protein and anti-FLAG mouse monoclonal antibody; their performance was comparable to that of protein G-Sepharose beads. This work demonstrates the potential for using cells expressing recombinant RsaA/GB1 in immunoassays, especially considering that protein G-Caulobacter cells are more cost-effective than protein G beads and exhibit a broader species and IgG isotype binding range than protein A.

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.

Transcriptional regulation of the S-layer protein type I secretion system inCaulobacter crescentus

The Gram-negative Caulobacter crescentus exports RsaA, the crystalline S-layer subunit protein using a dedicated type I secretion system. The protein and two transporter genes (rsaADE) are located together, comparable to the Escherichia coli type I hemolysin hlyCABD operon, where read through of a stem loop following hlyCA results in reduced transcription of the hlyBD. Using two genetic approaches and a direct assessment of transcription from regions 5' to the genes we learned that rsaD and rsaE were transcribed together as a separate transcript from rsaA. These results are contrary to previous assumptions about the rsaADE type I secretion gene control and add another theme to the area of type I secretion transcription regulation. It may be that to accommodate the high levels of RsaA secretion, the type I transporters must be transcribed independently from rsaA.

Comparison of S-layer secretion genes in freshwater caulobacters

Our freshwater caulobacter collection contains about 40 strains that are morphologically similar to Caulobacter crescentus. All elaborate a crystalline protein surface (S) layer made up of protein monomers 100-193 kDa in size. We conducted a comparative study of S-layer secretion in 6 strains representing 3 size groups of S-layer proteins: small (100-108 kDa), medium (122-151 kDa), and large (181-193 kDa). All contained genes predicted to encode ATP-binding cassette transporters and membrane fusion proteins highly similar to those of C. crescentus, indicating that the S-layer proteins were all secreted by a type I system. The S-layer proteins' C-termini showed unexpectedly low sequence similarity but contained conserved residues and predicted secondary structure features typical of type I secretion signals. Cross-expression studies showed that the 6 strains recognized secretion signals from C. crescentus and Pseudomonas aeruginosa and similarly that C. crescentus was able to secrete the S-layer protein C-terminus of 1 strain examined. Inactivation of the ATP-binding cassette transporter abolished S-layer protein secretion, indicating that the type I transporter is necessary for S-layer protein secretion. Finally, while all of the S-layer proteins of this subset of strains were secreted by type I mechanisms, there were significant differences in genome positions of the transporter genes that correlated with S-layer protein size.

Two outer membrane proteins are required for maximal type I secretion of the Caulobacter crescentus S-layer protein

Transport of RsaA, the crystalline S-layer subunit protein of Caulobacter crescentus, is mediated by a type I secretion mechanism. Two proteins have been identified that play the role of the outer membrane protein (OMP) component in the RsaA secretion machinery. The genes rsaF(a) and rsaF(b) were identified by similarity to the Escherichia coli hemolysin secretion OMP TolC by using the C. crescentus genome sequence. The rsaF(a) gene is located several kilobases downstream of the other transporter genes, while rsaF(b) is completely unlinked. An rsaF(a) knockout had approximately 56% secretion compared to wild-type levels, while the rsaF(b) knockout reduced secretion levels to approximately 79%. When expression of both proteins was eliminated, there was no RsaA secretion, but a residual level of approximately 9% remained inside the cell, suggesting posttranslational autoregulation. Complementation with either of the individual rsaF genes by use of a multicopy vector, which resulted in 8- to 10-fold overexpression of the proteins, did not restore RsaA secretion to wild-type levels, indicating that both rsaF genes were required for full-level secretion. However, overexpression of rsaF(a) (with normal rsaF(b) levels) in concert with overexpression of rsaA resulted in a 28% increase in RsaA secretion, indicating a potential for significantly increasing expression levels of an already highly expressing type I secretion system. This is the only known example of type I secretion requiring two OMPs to assemble a fully functional system.

The HfaB and HfaD adhesion proteins of Caulobacter crescentus are localized in the stalk

The differentiating bacterium Caulobacter crescentus produces two different cell types at each cell division, a motile swarmer cell and an adhesive stalked cell. The stalked cell harbours a stalk, a thin cylindrical extension of the cell surface. The tip of the stalk is decorated with a holdfast, an adhesive organelle composed at least in part of polysaccharides. The synthesis of the stalk and holdfast occur at the same pole during swarmer cell differentiation. Mutations in the hfaABDC gene cluster had been shown to disrupt the attachment of the holdfast to the tip of the stalk, but the role of individual genes was unknown. We used lacZ fusions of various DNA fragments from the hfaABDC region to show that these genes form an operon. In order to analyse the relative contribution of the different genes to holdfast attachment, mutations were constructed for each gene. hfaC was not required for holdfast attachment or binding to surfaces. The hfaA and hfaD mutants shed some holdfast material into the surrounding medium and were partially deficient in binding to surfaces. Unlike hfaA and hfaB mutants, hfaD mutants were still able to form rosettes efficiently. Cells with insertions in hfaB were unable to bind to surfaces, and lectin binding studies indicated that the hfaB mutants had the strongest holdfast shedding phenotype. We determined that HfaB and HfaD are membrane-associated proteins and that HfaB is a lipoprotein. Purification of stalks and cell bodies indicated that both HfaB and HfaD are enriched in the stalk as compared to the cell body. These results suggest that HfaB and HfaD, and probably HfaA, serve to anchor the holdfast to the tip of the stalk.

Identification of genes required for synthesis of the adhesive holdfast in Caulobacter crescentus

Adhesion to both abiotic and biotic surfaces by the gram-negative prothescate bacterium Caulobacter crescentus is mediated by a polar organelle called the "holdfast," which enables the bacterium to form stable monolayer biofilms. The holdfast, a complex polysaccharide composed in part of N-acetylglucosamine, localizes to the tip of the stalk (a thin cylindrical extension of the cell wall and membranes). We report here the isolation of adhesion mutants with transposon insertions in an uncharacterized gene cluster involved in holdfast biogenesis (hfs) as well as in previously identified polar development genes (podJ and pleC), and the holdfast attachment genes (hfa). Clean deletions of three of the four genes in the hfs gene cluster (hfsDAB) resulted in a severe holdfast biogenesis phenotype. These mutants do not bind to surfaces or to a fluorescently labeled lectin, specific for N-acetylglucosamine. Transmission electron microscopy indicated that the hfsDAB mutants fail to synthesize a holdfast at the stalk tip. The predicted hfs gene products have significant sequence similarity to proteins necessary for exopolysaccharide export in gram-negative bacteria. HfsA has sequence similarity to GumC from Xanthomonas campestris, which is involved in exopolysaccharide export in the periplasm. HfsD has sequence similarity to Wza from Escherichia coli, an outer membrane protein involved in secretion of polysaccharide through the outer membrane. HfsB is a novel protein involved in holdfast biogenesis. These data suggest that the hfs genes play an important role in holdfast export.

Caulobacter crescentus synthesizes an S-layer-editing metalloprotease possessing a domain sharing sequence similarity with its paracrystalline S-layer protein

Strains of Caulobacter crescentus elaborate an S-layer, a two-dimensional protein latticework which covers the cell surface. The S-layer protein (RsaA) is secreted by a type I mechanism (relying on a C-terminal signal) and is unusual among type I secreted proteins because high levels of protein are produced continuously. In efforts to adapt the S-layer for display of foreign peptides and proteins, we noted a proteolytic activity that affected S-layer monomers with foreign inserts. The cleavage was precise, resulting in fragments with an unambiguous N-terminal sequence. We developed an assay to screen for loss of this activity (i.e., presentation of foreign peptides without degradation), using transposon and traditional mutagenesis. A metalloprotease gene designated sap (S-layer-associated protease) was identified which could complement the protease-negative mutants. The N-terminal half of Sap possessed significant similarity to other type I secreted proteases (e.g., alkaline protease of Pseudomonas aeruginosa), including the characteristic RTX repeat sequences, but the C-terminal half which normally includes the type I secretion signal exhibited no such similarity. Instead, there was a region of significant similarity to the N-terminal region of RsaA. We hypothesize that Sap evolved by combining the catalytic portion of a type I secreted protease with an S-layer-like protein, perhaps to associate with nascent S-layer monomers to "scan" for modifications.

Development of small high-copy-number plasmid vectors for gene expression in Caulobacter crescentus

Caulobacter crescentus is a bacterium with a distinctive life cycle and so it is studied as a cell development model. In addition, we have adapted this bacterium for recombinant protein production and display based on the crystalline surface protein (S)-layer and its C-terminal secretion signal. We report here the development of small, high-copy-number plasmid vectors and methods for producing an obligate expression host. The vectors are based on a narrow-host-range colE1-replicon-based plasmid commonly used in Escherichia coli, to which was added the replication origin of the IncQ plasmid RSF1010. C. crescentus strains were modified to enable plasmid replication by introduction of the RSF1010 repBAC genes at the recA locus. The small (4.0-4.5 kb) plasmids were in high copy numbers in both C. crescentus and E. coli and amenable to rapid methods for plasmid isolation and DNA sequencing. The method for introducing repBAC is suitable for other C. crescentus strains or any bacterium with an adequately homologous recA gene. Application of the vector for protein expression, based on the type I secretion system of the S-layer protein, when compared to constructs in broad-host-range plasmids, resulted in reduced time and steps required from clone construction to recombinant protein recovery and increased protein yield.

Identification of lipopolysaccharide O antigen synthesis genes required for attachment of the S-layer of Caulobacter crescentus

The outer surface of Caulobacter crescentus consists of a two-dimensional crystalline protein lattice layer (S-layer). A fraction of the LPS has an O antigen polymer attached to the core to form a 'smooth' LPS (S-LPS), which is required for attachment of the protein S-layer to the outer-membrane surface. A method to screen for strains defective in LPS production, based on loss of S-layer attachment, was developed and applied to libraries of transposon-generated mutants. Eighteen distinct insertions were found with transposon interruptions in genes affecting S-LPS production, 12 of which were located near the S-layer subunit protein gene, rsaA, and its transporter genes. Sequence adjacent to transposon insertion points was determined and used to search a C. crescentus genome database. Twelve ORFs likely to be involved in S-LPS synthesis were identified. Seven of the predicted ORFs were linked to rsaA. Six of the putative genes had identity with proteins involved in synthesis of sugar residues, including five predicted to make perosamine. The remaining six ORFs were similar to glycosyltransferases involved in forming linkages between sugar residues in the O antigen, while one may be a transcription repressor. Other chemical and preliminary proton NMR studies of the S-LPS O antigen indicate that it contains an N-acetylated 4,6-dideoxy-4-aminohexose, but is not assembled as a simple, uniform homopolymer, consisting of several different linkages between sugar residues. The ORFs described here include homologues of all the enzymes involved in the synthesis of N-acetylperosamine, a 4,6-dideoxy-4-aminohexose. Overall, the data are consistent with the hypothesis that the O antigen of C. crescentus S-LPS consists primarily of N-acetylperosamine residues polymerized with multiple anomeric linkages.

Secretion of the Caulobacter crescentus S-layer protein: further localization of the C-terminal secretion signal and its use for secretion of recombinant proteins

The secretion signal of the Caulobacter crescentus S-layer protein (RsaA) was localized to the C-terminal 82 amino acids of the molecule. Protein yield studies showed that 336 or 242 C-terminal residues of RsaA mediated secretion of >50 mg of a cellulase passenger protein per liter to the culture fluids.

Characterization of high density monolayers of the biofilm bacteriumCaulobacter crescentus: Evaluating prospects for developing immobilized cell bioreactors

Caulobacters are biofilm-forming members of the natural flora of soil and aquatic environments, which exhibit several characteristics that make them attractive for development of high surface area microbial bioreactors or biosensors. Although caulobacters are well characterized genetically, little is known about their biofilm-forming characteristics as a monoculture, or their tolerance of bioreactor-like conditions. Here we investigated the ability of caulobacters to spontaneously form high-density monolayers on artificial surfaces under a variety of environmental conditions, using phase contrast image analysis to assess biofilm density, and epifluorescence with the vital stain DiBAC(tm) to assess viability. With adequate nutrition, extremely dense monolayers formed within 24-48 h, and maintained near 100% viability in experiments ranging up to 22 days. When areas were abraded to remove cells, repopulation occurred rapidly with characteristics similar to the population of a clean surface. When established monolayers were starved for nutrients, a significant fraction of the cells detached from the surface, and cells remaining on the surface no longer tested as viable. Within 4-6 h of nutrient restoration, however, cells in the monolayer again appeared normal and tested as 100% viable. This is the first demonstration that Caulobacter crescentus is stable and amenable to high density monolayer growth and resists starvation, though some cells may express a programmed response to detach from the surface under severe nutrient limitation.Key words: Caulobacter crescentus, biofilm characterization, image analysis, phase contrast, epifluorescence, vital stains.

Cell cycle control of a holdfast attachment gene in Caulobacter crescentus

Attachment to surfaces by the prosthecate bacterium Caulobacter crescentus is mediated by an adhesive organelle, the holdfast, found at the tip of the stalk. Indirect evidence suggested that the holdfast first appears at the swarmer pole of the predivisional cell. We used fluorescently labeled lectin and transmission electron microscopy to detect the holdfast in different cell types. While the holdfast was readily detectable in stalked cells and at the stalked poles of predivisional cells, we were unable to detect the holdfast in swarmer cells or at the flagellated poles of predivisional cells. This suggests that exposure of the holdfast to the outside of the cell occurs during the differentiation of swarmer to stalked cells. To investigate the timing of holdfast synthesis and exposure to the outside of the cell, we have examined the regulation of a holdfast attachment gene, hfaA. The hfaA gene is part of a cluster of four genes (hfaABDC), identified in strain CB2A and involved in attachment of the holdfast to the polar region of the cell. We have identified the hfaA gene in the synchronizable C. crescentus strain CB15. The sequence of the CB2A hfaA promoter suggested that it was regulated by sigma54. We show that the transcription of hfaA from either strain is not dependent on sigma54. Using a hfaA-lacZ fusion, we show that the transcription of hfaA is temporally regulated during the cell cycle, with maximal expression in late-predivisional cells. This increase in expression is largely due to the preferential transcription of hfaA in the swarmer pole of the predivisional cell.

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.

Factors controlling in vitro recrystallization of the Caulobacter crescentus paracrystalline S-layer

The S-layer of Caulobacter is a two-dimensional paracrystalline array on the cell surface composed of a single protein, RsaA. We have established conditions for preparation of stable, soluble protein and then efficient in vitro recrystallization of the purified protein. Efficient recrystallization and long range order could not be obtained with pure protein only, though it was apparent that calcium was required for crystallization. Recrystallization was obtained when lipid vesicles were provided, but only when the vesicles contained the specific species of Caulobacter smooth lipopolysaccharide (SLPS) that previous studies implicated as a requirement for attaching the S-layer to the cell surface. The specific type of phospholipids did not appear critical; phospholipids rather different from those present in Caulobacter membranes or archaebacterial tetraether lipids worked equally well. The source of LPS was critical; rough and smooth variants of Salmonella typhimurium LPS as well as the rough form of Caulobacter LPS were ineffective. The requirement for calcium ions for recrystallization was further evaluated; strontium ions could substitute for calcium, and to a lesser extent, cobalt, barium, manganese and magnesium ions also stimulated crystallization. On the other hand, nickel and cadmium provided only weak crystallization stimulation, and zinc, copper, iron, aluminum ions, and the monovalent potassium, sodium, and lithium ions were ineffective. The recrystallization could also be reproduced with Langmuir-Blodgett lipid monolayers at an air-water interface. As with the vesicle experiments, this was only successful when SLPS was incorporated into the lipid mix. The best method for RsaA preparation, leading to apparently monomeric protein that was stable for many months, was an extraction with a low pH aqueous solution. We also achieved recrystallization, albeit at lower efficiency, using RsaA protein solubilized by 8 M urea, a method which allows retrieval of protein from inclusions, when expressed as heterologous protein in Escherichia coli or when retrieved as shed, precipitated protein from certain mutant caulobacters. In summary, the clarification of recrystallization methods has confirmed the requirement of SLPS as a surface attachment component and suggests that its presence in a membrane-like structure greatly stimulates the extent and quality of S-layer formation. The in vitro approach allowed the demonstration that specific ions are capable of participating in crystallization and now provides an assay for the crystallization potential of modified S-layer proteins, whether they were produced in or can be secreted by caulobacters.

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.

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.

Characterization of mutants of Caulobacter crescentus defective in surface attachment of the paracrystalline surface layer

Strains of Caulobacter crescentus express a paracrystalline surface layer (S-layer) consisting of the protein RsaA. Mutants of C. crescentus NA1000 and CB2, isolated for their ability to grow in the absence of calcium ions, uniformly no longer had the S-layer attached to the cell surface. However, RsaA was still produced, and when colonies grown on calcium-sufficient medium were examined, large two-dimensional arrays of S-layer were found intermixed with the cells. Such arrays were not found in calcium-deficient medium even when high levels of magnesium ions were provided. The arrays could be disrupted with divalent ion chelators and more readily with the calcium-selective ethylene glycol-bis (beta-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA). Thus, the outer membrane surface was not needed as a template for self-assembly, but calcium likely was. The cell surface and S-layer gene of assembly-defective mutants of NA1000 were examined to determine the basis of the S-layer surface attachment defect. Mutants had no detectable alteration in the rough lipopolysaccharide (LPS) or a characterized capsular polysaccharide, but another polysaccharide molecule was greatly reduced or absent in all calcium-independent mutants. The molecule was shown to be a smooth LPS with a core sugar and fatty acid complement identical to those of the rough LPS and an O polysaccharide of homogeneous length, tentatively considered to be composed of 4,6-dideoxy-4-amino hexose, 3,6-dideoxy-3-amino hexose, and glycerol in equal proportions. This molecule (termed SLPS) was detectable by surface labeling with a specific antiserum only when the S-layer was not present. The rsaA genes from three calcium-independent mutants were cloned and expressed in an S-layer-negative, SLPS-positive strain. A normal S-layer was produced, ruling out defects in rsaA in these cases. It is proposed that SLPS is required for S-layer surface attachment, possibly via calcium bridging. The data support the possibility that calcium binding is required to prevent an otherwise lethal effect of SLPS. If true, mutations that eliminate the O polysaccharide of SLPS eliminate the lethal effects of calcium-deprived SLPS, at the expense of S-layer attachment.

Identification of genes affecting production of the adhesive holdfast of a marine caulobacter

Caulobacters are stalked bacteria that produce a structure termed a holdfast which enables firm attachment to surfaces. Tn5 insertion mutagenesis was used to identify genes affecting holdfast production or function in the marine strain MCS6. Twelve thousand Tn5 insertion mutants were screened for adhesion defects by an assay involving the attachment of cells to polystyrene microtiter dish wells. Among adhesion-defective mutants, those with multiple polar (pleiotropic) defects were excluded and the remainder were examined for the presence of holdfast. Forty-one mutants that produced no detectable holdfast or a significantly reduced amount were found. Southern blot and pulsed-field gel electrophoresis analyses indicated that 11 unique Tn5 insertions were clustered in three regions of the genome. In addition, 71 mutants that adhered poorly or not at all to polystyrene, yet still produced a holdfast, were found. Southern blot and pulsed-field gel electrophoresis analyses of 15 of these mutants showed eight unique Tn5 insertion sites clustered in two additional regions of the genome. An assay involving attachment to glass treated with siloxane chemicals (producing surfaces with varying degrees of hydrophobicity or hydrophilicity) was used to attempt characterization of this phenotype. Unexpectedly, no simple pattern of differences in binding between the mutants and wild-type caulobacters was found. In particular, no reduction in the ability of the mutants to bind to hydrophobic surfaces was noted. Complementation with cosmid clones was successful in nearly all cases and confirmed the designation of five genomic regions of holdfast-related genes. No detectable cross-hybridization was observed with several holdfast-related gene regions from a freshwater caulobacter, providing further evidence that the marine and freshwater caulobacters are genetically distinct.

Characterization of the cspB gene encoding PS2, an ordered surface-layer protein in Corynebacterium glutamicum

PS2 is one of two major proteins detected in the culture media of various Corynebacterium glutamicum strains. The coding and promoter regions of the cspB gene encoding PS2 were cloned in lambda gt11 using polyclonal antibodies raised against PS2 for screening. Expression of the cspB gene in Escherichia coli led to the production of a major anti-PS2 labelled peptide of 63,000 Da, corresponding presumably to the mature form of PS2. It was detected in the cytoplasm, periplasm and surrounding medium of E. coli. Three other slower migrating bands of 65,000 68,000 and 72,000 Da were detected. The largest one probably corresponds to the precursor form of PS2 in E. coli. Analysis of the nucleotide sequence revealed an open reading frame (ORF) of 1533 nucleotides. The deduced 510-amino-acid polypeptide had a calculated molecular mass of 55,426 Da. According to the predicted amino acid sequence, PS2 is synthesized with a N-terminal segment of 30-amino-acid residues reminiscent of eukaryotic and prokaryotic signal peptides, and a hydrophobic domain of 21 residues near the C-terminus. Although no significant homologies were found with other proteins, it appears that some characteristics and the amino acid composition of PS2 share several common features with surface-layer proteins. The cspB gene was then disrupted in C. glutamicum by gene replacement. Freeze-etching electron microscopy performed on the wild-type strain indicated that the cell wall of C. glutamicum is covered with an ordered surface of proteins (surface layer, S-layer) which is in very close contact with other cell-wall components. These structures are absent from the cspB-disrupted strain but are present after reintroduction of the cspB gene on a plasmid into this mutant. Thus we demonstrate that the S-layer protein is the product of the cspB gene.

Identification, isolation, and structural studies of the outer membrane lipopolysaccharide of Caulobacter crescentus

The lipopolysaccharide (LPS) of the outer membrane of Caulobacter crescentus was purified and analyzed. Two distinct strains of the species, NA 1000 and CB2A, were examined; despite differences in other membrane-related polysaccharides, the two gave similar LPS composition profiles. The LPS was the equivalent of the rough LPS described for other bacteria in that it lacked the ladder of polysaccharide-containing species that results from addition of variable amounts of a repeated sequence of sugars, as detected by gel electrophoresis in smooth LPS strains. The purified LPS contained two definable regions: (i) an oligosaccharide region, consisting of an inner core of three residues of 2-keto-3-deoxyoctonate, two residues of alpha-L-glycero-D-mannoheptose, and one alpha-D-glycero-D-mannoheptose unit and an outer core region containing one residue each of alpha-D-mannose, alpha-D-galactose, and alpha-D-glucose, with the glucose likely phosphorylated and (ii) a region equivalent to the lipid A region of the archetype, consisting primarily of an esterified fatty acid, 3-OH-dodecanoate. The lipid A-like region was resistant to conclusive analysis; in particular, although a variety of analytical methods were used, no amino sugars were detected, as is found in the lipid A of the LPS of most bacteria.

The S-layer of Caulobacter crescentus: three-dimensional image reconstruction and structure analysis by electron microscopy

The regular surface protein structure (S-layer) of Caulobacter crescentus was analyzed by electron microscopy and three-dimensional image reconstruction to a resolution of 2 nm. Projections showed that the S-layer is an array of ring structures, each composed of six subunits that are arranged on a lattice with p6 symmetry. Three-dimensional reconstructions showed that the ring subunits were approximately rod-shaped structures and were perpendicular to the plane of the array, with a linker arm emanating from approximately the middle of the rod, accounting for the connections between the rings. The calculated subunit mass was ca. 100 kDa, very close to the size of RsaA (the protein known to be at least the predominant species in the S-layer) predicted from the DNA sequence of the rsaA gene. The core region of the rings creates an open pore 2.5 to 3.5 nm in diameter. The size of the gaps between the neighboring unit cells is in the same range, suggesting a uniform porosity predicted to exclude molecules larger than ca. 17 kDa. Attempts to remove membrane material from S-layer preparations with detergents revealed that the structure spontaneously rearranged into a mirror-image double layer. Negative-stain and thin-section electron microscopy examination of colonies of C. crescentus strains with a mutation in a surface molecule involved in the attachment of the S-layer showed that shed RsaA protein organized into large sheets. The sheets in turn organized into stacks that tended to accumulate near the upper surface of the colony. Image reconstruction indicated that these sheets were also precise mirror-image double layers, and thickness measurements obtained from thin sections were consistent with this finding. The sheets were absent when these mutant strains were grown without calcium, supporting other data that calcium is involved in attachment of the S-layer to a surface molecule and perhaps in subunit-subunit interactions. We propose that when the membrane is removed from S-layer fragments by detergents or the attachment-related surface molecule is absent, the attachment sites of the S-layer align precisely to form a double layer via a calcium interaction.

The phylogeny of marine and freshwater caulobacters reflects their habitat

Caulobacter is a distinctive genus of prosthecate bacteria. Because caulobacters adhere to surfaces and are found in diverse locales, their role in oligotrophic environments and bacterial biofilm communities is of interest. The phylogenetic relationships of a group of marine and freshwater caulobacters were examined in part to address whether the taxonomic grouping of these bacteria (based primarily on morphological characters) was consistent with 16S rRNA sequence divergence. The caulobacters examined (9 marine and 11 freshwater species or strains) were affiliated with the alpha proteobacteria. They made up a diverse yet, with the possible exception of a strain of Caulobacter subvibrioides, coherent assemblage. The diversity was most apparent in a comparison of freshwater and marine isolates; an early divergence within the main caulobacter lineage generally corresponded to strains isolated from freshwater and marine habitats. The marine caulobacter assemblage was not exclusive; it also embraced strains of marine hyphomonads and Rhodobacter capsulatus. We hypothesize that these genera are derived from more ancestral caulobacters. Overall, the data are consistent with the interpretation that all of the caulobacters examined, with the possible exception of C. subvibrioides, are ancestrally related, albeit anciently, and that most often division by terrestrial and marine habitats corresponds to an early evolutionary divergence within the genus.

Isolation and comparison of the paracrystalline surface layer proteins of freshwater caulobacters

Several methods for isolation of the paracrystalline surface (S) layer protein (RsaA) of Caulobacter crescentus CB15A were evaluated. Treatment of cells with HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid) buffer at pH 2 was the most effective means of selectively removing RsaA from cells, and after neutralization, the protein was capable of reassembling into a paracrystalline structure. Ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid treatment could also be used to extract RsaA and yielded protein capable of reassembly. The success of the methods was likely related to disruption of calcium-mediated bonding; calcium was required for recrystallization, while magnesium and strontium ions were ineffective. Antibody was raised against purified RsaA and, along with the S-layer extraction techniques, was used to evaluate 42 strains of caulobacters isolated from a variety of aquatic and wastewater treatment locations. A single characteristic protein could be isolated from the 35 strains that produced an S layer; with one exception, no proteins were extracted from strains that had no S layer. The presumed S-layer proteins ranged in size from 100 to 193 kDa. All of these proteins specifically reacted with anti-RsaA serum by Western immunoblot analysis. In strain CB15A, a specific S-layer-associated oligosaccharide has been proposed to be involved in a calcium-mediated attachment of the S layer to the cell surface. This molecule was detected by Western immunoblotting with a specific antiserum and on polyacrylamide gels stained for polysaccharides. A comparable band was found in all S-layer-producing strains and for most, S-layer-associated oligosaccharide-specific antibody reacted with them in Western analysis. Overall, in freshwater caulobacters at least portions of their S-layer structures appear to be strongly conserved entities, as well as the means of attachment to the cell surface.

Analysis of a Caulobacter crescentus gene cluster involved in attachment of the holdfast to the cell

Caulobacter crescentus firmly adheres to surfaces with a structure known as the holdfast, which is located at the flagellar pole of swarmer cells and at the stalk tip in stalked cells. A three-gene cluster (hfaAB and hfaC) is involved in attachment of the holdfast to the cell. Deletion and complementation analysis of the hfaAB locus revealed two genes in a single operon; both were required for holdfast attachment to the cell. Sequence analysis of the hfaAB locus showed two open reading frames with the potential to encode proteins of 15,000 and 26,000 Da, respectively. A protein migrating with an apparent size of 21 kDa in gel electrophoresis was encoded by the hfaA region when expressed in Escherichia coli under the control of the lac promoter, but no protein synthesis could be detected from the hfaB region. S1 nuclease analysis indicated that transcription of the hfaAB locus was initiated from a region containing a sequence nearly identical to the consensus for C. crescentus sigma 54-dependent promoters. In addition, a sequence with some similarity to ftr sequences (a consensus sequence associated with other Caulobacter sigma 54-dependent genes) was identified upstream of the hypothesized sigma 54 promoter. At least one of the hfaAB gene products was required for maximal transcription of hfaC. The sequence of hfaB showed some similarity to that of transcriptional activators of other bacteria. The C-terminal region of the putative gene product HfaA was found to be homologous to PapG and SmfG, which are adhesin molecules of enteropathogenic E. coli and Serratia marcescens, respectively. This information suggests that the protein encoded by the hfaA locus may have a direct role in the attachment of the holdfast to the cell, whereas hfaB may be involved in the positive regulation of hfaC.

Identification, isolation, and structural studies of extracellular polysaccharides produced by Caulobacter crescentus

Caulobacters are adherent prosthecate bacteria that are members of bacterial biofouling communities in many environments. Investigation of the cell surface carbohydrates produced by two strains of the freshwater Caulobacter crescentus, CB2A and CB15A, revealed a hitherto undetected extracellular polysaccharide (EPS) or capsule. Isolation and characterization of the EPS fractions showed that each strain produced a unique neutral EPS which could not be readily removed from the cell surface by washing. Monosaccharide analysis showed that the main CB2A EPS contained D-glucose, D-gulose, and D-fucose in a ratio of 3:1:1, whereas the CB15A EPS fraction contained D-galactose, D-glucose, D-mannose, and D-fucose in approximately equal amounts. Methylation analysis of the main CB2A EPS showed the presence of terminal glucose and gulose groups, 3-linked fucosyl, and two 3,4-linked glucosyl units, thus confirming the pentasaccharide repeating unit indicated by 1H nuclear magnetic resonance analysis. Similar studies of the CB15A EPS revealed a tetrasaccharide repeating unit consisting of terminal galactose, 4-linked fucosyl, 3-linked glucosyl, and 3,4-linked mannosyl residues. EPS was not detectable by thin-section electron microscopy techniques, including some methods designed to preserve or enhance capsules, nor was the EPS readily detected on the cell surface by scanning electron microscopy when conventional fixation techniques were used; however, a structure consistent with EPS was revealed when samples were prepared by cryofixation and freeze-substitution methods.

Cloning and expression in Escherichia coli of the structural gene coding for the monomeric protein of the S layer of Thermus thermophilus HB8

The gene coding for the 100 kDa monomeric protein (P100) of the S layer of Thermus thermophilus HB8 has been cloned in the Escherichia coli plasmid vector pUC9. Recombinant plasmids were selected by colony screening with anti-P100 rabbit antiserum. The gene, named slpA (for surface layer protein A), was identified in a bacterial clone harboring a hybrid plasmid, pMF4, with a 5.8-kbp insert. This plasmid consistently expressed a protein specifically recognized by anti-P100 antiserum. Expression was apparently independent of Plac, indicating that the promoter for P100 is functional in E. coli. Most E. coli strains transformed with plasmids containing the 5.8-kbp insert cloned in pMF4 expressed two proteins with apparent masses of 52 and 50 kDa, which were strongly recognized by anti-P100 antiserum in Western immunoblots. The 52-kDa fragment could be overproduced, and the sequence of the N-terminal undecapeptide, determined by microsequencing, indicated that it could correspond to the N-terminal domain of P100. Expression of slpA in lon mutants of E. coli led to accumulation of a protein indistinguishable from native P100, indicating that the complete gene was cloned and that the product of lon, protease La, was involved in proteolytic degradation of P100 synthesized in E. coli.

A transducing bacteriophage for Caulobacter crescentus uses the paracrystalline surface layer protein as a receptor

The bacteriophage phi Cr30, a transducing phage for Caulobacter crescentus strains, required the paracrystalline surface (S) layer for infectivity. Wild-type strains were phage resistant when rsaA, the gene for the 130K S-layer protein, was interrupted with an antibiotic resistance cassette. Strains that had lost the S layer by mutation were phage resistant, as were mutants that produce an S layer but which do not attach the structure to the cell surface. Phage sensitivity was restored to 130K-protein-deficient strains by introducing rsaA on a plasmid. Spontaneous phage-resistant strains produced expected phenotypes as follows (in order of decreasing frequency): S-layer cell attachment defects, no S layer, or an S layer that was wild type in appearance.

Repression of the cell wall protein gene operon in Bacillus brevis 47 by magnesium and calcium ions

Transcription from P2, one of the major promoters of the cell wall protein gene operon of Bacillus brevis 47, was markedly enhanced at the early stationary phase of growth. MgCl2, when added at 1 to 5 mM to the medium, inhibited this enhancement of transcription as well as shedding of the cell wall protein layers from the cell surface. MgSO4 or CaCl2 showed an effect similar to that of MgCl2. The possible coordination of the cell wall structure with regulation of the cell wall protein genes is discussed.

Effect of paracrystalline protein surface layers on predation by Bdellovibrio bacteriovorus

We determined that paracrystalline protein surface arrays (S layers) protected gram-negative eubacteria from predation by Bdellovibrio bacteriovorus. Aquaspirillum serpens VHA and MW5 and Aquaspirillum sinuosum were resistant to predation by B. bacteriovorus 6-5-S when fully covered by their S layers. The S layer of Aeromonas salmonicida A449 protected the cells from predication by B. bacteriovorus 109J. A predacious, plaque-forming vibrio that lysed an S-layer- variant of Caulobacter crescentus but was not predacious on the parental strain which possessed an S layer was isolated from raw sewage. Since S layers are stable components of many bacterial surfaces in nature, they can provide this protective function in both aquatic and terrestrial habitats where Bdellovibrio spp. are found.

Characterization of caulobacters isolated from wastewater treatment systems

Caulobacters are generally assumed to be found only in environments of low organic content; however, we readily isolated strains from a variety of sewage treatment system designs and locations, and 33 distinct strains were characterized. Most were morphologically similar, having the crescent-shaped cell body, short stalk, and hexagonally packed, paracrystalline surface (S) layer characteristic of several Caulobacter crescentus laboratory strains. Upon closer examination, they were distinguishable on the basis of protein band profiles on polyacrylamide gel electrophoresis, gross colony characteristics, or holdfast composition or by DNA restriction fragment length polymorphism analysis with flagellin and S-layer gene probes. Most of the isolates contained one or more high-molecular-weight plasmids and were resistant to a number of antibiotics, characteristics generally not shared with caulobacters isolated from other sources. Six of the 33 strains were retained because they did not fit the typical isolate profile; these strains are overrepresented in our collection compared with their relative proportion in wastewater treatment systems. By colony hybridization and restriction fragment length polymorphism analysis, all of these and one typical isolate showed less homology than the others to the surface array gene of a laboratory strain (C. crescentus CB15), and three hybridized less strongly with the flagellin gene from the same strain. In sum, although the strains were distinguishable, caulobacters from the wastewater treatment systems we examined were relatively homogenous, were similar to characterized laboratory strains, and, with exceptions, could probably be reliably detected as a group by gene probes derived from C. crescentus strains.

Transformation of freshwater and marine caulobacters by electroporation

We performed plasmid electrotransformation of Caulobacter crescentus strains and obtained up to 3 x 10(8) transformants per micrograms of pKT230. The presence and integrity of the paracrystalline protein surface (S) layer influenced electroporation; caulobacters lacking the S layer were electrotransformed 10 times more efficiently than caulobacters possessing the S layers. A procedure yielding 1,500 transformants per micrograms of pKT230 was developed for a marine caulobacter. Electroporation was used in combination with several genetic techniques, including introduction of ligation mixtures, suicide transposon mutagenesis, gene replacement, and plasmid electrotransfer from Escherichia coli to caulobacters.

High-level expression vectors for Caulobacter crescentus incorporating the transcription/translation initiation regions of the paracrystalline surface-layer-protein gene

A number of plasmid vectors were constructed for high-level gene expression in the dimorphic gram-negative bacterium Caulobacter crescentus. These vectors incorporate the transcription and translation initiation regions of the C. crescentus CB15A rsaA gene, which codes for the abundantly synthesized protein comprising the bacterium's paracrystalline surface layer. The expression vectors are based on the broad-host-range IncQ plasmid RSF1010 (R300B) and incorporate the rsaA promoter and transcription start site. Some vectors also contain translation initiation information; these can result in the addition of as little as a single glycine residue to the protein encoded by the cloned segment. The vectors can be introduced into C. crescentus by electroporation at high frequency (ranging up to 10(6)-10(7) electroporants/micrograms DNA with surface-layer-deficient mutant C. crescentus CB2A) or conjugal transfer. They range in size from 10 to 12 kb, specify either chloramphenicol or kanamycin resistance, and possess the restriction sites EcoRI, BamHI, KpnI, and SstI for cloning genes downstream of the rsaA gene sequences. For a number of the vectors, the complete nucleotide sequence is known. A comparison was made between the expression of an endoglucanase gene from these plasmids in C. crescentus CB2A and CB15A and similar constructions under the control of lacZ alpha transcription and translation initiation signals carried on a pUC9 vector in an Escherichia coli host. The two expression systems compared favorably; cell lysates prepared from C. crescentus CB2A exhibited 40% of the endoglucanase activity of similarly prepared lysates from E. coli JM101. Lysates prepared from C. crescentus CB15A exhibited only 8% of the endoglucanase activity of E. coli lysates.

Antigenic differences among Campylobacter fetus S-layer proteins

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of S-layer proteins extracted from Campylobacter fetus strains by using acid glycine buffer showed that the predominant S-layer proteins of different strains had subunit molecular weights in the range of 90,000 to 140,000. Electron microscopy revealed oblique S-layer lattices with a spacing of approximately 5.6 nm (gamma = 75 degrees) on wild-type strains VC1, VC119, VC202, and VC203. Three variants of C. fetus VC119 producing a predominant S-layer subunit protein of different molecular weight (Mr) from that of the parent were also examined. Each variant produced an oblique lattice morphologically indistinguishable from that of the parent. Amino-terminal sequence analysis showed that the S-layer proteins of the VC119 parent and variants were identical up to residue 18 and that this sequence differed from but was related to the first 16 N-terminal residues shared by the S-layer proteins of the three other wild-type C. fetus isolates. Western immunoblot analysis with an antiserum prepared to the VC119 protein and an antiserum prepared to C. fetus 84-40 LP (Z. Pei, R. T. Ellison, R. V. Lewis, and M. J. Blaser, J. Biol. Chem. 263:6416-6420, 1988) showed that strains of C. fetus were capable of producing S-layer proteins with at least four different antigenic specificities. Immunoelectron microscopy with antiserum to the VC119 S-layer protein showed that C. fetus cultures contained cells with immunoreactive oblique S-layer lattices as well as cells with oblique S-layer lattices which did not bind antibody. This suggests that C. fetus S-layer proteins undergo antigenic variation. Thermal denaturation experiments indicated that the antigenicity conferred by the surface-exposed C. fetus S-layer epitopes was unusually resistant to heat, and the thermal stability appeared to be due to the highly organized lattice structure of the S. layer. Protease digestion of purified VC119 S-layer protein revealed a trypsin-, chymotrypsin-, and endoproteinase Glu-C-resistant domain with an apparent Mr of 110,000, which carried the majority of the epitopes of the S-layer protein, and a small enzyme-sensitive domain. The trypsin- and chymotrypsin-resistant polypeptides shared an overlapping sequence which differed from the N-terminal sequence of the intact S-layer protein.

Identification of genes affecting production of the adhesion organelle of Caulobacter crescentus CB2

Transposon (Tn5) mutagenesis was used to identify regions in the genome involved with production, regulation, or attachment to the cell surface of the adhesive holdfast of the freshwater bacterium Caulobacter crescentus CB2. A total of 12,000 independently selected transposon insertion mutants were screened for defects in adhesion to cellulose acetate; 77 mutants were detected and examined by Southern blot hybridization mapping methods and pulsed-field gel electrophoresis. Ten unique sites of Tn5 insertion affecting holdfast function were identified that were clustered in four regions of the genome. Representative mutants of the 10 Tn5 insertion sites were examined by a variety of methods for differences in their phenotype leading to the loss of adhesiveness. Four phenotypes were identified: no holdfast production, production of a smaller or an altered holdfast, production of a holdfast that was unable to remain attached to the cell, and a fourth category in which a possible alteration of the stalk was related to impaired adhesion of the cell. With the possible exception of the last class, no pleiotropic mutants (those with multiple defects in the polar region of the cell) were detected among the adhesion-defective mutants. This was unexpected, since holdfast deficiency is often a characteristic of pleiotropic mutants obtained when selecting for loss of other polar structures. Overall, the evidence suggests that we have identified regions containing structural genes for the holdfast, genes involved with proper attachment or positioning on the caulobacter surface, and possibly regions that regulate the levels of holdfast production.

Conserved structures of cell wall protein genes among protein-producing Bacillus brevis strains

Bacillus brevis HPD31 contains a surface (S)-layer protein, termed the HWP, which forms a hexagonal array in the cell wall. The 5' region of the HWP gene was isolated from a DNA library constructed in bacteriophage vector EMBL3 from a partial BamHI digest of the chromosomal DNA. The 3' region contained in a 2.7-kilobase BglII fragment of the DNA was cloned into Escherichia coli, using pUC118 as a vector. On the basis of the chemically determined N-terminal amino acid sequence, the HWP gene was found to encode a polypeptide consisting of 1,087 amino acid residues with a signal peptide of 53 or 23 amino acid residues. The deduced amino acid composition was similar to the chemical amino acid compositions of other S-layer proteins in the predominance of acidic relative to basic amino acids and in the very low content of sulfur-containing amino acids. The deduced amino acid sequence showed high homology (78%) with that of the middle wall protein of B. brevis 47. Furthermore, the multiple 5' ends of the HWP gene transcripts detected on S1 nuclease analysis closely resembled those of the middle wall protein gene transcripts. This complex structure was also conserved (greater than 85%) in the regulatory regions of two other cell wall protein genes isolated from B. brevis HPD52 and HP033, suggesting that the synthesis of the cell wall proteins is intricately regulated through a similar mechanism in protein-producing B. brevis.

Attachment of the adhesive holdfast organelle to the cellular stalk of Caulobacter crescentus

Caulobacters attach to surfaces in the environment via their holdfasts, attachment organelles located at the base of the flagellum in swarmer cells and later at the end of the cellular stalk in the stalked cells which develop from the swarmer cells. There seems to be little specificity with respect to the types of surfaces to which holdfasts adhere. A notable exception is that the holdfast of one cell does not adhere to the cell surface of another caulobacter, except by joining holdfasts, typically forming "rosettes" of stalked cells. Thus, the localized adhesion of the holdfasts to the cells is in some way a specialized attachment. We investigated this holdfast-cell attachment by developing an adhesion screening assay and analyzing several mutants of Caulobacter crescentus CB2A selected to be defective in adhesion. One class of mutants made a normal holdfast by all available criteria, yet the attachment to the cell was very weak, such that the holdfast was readily shed. Another class of mutants made no holdfast at all, but when mixed with a wild-type strain, a mutant of this class participated in rosette formation. The mutant could also attach to the discarded holdfast produced by a shedding mutant. In addition, when rosettes composed of holdfast-defective and wild-type cells were examined, an increase in the number of holdfast-defective cells was correlated with a decrease in the ability of the holdfast material at the center of the rosette to bind colloidal gold particles. Gold particles are one type of surface to which holdfasts adhere well, suggesting that the stalk end and the colloidal gold particles occupy the same sites on the holdfast substance. Taken together, the data support the interpretation that there is a specialized attachment site for the holdfast at the base of the flagellum which later becomes the end of the stalk, but not a specialized region of the holdfast for attachment to this site. Also, attachment to the cell is accomplished by bond formations that occur not only at the time of holdfast production. Thus, we propose that the attachment of the holdfast to the cell is a true adhesion process and that the stalk tip and base of the flagellum must have compositions distinctly different from that of the remainder of the caulobacter cell surface.

Use of transmissible plasmids as cloning vectors in Caulobacter crescentus

Cloning vectors for studies of Caulobacter crescentus genes should be transferrable between Escherichia coli and C. crescentus since a transformation system has not been developed for C. crescentus. We have tested a large number of vectors containing IncP or IncQ replicons and found that many of the vectors containing IncQ replicons, and all but one of the vectors containing IncP replicons, are readily transferred by conjugation into C. crescentus. All of the plasmids tested were maintained in C. crescentus at 1 to 5 copies per cell, but plasmids containing IncP replicons were more stable than plasmids containing IncQ replicons. Further studies with a derivative of the IncQ plasmid R300B showed that when a promoterless kanamycin (Km)-resistance gene (npt2) was inserted into the intercistronic region of the sul-aphC (SuR-SmR) operon, Km resistance was expressed only when the npt2 gene was inserted such that it would be transcribed from the sul promoter. These data indicate that R300B does not contain sequences which would provide promoter function in C. crescentus in the orientation opposite to that of the sul operon and that any genes cloned in this orientation would require native promoters for expression. To provide greater versatility for cloning into R300B, additional vectors were constructed by the addition of multiple cloning sites in the intercistronic region of the sul-aphC operon. In addition, chromosomal DNA libraries were constructed in R300B and in the cosmid vector pLAFR1-7. Specific clones from these libraries containing genes of interest were identified by complementation of the appropriate C. crescentus mutants.

Transcriptional analysis of the major surface array gene of Caulobacter crescentus

The major component of the paracrystalline surface array of Caulobacter crescentus CB15 and one of the most abundant cellular proteins is a protein designated 130K. We have determined the DNA sequence of the 5' portion of the 130K gene, including the N-terminal one-third of the protein coding region, and analyzed the transcription of the gene. The site of transcription initiation was determined by S1 mapping of Caulobacter RNA. Although the DNA sequence upstream from the transcription start site showed significant homology to the consensus promoter sequences of Escherichia coli, S1 analysis of RNA from E. coli carrying the 130K gene on a plasmid indicated that the 130K promoter was not transcribed by E. coli RNA polymerase in vivo. Quantitative S1 analysis of RNA isolated from synchronously growing Caulobacter cells suggested that this promoter was not under developmental regulation; the amount of 130K transcript varied no more than 1.5-fold during the cell cycle. The length of the 130K mRNA was determined to be 3.3 kilobases by Northern (RNA blot) analysis, indicating that the 130K mRNA is not part of a polycistron. The amino acid sequence predicted from the DNA sequence agreed well with the N-terminal amino acid sequence determined by sequencing of the 130K protein. The 130K protein appears to be synthesized without an N-terminal leader sequence, but the N-terminal 20 amino acids are relatively hydrophobic and may function like a signal sequence during transmembrane translocation.

Use of pulsed-field-gradient gel electrophoresis to construct a physical map of the Caulobacter crescentus genome

The restriction enzyme DraI cleaves the Caulobacter crescentus genome into at least 35 fragments which have been resolved in agarose gels using pulsed-field-gradient gel electrophoresis (PFGE). When digests were performed using DNA from strains containing Tn5 insertion mutations, altered band migrations were observed. Using PFGE with the appropriate pulse times, size differences as small as 2% could be resolved in large fragments. Using this approach, we have constructed a partial physical map of the genome which correlates well with the C. crescentus genetic map and have shown the size of the genome to be approx. 3800 kb. Using hybridization with cloned genes, we have determined the map locations of five previously unmapped genes. In addition, we have shown that PFGE can be used to rapidly determine the map locations of new insertion mutations or the sizes of deletion mutations.