Radioimmunological screening method for specific membrane proteins

Role of OmpA1 and OmpA2 in Aggregatibacter actinomycetemcomitans and Aggregatibacter aphrophilus serum resistance

Aggregatibacter actinomycetemcomitans and Aggregatibacter aphrophilus belong to the HACEK group of fastidious Gram-negative organisms, a recognized cause of infective endocarditis. A. actinomycetemcomitans is also implicated in aggressive forms of periodontitis. We demonstrated that A. aphrophilus strains, as A. actinomycetemcomitans are ubiquitously serum resistant. Both species encode two Outer membrane protein A paralogues, here denoted OmpA1 and OmpA2. As their respective pangenomes contain several OmpA1 and OmpA2 alleles, they represent potential genotypic markers. A naturally competent strain of A. actinomycetemcomitans and A. aphrophilus, respectively were used to elucidate if OmpA1 and OmpA2 contribute to serum resistance. Whereas OmpA1 was critical for survival of A. actinomycetemcomitans D7SS in 50% normal human serum (NHS), serum resistant ompA1 mutants were fortuitously obtained, expressing enhanced levels of OmpA2. Similarly, OmpA1 rather than OmpA2 was a major contributor to serum resistance of A. aphrophilus HK83. Far-Western blot revealed that OmpA1^AA, OmpA2^AA, and OmpA1^AP can bind to C4-binding protein, an inhibitor of classical and mannose-binding lectin (MBL) complement activation. Indeed, ompA1 mutants were susceptible to these pathways, but also to alternative complement activation. This may at least partly reflect a compromised outer membrane integrity but is also consistent with alternative mechanisms involved in OmpA-mediated serum resistance.

Detection of Mutations Affecting Heterogeneously Expressed Phenotypes by Colony Immunoblot and Dedicated Semi-Automated Image Analysis Pipeline

To understand how bacteria evolve and adapt to their environment, it can be relevant to monitor phenotypic changes that occur in a population. Single cell level analyses and sorting of mutant cells according to a particular phenotypic readout can constitute efficient strategies. However, when the phenotype of interest is expressed heterogeneously in ancestral isogenic populations of cells, single cell level sorting approaches are not optimal. Phenotypic heterogeneity can for instance make no-expression mutant cells indistinguishable from a subpopulation of wild-type cells transiently not expressing the phenotype. The analysis of clonal populations (e.g., isolated colonies), in which the average phenotype is measured, can circumvent this issue. Indeed, no-expression mutants form negative populations while wild-type clones form populations in which average expression of the phenotype yields a positive signal. We present here an optimized colony immunoblot protocol and a semi-automated image analysis pipeline (ImageJ macro) allowing for rapid detection of clones harboring mutations that affect the heterogeneous (i.e., bimodal) expression of the Type Three Secretion System-1 (TTSS-1) in Salmonella enterica serovar Typhimurium. We show that this protocol can efficiently differentiate clones expressing TTSS-1 at various levels in mixed populations. We were able to detect the emergence of hilC mutants in which the proportion of cells expressing TTSS-1 was reduced compared to the ancestor. We could also follow changes in the frequency of different mutants during long-term infections. This demonstrates that our protocol constitutes a tractable approach to assess semi-quantitatively the evolutionary dynamics of heterogeneous phenotypes, such as the expression of virulence genes, in bacterial populations.

Perinuclear localization of internalized outer membrane vesicles carrying active cytolethal distending toxin from Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans is implicated in aggressive forms of periodontitis. Similarly to several other Gram-negative species, this organism produces and excretes a cytolethal distending toxin (CDT), a genotoxin associated with cell distention, G2 cell cycle arrest, and/or apoptosis in many mammalian cell types. In this study, we have identified A. actinomycetemcomitans outer membrane vesicles (OMVs) as a vehicle for simultaneous delivery of multiple proteins, including CDT, into human cells. The OMV proteins were internalized in both HeLa cells and human gingival fibroblasts (HGF) via a mechanism of OMV fusion with lipid rafts in the plasma membrane. The active toxin unit, CdtB, was localized inside the nucleus of the intoxicated cells, whereas OmpA and proteins detected using an antibody specific to whole A. actinomycetemcomitans serotype a cells had a perinuclear distribution. In accordance with a tight association of CdtB with OMVs, vesicles isolated from A. actinomycetemcomitans strain D7SS (serotype a), in contrast to OMVs from a D7SS cdtABC mutant, induced a cytolethal distending effect on HeLa and HGF cells, indicating that OMV-associated CDT was biologically active. Association of CDT with OMVs was also observed in A. actinomycetemcomitans isolates belonging to serotypes b and c, indicating that OMV-mediated release of CDT may be conserved in A. actinomycetemcomitans. Although the role of A. actinomycetemcomitans OMVs in periodontal disease has not yet been elucidated, our present data suggest that OMVs could deliver biologically active CDT and additional virulence factors into susceptible cells of the periodontium.

2D proteome analysis initiates new insights on the Salmonella Typhimurium LuxS protein

Background

Quorum sensing is a term describing a bacterial communication system mediated by the production and recognition of small signaling molecules. The LuxS enzyme, catalyzing the synthesis of AI-2, is conserved in a wide diversity of bacteria. AI-2 has therefore been suggested as an interspecies quorum sensing signal. To investigate the role of endogenous AI-2 in protein expression of the Gram-negative pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium), we performed a 2D-DIGE proteomics experiment comparing total protein extract of wildtype S. Typhimurium with that of a luxS mutant, unable to produce AI-2.

Results

Differential proteome analysis of wildtype S. Typhimurium versus a luxS mutant revealed relatively few changes beyond the known effect on phase 2 flagellin. However, two highly differentially expressed protein spots with similar molecular weight but differing isoelectric point, were identified as LuxS whereas the S. Typhimurium genome contains only one luxS gene. This observation was further explored and we show that the S. Typhimurium LuxS protein can undergo posttranslational modification at a catalytic cysteine residue. Additionally, by constructing LuxS-βla and LuxS-PhoA fusion proteins, we demonstrate that S. Typhimurium LuxS can substitute the cognate signal peptide sequences of β-lactamase and alkaline phosphatase for translocation across the cytoplasmic membrane in S. Typhimurium. This was further confirmed by fractionation of S. Typhimurium protein extracts, followed by Western blot analysis.

Conclusion

2D-DIGE analysis of a luxS mutant vs. wildtype Salmonella Typhimurium did not reveal new insights into the role of AI-2/LuxS in Salmonella as only a small amount of proteins were differentially expressed. However, subsequent in depth analysis of the LuxS protein itself revealed two interesting features: posttranslational modification and potential translocation across the cytoplasmic membrane. As the S. Typhimurium LuxS protein does not contain obvious signal motifs, it is speculated that LuxS is a new member of so called moonlighting proteins. These observations might have consequences in future studies on AI-2 quorum signaling in S. Typhimurium.

Analysis of the sfaX(II) locus in the Escherichia coli meningitis isolate IHE3034 reveals two novel regulatory genes within the promoter-distal region of the main S fimbrial operon

We describe the expression and regulation of the gene sfaX(II) located near the Sfa(II) fimbrial determinant in the newborn meningitis Escherichia coli (NMEC) isolate IHE3034. sfaX(II) belongs to a gene family, the 17-kDa genes, typically located downstream (300-3000bp) of different fimbrial operons found in E. coli isolates of uropathogenic and newborn meningitis origin. Using transcriptional sfaX(II) reporter gene fusions we found that different environmental conditions commonly affecting expression of fimbrial genes also affected sfaX(II) expression. Analysis of the sfaX(II) transcripts showed that the gene is part of the main fimbrial operon as it is transcribed together with the rest of the fimbrial genes. In addition, the sfaX(II) gene can be expressed from a more proximal promoter and is found to be subject to strong down-regulation by the nucleoid protein H-NS. Studies with an sfaX(II) mutant derivative of IHE3034 did not reveal effects on Sfa(II) fimbrial biogenesis as monitored by e.g. immunofluorescence microscopy. Nevertheless, a mutation in sfaX(II) resulted in altered expression of other surface components. Moreover, we define a new gene, sfaY(II), coding for a putative phosphodiesterase that is located in between the sfaX(II) gene and the fimbrial biogenesis genes. Our studies by ectopic expression of sfaY(II) in Vibrio cholerae showed that the gene product caused reduced biofilm formation and it is proposed that sfaY(II) can influence cyclic-di-GMP turnover in the bacteria. Our findings demonstrate that the operons typical for S-fimbriae of extraintestinal pathogenic E. coli include previously unrecognized novel regulatory genes.

Proinflammatory effect in whole blood by free soluble bacterial components released from planktonic and biofilm cells

BACKGROUND

Aggregatibacter actinomycetemcomitans is an oral bacterium associated with aggressive forms of periodontitis. Increasing evidence points to a link between periodontitis and cardiovascular diseases, however, the underlying mechanisms are poorly understood. This study investigated the pathogenic potential of free-soluble surface material, released from live planktonic and biofilm A. actinomycetemcomitans cells.

RESULTS

By employing an ex vivo insert model (filter pore size 20 nm) we demonstrated that the A. actinomycetemcomitans strain D7S and its derivatives, in both planktonic and in biofilm life-form, released free-soluble surface material independent of outer membrane vesicles. This material clearly enhanced the production of several proinflammatory cytokines (IL-1 beta, TNF-alpha, IL-6, IL-8, MIP-1 beta) in human whole blood, as evidenced by using a cytokine antibody array and dissociation-enhanced-lanthanide-fluorescent-immunoassay. In agreement with this, quantitative real-time PCR indicated a concomitant increase in transcription of each of these cytokine genes. Experiments in which the LPS activity was blocked with polymyxin B showed that the stimulatory effect was only partly LPS-dependent, suggesting the involvement of additional free-soluble factors. Consistent with this, MALDI-TOF-MS and immunoblotting revealed release of GroEL-like protein in free-soluble form. Conversely, the immunomodulatory toxins, cytolethal distending toxin and leukotoxin, and peptidoglycan-associated lipoprotein, appeared to be less important, as evidenced by studying strain D7S cdt/ltx double, and pal single mutants. In addition to A. actinomycetemcomitans a non-oral species, Escherichia coli strain IHE3034, tested in the same ex vivo model also released free-soluble surface material with proinflammatory activity.

CONCLUSION

A. actinomycetemcomitans, grown in biofilm and planktonic form, releases free-soluble surface material independent of outer membrane vesicles, which induces proinflammatory responses in human whole blood. Our findings therefore suggest that release of surface components from live bacterial cells could constitute a mechanism for systemic stimulation and be of particular importance in chronic localized infections, such as periodontitis.

Active cytotoxic necrotizing factor 1 associated with outer membrane vesicles from uropathogenic Escherichia coli

Cytotoxic necrotizing factor type 1 (CNF1) is one of the virulence factors produced by uropathogenic Escherichia coli (UPEC). How this toxin is translocated from the bacterial cytoplasm to the surrounding environment is not well understood. Our data suggest that CNF1 may be regarded as a secreted protein, since it could be detected in culture supernatants. Furthermore, we found that CNF1 was tightly associated to outer membrane vesicles, suggesting that such vesicles play a role in the secretion of this protein. Interestingly, vesicle samples containing CNF1 could exert the effects known for this protein on HeLa cell cultures, showing that CNF1 is transported by vesicles in its active form. Taken together, our results strongly suggest that outer membrane vesicles could be a means for the bacteria to deliver CNF1 to the environment and to the infected tissue. In addition, our results indicate that the histone-like nucleoid structuring protein H-NS has a role in the downregulation of CNF1 production and that it affects the outer membrane vesicle release in UPEC strain J96.

Release of the type I secreted alpha-haemolysin via outer membrane vesicles from Escherichia coli

The alpha-haemolysin is an important virulence factor commonly expressed by extraintestinal pathogenic Escherichia coli. The secretion of the alpha-haemolysin is mediated by the type I secretion system and the toxin reaches the extracellular space without the formation of periplasmic intermediates presumably in a soluble form. Surprisingly, we found that a fraction of this type I secreted protein is located within outer membrane vesicles (OMVs) that are released by the bacteria. The alpha-haemolysin appeared very tightly associated with the OMVs as judged by dissociation assays and proteinase susceptibility tests. The alpha-haemolysin in OMVs was cytotoxically active and caused lysis of red blood cells. The OMVs containing the alpha-haemolysin were distinct from the OMVs not containing alpha-haemolysin, showing a lower density. Furthermore, they differed in protein composition and one component of the type I secretion system, the TolC protein, was found in the lower density vesicles. Studies of natural isolates of E. coli demonstrated that the localization of alpha-haemolysin in OMVs is a common feature among haemolytic strains. We propose an alternative pathway for the transport of the type I secreted alpha-haemolysin from the bacteria to the host cells during bacterial infections.

Vesicle-mediated export and assembly of pore-forming oligomers of the enterobacterial ClyA cytotoxin

The ClyA protein is a pore-forming cytotoxin expressed by Escherichia coli and some other enterobacteria. It confers cytotoxic activity toward mammalian cells, but it has remained unknown how ClyA is surface exposed and exported from bacterial cells. Outer-membrane vesicles (OMVs) released from the bacteria were shown to contain ClyA protein. ClyA formed oligomeric pore assemblies in the OMVs, and the cytotoxic activity toward mammalian cells was considerably higher than that of ClyA protein purified from the bacterial periplasm. The redox status of ClyA correlated with its ability to form the oligomeric pore assemblies. In bacterial cells with a defective periplasmic disulphide oxidoreductase system, the ClyA protein was phenotypically expressed in a constitutive manner. The results define a vesicle-mediated transport mechanism in bacteria, and our findings show that the localization of proteins to OMVs directly may contribute to the activation and delivery of pathogenic effector proteins.

Selection of peptide entry motifs by bacterial surface display

Surface display technologies have been established previously to select peptides and polypeptides that interact with purified immobilized ligands. In the present study, we designed and implemented a surface display-based technique to identify novel peptide motifs that mediate entry into eukaryotic cells. An Escherichia coli library expressing surface-displayed peptides was combined with eukaryotic cells and the gentamicin protection assay was performed to select recombinant E. coli, which were internalized into eukaryotic cells by virtue of the displayed peptides. To establish the proof of principle of this approach, the fibronectin-binding motifs of the fibronectin-binding protein A of Staphylococcus aureus were inserted into the E. coli FhuA protein. Surface expression of the fusion proteins was demonstrated by functional assays and by FACS analysis. The fibronectin-binding motifs were shown to mediate entry of the bacteria into non-phagocytic eukaryotic cells and brought about the preferential selection of these bacteria over E. coli expressing parental FhuA, with an enrichment of 100000-fold. Four entry sequences were selected and identified using an S. aureus library of peptides displayed in the FhuA protein on the surface of E. coli. These sequences included novel entry motifs as well as integrin-binding Arg-Gly-Asp (RGD) motifs and promoted a high degree of bacterial entry. Bacterial surface display is thus a powerful tool to effectively select and identify entry peptide motifs.

Ligand-mediated protection against phage lysis as a positive selection strategy for the enrichment of epitopes displayed on the surface of E. coli cells

We present a novel strategy, termed CISTEM, which allows direct in vivo screening of polypeptides displayed on the surface of E. coli cells by a combination of ligand-mediated protection and phage-mediated selection. The effectiveness of this new approach was demonstrated by displaying the T7.tag on the surface of E. coli as a fusion with the outer membrane protein A, the receptor for bacteriophage K3. A monoclonal T7.tag antibody was used as protective ligand for T7.tag-displaying cells and phage K3 for the elimination of unprotected cells. When populations of bacteria, containing between 6 to 10,000 cells displaying the T7.tag and approximately 10(8) cells displaying an unrelated OmpA fusion protein, were infected with phage K3, specific and antibody-dependent survival of T7.tag displaying cells was observed, yielding an enrichment factor of up to 10(7)-fold. The CISTEM technology was used to select sequences from a T7.tag-based, randomised library and the results were compared to those obtained from selection by MACS with the same library. Together, these results reveal a novel in vivo screening strategy in which an E. coli phage receptor is used as display plafform and selection is performed in suspension upon addition of a protective ligand and a bacteriophage. Extentions and modifications of the basic strategy should lead to novel applications for the identification of protein-ligand interactions.

Bacterial phage receptors, versatile tools for display of polypeptides on the cell surface

Four outer membrane proteins of Escherichia coli were examined for their capabilities and limitations in displaying heterologous peptide inserts on the bacterial cell surface. The T7 tag or multiple copies of the myc epitope were inserted into loops 4 and 5 of the ferrichrome and phage T5 receptor FhuA. Fluorescence-activated cell sorting analysis showed that peptides of up to 250 amino acids were efficiently displayed on the surface of E. coli as inserts within FhuA. Strains expressing FhuA fusion proteins behaved similarly to those expressing wild-type FhuA, as judged by phage infection and colicin sensitivity. The vitamin B(12) and phage BF23 receptor BtuB could display peptide inserts of at least 86 amino acids containing the T7 tag. In contrast, the receptors of the phages K3 and lambda, OmpA and LamB, accepted only insertions in their respective loop 4 of up to 40 amino acids containing the T7 tag. The insertion of larger fragments resulted in inefficient transport and/or assembly of OmpA and LamB fusion proteins into the outer membrane. Cells displaying a foreign peptide fused to any one of these outer membrane proteins were almost completely recovered by magnetic cell sorting from a large pool of cells expressing the relevant wild-type platform protein only. Thus, this approach offers a fast and simple screening procedure for cells displaying heterologous polypeptides. The combination of FhuA, along with with BtuB and LamB, should provide a comprehensive tool for displaying complex peptide libraries of various insert sizes on the surface of E. coli for diverse applications.

Alterations in protein expression caused by the hha mutation in Escherichia coli: influence of growth medium osmolarity

The Hha protein belongs to a new family of regulators involved in the environmental regulation of virulence factors. The aim of this work was to study the effect of the hha mutation on the overall protein pattern of Escherichia coli cells by two-dimensional polyacrylamide gel electrophoresis. The growth medium osmolarity clearly influenced the effect of the hha mutation. The number of proteins whose expression was altered in hha cells, compared with wild-type cells, was three times larger at a high osmolarity than at a low osmolarity. Among the proteins whose expression was modified by the hha allele, both OmpA and protein IIAGlc of the phosphotransferase system could be identified. As this latter enzyme participates in the regulation of the synthesis of cyclic AMP and hence influences the catabolite repression system, we tested whether the expression of the lacZ gene was also modified in hha mutants. This was the case, suggesting that at least some of the pleiotropic effects of the hha mutation could be caused by its effect on the catabolite repression system.

A pplications of genetics to studies of bacterial virulence

Classical genetic techniques have made it possible in many instances to discern which bacterial factors are directly involved in causation of infection, as opposed to those that are associated with but do not directly contribute to virulence. By use of these methods as well as monoclonal antibodies, recombinant DNA, and other new techniques it has been shown that bacterial virulence is complicated, with involvement of many different bacterial factors at each step of infection; bacterial factors that facilitate one step of infection may actually impede a subsequent step. Interestingly, a large number of genes involved in toxin production or bacterial cell-surface structure are carried on unstable elements (phage, plasmids). In addition, many chromosomal genes affecting surface antigens or appendages involved in pathogenesis are subject to high-frequency variation, enabling the bacterium to adapt rapidly to different ecological niches or to evade host immunological defences. Genetic approaches have greatly increased our appreciation for the sophistication of successful bacterial pathogens, and are rapidly being used to create exciting new vaccines.

Characterization of the helper proteins for the assembly of tail fibers of coliphages T4 and lambda

Assembly of tail fibers of coliphage T4 requires the action of helper proteins. In the absence of one of these, protein 38 (p38), p37, constituting the distal part of the long tail fiber, fails to oligomerize. In the absence of the other, p57, p34 (another component of the long tail fiber), p37, and p12 (the subunit of the short tail fiber) remain unassembled. p38 can be replaced by the Tfa (tail fiber assembly) protein (pTfa) of phage lambda, which has the advantage of remaining soluble even when produced in massive amounts. The mechanisms of action of the helpers are unknown. As a first step towards elucidation of these mechanisms, p57 and pTfa have been purified to homogeneity and have been crystallized. The identity of gene 57 (g57), not known with certainty previously, has been established. The 79-residue protein p57 represents a very exotic polypeptide. It is oligomeric and acidic (an excess of nine negative charges). It does not contain Phe, Trp, Tyr, His, Pro, and Cys. Only 25 N-terminal residues were still able to complement a g57 amber mutant, although with a reduced efficiency. In cells overproducing the protein, it assumed a quasi-crystalline structure in the form of highly ordered fibers. They traversed the cells longitudinally (and thus blocked cell division) with a diameter approaching that of the cell and with a hexagonal appearance. The 194-residue pTfa is also acidic (an excess of 13 negative charges) and is likely to be dimeric.

Aeromonas salmonicida possesses two genes encoding homologs of the major outer membrane protein, OmpA

Two homologs of the outer membrane protein OmpA were identified in Aeromonas salmonicida by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and amino-terminal sequence analyses. An A. salmonicida genomic DNA library was constructed by using lambda GEM-11 and recombinant phage carrying both genes ompAI and ompAII) selected by immunoscreening. A 5.0-kb BamHI fragment containing the two genes in tandem was subcloned in pBluescript and used for further subcloning and sequencing of the genes. The encoded proteins (Mr = 33,564 and 32,536 for mature OmpAI and OmpAII, respectively) had only 64% identity with each other and otherwise had the highest level of homology to OmpA proteins from the members of the family Enterobacteriaceae. Based on the Escherichia coli OmpA model, an eight-stranded amphipathic beta-barrel model for the membrane assembly of the N-terminal half of OmpAI and OmpAII was predicted. Most variation between the two proteins was localized to the predicted surface loops and periplasmic turns, while the transmembrane strands and C-terminals domains were highly conserved. Expression of ompAI and ompAII separately in E. coli indicated that both genes could be independently transcribed from their own promoters and that both gene products were assembled into the E. coli outer membrane. A survey of different Aeromonas spp. by PCR revealed that possession of two tandem ompA genes was widespread among this genus. This is the first report of any bacterial species possessing two genes for homologs of this major outer membrane protein.

A periplasmic protein (Skp) of Escherichia coli selectively binds a class of outer membrane proteins

A search was performed for a periplasmic molecular chaperone which may assist outer membrane proteins of Escherichia coli on their way from the cytoplasmic to the outer membrane. Proteins of the periplasmic space were fractionated on an affinity column with sepharose-bound outer membrane porin OmpF. A 17 kDa polypeptide was the predominant protein retained by this column. The corresponding gene was found in a gene bank; it encodes the periplasmic protein Skp. The protein was isolated and it could be demonstrated that it bound outer membrane proteins, following SDS-PAGE, with high selectivity. Among these were OmpA, OmpC, OmpF and the maltoporin LamB. The chromosomal skp gene was inactivated by a deletion causing removal of most of the signal peptide plus 107 residues of the 141-residue mature protein. The mutant was viable but possessed much-reduced concentrations of outer membrane proteins. This defect was fully restored by a plasmid-borne skp gene which may serve as a periplasmic chaperone.

Location and unusual membrane topology of the immunity protein of the Escherichia coli phage T4

The immunity protein (Imm) encoded by the Escherichia coli phage T4 effects exclusion of phage superinfecting cells already infected with T4. The 83-residue polypeptide possesses two long lipophilic areas (from residues 3 to 32 and 37 to 65) interrupted by a hydrophilic stretch including two positively charged residues. The charge distribution of the protein very strongly suggested that it is a plasma membrane protein with the C terminus facing the periplasm. While it could be shown that the expected location was correct, fusions of Imm to alkaline phosphatase or beta-galactosidase showed that the C terminus was at the cytosolic side of the membrane. Also, concerning function, there was almost no structural specificity to this part of the protein. Even removal of the two positively charged residues did not completely abolish function. Evidence suggesting that Imm is associated with the membrane at specific sites is presented. It is proposed that Imm is localized to the membrane with the help of a receptor and that, therefore, it does not follow the established rules for the topology of other membrane proteins. The results also suggest that Imm acts indirectly, possibly by altering the conformation of a component of a phage DNA injection site.

Lysis protein T of bacteriophage T4

Lysis protein T of phage T4 is required to allow the phage's lysozyme to reach the murein layer of the cell envelope and cause lysis. Using fusions of the cloned gene t with that of the Escherichia coli alkaline phosphatase or a fragment of the gene for the outer membrane protein OmpA, it was possible to identify T as an integral protein of the plasma membrane. The protein was present in the membrane as a homooligomer and was active at very low cellular concentrations. Expression of the cloned gene t was lethal without causing gross leakiness of the membrane. The functional equivalent of T in phage lambda is protein S. An amber mutant of gene S can be complemented by gene t, although neither protein R of lambda (the functional equivalent of T4 lysozyme) nor S possess any sequence similarity with their T4 counterparts. The murein-degrading enzymes (including that of phage P22) have in common a relatively small size (molecular masses of ca. 18,000) and a rather basic nature not exhibited by other E. coli cystosolic proteins. The results suggest that T acts as a pore that is specific for this type of enzyme.

Requirement of the SecB chaperone for export of a non-secretory polypeptide in Escherichia coli

The SecB protein of Escherichia coli is a cytosolic component of the export machinery which can prevent some precursors from prematurely folding into export-incompatible conformations by binding to the newly synthesised polypeptide. The feature(s) of target proteins recognised by SecB, however, are unclear and have been a matter of controversy. Also, it has not been asked if binding of SecB is specific for secretory proteins. We demonstrate here that a non-secretory polypeptide, a fragment of a tail fiber protein of phage T4, fused to the signal peptide of the outer membrane protein OmpA has a very strong SecB requirement for export and that the signal peptide itself cannot, at least not alone, be responsible for this action of SecB. The data reported, together with those of the literature, suggest that SecB recognizes the polypeptide backbone of the target protein.

Suitability of different silver enhancement methods applied to 1 nm colloidal gold particles: an immunoelectron microscopic study

In order to exploit the recently introduced 1 nm gold colloids in routine electron microscopic labeling experiments, an efficient enhancement step for a better visualization of this small marker is a prerequisite. Efficiency and reproducibility of enhancement as well as growth homogeneity of gold particles were evaluated for three different silver intensifying solutions: silver lactate/hydroquinone/gum arabic (Danscher, 1981), Ilford L4/Metol (Bienz et al., 1986), and the commercially available IntenSE M silver enhancement kit (Janssen Pharmaceutica). The best results were obtained by using the silver lactate/hydroquinone/gum arabic mixture. The quality of enhancement of the IntenSE M kit was considerably increased by the addition of the protective colloid gum arabic.

The TraT lipoprotein as a vehicle for the transport of foreign antigenic determinants to the cell surface of Escherichia coli K12: structure-function relationships in the TraT protein

The TraT protein is a surface-exposed lipoprotein, specified by plasmids of the IncF group, that mediates serum resistance and surface exclusion. The structure and function of the TraT protein determined by plasmid R6-5 was probed by genetic insertion of a foreign antigenic determinant, the C3 epitope of polio virus, at residues 61, 125, 180, 200 or 216 of the protein. The chimaeric proteins were transported to the outer membrane and, in three cases, immunoassays with an anti-C3 monoclonal antibody indicated that the C3 epitope was exposed on the cell surface. Three of the hybrids, with insertions at residues 125, 180 and 200, assembled into the trypsin-resistant oligomeric form characteristic of the wild-type protein, which suggested that these regions are not involved in TraT subunit:subunit interactions. Additionally, the hybrid protein carrying the C3 epitope at position 180 functioned in a genetic suppression assay and retained partial surface-exclusion activity. Thus, its localization, folding and organization does not appear to be grossly altered from that of the wild-type protein. Applications of the protein for the transport of foreign antigenic determinants to the cell surface are discussed.

The toxin-coregulated pilus (TCP) of Vibrio cholerae: molecular cloning of genes involved in pilus biosynthesis and evaluation of TCP as a protective antigen in the infant mouse model

A serum containing antibodies to non-lipopolysaccharide (non-LPS) protective antigens of Vibrio cholerae has been used, after extensive absorption, to facilitate the cloning of genes involved in the synthesis of toxin-coregulated pili (TCP). A gene bank was constructed from V. cholerae Z17561 DNA using a mobilizable cosmid vector in Escherichia coli, and subsequently transferred by conjugation into V. cholerae O17. This strain does not produce TCP in vitro and lacks non-LPS protective antigens. Eight positive clones were isolated, and of these, four produced TCP as determined by electron microscopic and immunoblotting analyses. TCP-positive O17 clones were 70-fold more virulent than TCP-negative clones or O17 in the infant mouse cholera model. Only the former could remove protective antibodies from the clone-probing serum by absorption. As a corollary, serum containing antibodies to TCP protected mice from challenge with TCP-positive clones, but not with TCP-negative clones or O17. Our data indicate that TCP can function as both a virulence determinant and a protective antigen in the infant mouse model.

A 20-kilodalton protein is required for efficient production of the Bacillus thuringiensis subsp. israelensis 27-kilodalton crystal protein in Escherichia coli

The 27-kilodalton (kDa) mosquitocidal protein gene from Bacillus thuringiensis subsp. israelensis has been cloned as a 10-kilobase (kb) HindIII fragment from plasmid DNA; efficient expression in Escherichia coli KM1 depends on a region of DNA located approximately 4 kb upstream (K. McLean and H. R. Whiteley, J. Bacteriol. 169:1017-1023, 1987). We have cloned the upstream DNA region and show that it contains a complete open reading frame (ORF) encoding a protein with a molecular mass of 19,584 Da. Sequencing of adjacent stretches of DNA revealed two partial ORFs: one has 55.2% identity in an overlap of 319 amino acids to the putative transposase of IS231 of B. thuringiensis subsp. thuringiensis, and the other, a 78-codon partial ORF, may be the carboxyl terminus of the 67-kDa protein previously observed in maxicells of strain KM1. A 0.8-kb fragment containing only the 20-kDa protein gene greatly enhanced the expression of the 27-kDa protein in E. coli. The introduction of nonsense codons into the 20-kDa protein gene ORF abolished this effect, indicating that the gene product, not the mRNA or DNA, is required for the enhancement. The effect of the 20-kDa protein gene on various fusions of lacZ to the 27-kDa protein gene suggests that the 20-kDa protein acts after the initiation of translation of the 27-kDa protein gene.

Highly repressible expression system for cloning genes that specify potentially toxic proteins

A highly repressible expression vector system that allows the cloning of potentially deleterious genes has been constructed. Undesired expression of a cloned gene was prevented (i) at the level of initiation of transcription, by the presence of the strong but highly repressible leftward promoter of bacteriophage lambda, lambda pL, and (ii) at the level of transcript elongation or translation, through synthesis of antisense RNA complementary to the mRNA of the cloned gene. The system was tested by measuring the inhibition of expression of traT, the gene for the TraT major outer membrane lipoprotein. Direct detection and functional assays indicated that an essentially complete inhibition of traT expression was obtained. As a further test of the system, the gene encoding the EcoRI restriction endonuclease was cloned in the absence of the gene of the corresponding protective EcoRI modification methylase. Transformants harboring this construct were only viable when both repression controls were operational.

Expression in Escherichia coli of a cloned crystal protein gene of Bacillus thuringiensis subsp. israelensis

A ca. 10-kilobase (kb) HindIII fragment of plasmid DNA from Bacillus thuringiensis subsp. israelensis was cloned into plasmid pUC9 and transformed into Escherichia coli. Extracts of the recombinant strain contained a 27-kilodalton (kDa) peptide that reacted with antibodies to a 27-kDa peptide isolated from crystals produced by B. thuringiensis subsp. israelensis. Extracts of the recombinant strain were hemolytic and toxic to Aedes aegypti larvae. Full expression of the 27-kDa peptide required the presence of a ca. 0.8-kb region of DNA located 4 kb upstream from the structural gene; the 0.8-kb region could be present in cis or trans relative to the gene and apparently acted post-transcriptionally. Analysis of maxicells showed that the 10-kb insert also coded for peptides of 67, 20, and 16 kDa; data obtained with different subclones suggest that the 20-kDa peptide is encoded in the 0.8-kb DNA region.

Nucleotide sequence of the gene for the peptidoglycan-associated lipoprotein of Escherichia coli K12

During attempts to clone the gene for the Escherichia coli outer membrane protein III another gene was recovered. Its nucleotide sequence was determined and the deduced amino acid sequence showed that the gene does not encode protein III. It codes for a 173-residue polypeptide; 21 NH2-terminal residues are typical for a signal peptide. The sequence around the putative site (Ala-Cys) for removing this peptide, Ala-Ile-Ala-Ala-Cys-Ser-Ser-Asn, is highly homologous to that of the major cell envelope lipoprotein (Braun lipoprotein) surrounding its processing site; it is also homologous to the consensus pentapeptide Leu-Leu-Ala-Gly-Cys present in other lipoproteins of gram-negative bacteria. It could be shown that the gene expresses a lipoprotein with all the properties, including the amino acid composition, of the peptidoglycan-associated lipoprotein (PAL) [Mizuno, T. (1979) J. Biochem. (Tokyo) 86, 991-1000]. Therefore, the cloned gene is the pal gene. The protein does not contain hydrophobic regions which would serve as a membrane anchor. Tandemly repeated amino acid sequences exist at and near the NH2-terminus of the mature protein which are homologous to such repeats in the Braun lipoprotein, suggesting a common origin of this part of the two proteins. Attempts to place a transposon into the pal gene were unsuccessful. Hence the complete absence of the protein may be lethal and its function remains unknown.

Receptor specificity of the short tail fibres (gp12) of T-even type Escherichia coli phages

Short tail fibres of T-even like phages are involved in host recognition. To determine the specificity of the fibres, the region containing gene 12 of phages T2, K3, and K3hx was cloned. The genes 11, 12, wac, and 13, coding for the baseplate outer wedge, short tail fibres, collar wishes, and a head completion component, respectively, were localized on the cloned fragments. Plasmid-encoded gene 12 could be expressed without helper phage. Efficient expression of gene 12 from T2 and K3hx made an extraction of protein 12 possible. Hybrid phages obtained by in vitro complementation, recombination analysis and protein 12 binding to host range mutant bacteria excluded a role of the short tail fibres from T2, K3 or K3hx in the recognition of outer membrane proteins. Binding patterns of protein 12 to different Escherichia coli lipopolysaccharide mutants and inhibition of binding of protein 12 by a monoclonal antibody against the core region of E. coli K12 lipopolysaccharide suggested that heptose residues are necessary for efficient binding. The binding site of the same monoclonal antibody is different from the short tail fibre binding site in an E. coli B strain suggesting different binding specificities of protein 12. Thus, the ability of different bacterial strains to inactivate phage could be related to differences in the binding specificity of the short tail fibres for the lipopolysaccharides of these bacteria.

Transverse sectioning of plastic-embedded immunolabeled cryosections: morphology and permeability to protein A-colloidal gold complexes

In order to provide data for meaningful interpretation and quantitation of immunogold labeling on cryosections their morphology and permeability to protein A-gold were evaluated: We studied plastic sections of immunogold-labeled ultrathin and semithick cryosections cut perpendicular to the original cryosection plane. Various soluble and insoluble antigens in different specimens (hemoglobin and histone H5 in chicken erythrocytes, tubulin in Leishmania cells, and outer membrane protein OmpA in Escherichia coli) were fixed with glutaraldehyde-formaldehyde, formaldehyde, or periodate-lysine-paraformaldehyde and incubated with specific antibodies and protein A-gold of different sizes. The cryosection surface may be rough or smooth depending both on the sectioned material and on dehydration and drying artifacts or possibly on the cutting process itself. Well-preserved sections are capable of withstanding considerable deformation without showing clefts or cracks. If the sectioned specimen is sufficiently fixed, protein A-gold is not able to enter the IgG-labeled sections significantly but follows surface irregularities. However, gold particles can be detected within visibly damaged sections.

Molecular cloning and expression in Escherichia coli K-12 of the gene for a hemagglutinin from Vibrio cholerae

Using antiserum to the purified soluble hemagglutinin we isolated an Escherichia coli K-12 clone expressing the gene for a hemagglutinin from Vibrio cholerae 569B. The plasmid present in this clone was designated pPM471. By deletion analysis with both specific restriction endonucleases and Bal 31 nuclease, we localized the gene, to a 0.72-kilobase region of DNA, implying a molecular weight of less than 27,000 for the protein. Analysis in E. coli K-12 minicells of plasmids containing the cloned gene and deletion derivatives of these plasmids identified a protein of 24,000 daltons correlating with hemagglutinating activity. Using the cloned gene as a probe, we demonstrated the presence of homologous DNA in a variety of V. cholerae strains including both biotypes. Furthermore, by screening gene banks in E. coli K-12 of V. cholerae El Tor O17, we isolated several El Tor clones containing this region of DNA and also expressing hemagglutinating activity.

Cloning and surface expression in Escherichia coli of a structural gene encoding a surface protein of Haemophilus influenzae type b

Recombinant DNA technology was used to clone a gene coding for a surface protein of Haemophilus influenzae type b (Hib) into Escherichia coli. Chromosomal DNA from a clinical isolate of Hib was cleaved with EcoRI and ligated into plasmid vectors containing three different translational reading frames. E. coli carrying recombinant plasmids were screened in a colony blot-radioimmunoassay system by using murine monoclonal antibodies (mabs) directed against cell surface-exposed proteins of Hib. mab 7B2, which is specific for a Hib surface protein with an apparent molecular weight of 27,000 (27K), reacted with several recombinant strains of E. coli. Restriction analysis revealed the presence of a 9.1-kilobase DNA insert in each of these recombinant plasmids and also determined that both transcription and translation of the Hib gene(s) coding for the 7B2-reactive antigen were not dependent on the lac operator and promoter of the vectors. Radioimmunoprecipitation and Western blot analyses showed that the antigenic determinant recognized by mab 7B2 in these recombinant E. coli was present in a 27K protein. In addition, this 27K protein was shown to be both localized on the surface of these E. coli cells and accessible to antibody.

Colonization factor antigen II (CFA/II) of enterotoxigenic Escherichia coli: molecular cloning of the CS3 determinant

The genes for the cell surface associated antigen CS3, produced by CFA/II type enterotoxigenic Escherichia coli, have been cloned in the plasmid vector pBR322 to produce a family of recombinant plasmids. These plasmids contain a series of HindIII fragments of which a fragment of 4.6 kb is common to all those expressing CS3. One of these plasmids, pPM474, has been subjected to mutagenesis with Tn1725 and deletions generated using Bal31. This has defined a minimum region of 3.75 kb necessary for the production of CS3 on the cell surface and implying genetic complexity as has been observed with other fimbrial antigens. Analysis of the plasmid encoded proteins in E. coli K-12 minicells has confirmed this complexity.

Purification of the 25-kDa Vibrio cholerae major outer-membrane protein and the molecular cloning of its gene: ompV

The 25-kDa peptidoglycan-associated outer-membrane protein and most likely porin of Vibrio cholerae is a major immunogenic species. It has been purified by ion-exchange elution on hydroxyapatite followed by gel filtration on Bio-Gel P150 both in the presence of sodium dodecyl sulfate. This protein, of greater than 90% purity as judged by Western blotting, has been used to raise antibodies in rabbits. The antisera were then used to screen V. cholerae gene banks, constructed in Escherichia coli K12, and this has enabled us to isolate several colonies harbouring the cloned gene. The plasmids in these colonies have been designated pPM451, pPM455 and pPM472. These plasmids have a 5.3 X 10(3)-base BamHI fragment of V. cholerae DNA in common. Restriction endonuclease mapping of these plasmids has been performed and the protein identified both by Western blot analysis and in E. coli K12 minicells. The protein is not efficiently expressed in E. coli K12. It is proposed to use the name ompV to describe the structural gene, present in the cloned DNA, for this V. cholerae outer membrane protein.

Use of a replica technique to isolate muscle cell lines defective in expressing the acetylcholine receptor

We have isolated genetic variants of the C2 muscle cell line that are defective in expressing the acetylcholine receptor (AcChoR). Because the AcChoR is expressed only after C2 myoblasts have fused to form myotubes, we employed a replica technique to detect the variants. This technique yields two copies of each clone, one of which can be used for screening and the other, as a source of dividing cells. In a screening of about 10,000 clones derived from mutagenized cells, we found 2 that fused normally and expressed normal levels of acetylcholinesterase but had reduced amounts of AcChoR on their surface. One of these also had a reduced level of intracellular AcChoR, but, in the other, the amount of intracellular AcChoR was 5-fold higher than normal. Several variants were found that failed to fuse and had reduced levels of both AcChoR and acetylcholinesterase. Though we relied on 125I-labeled alpha-bungarotoxin to distinguish wild-type from deficient clones, we found that an antiserum to the AcChoR, followed by a biotinylated second antibody and a horseradish peroxidase-avidin complex, could also be used. Therefore, it should be possible to obtain muscle cell variants defective in the expression of a variety of proteins for which specific antibodies are available.

Molecular cloning using immune sera of a 22-kDal minor outer membrane protein of Vibrio cholerae

Using antisera prepared against live Vibrio cholerae we have selected several recombinant DNA clones, plasmids pPM440, pPM450 and pPM460, encoding the gene for a 22-kDal V. cholerae peptidoglycan-associated-outer-membrane protein. This is a minor protein in V. cholerae but is expressed in large amounts when the cloned gene is present in Escherichia coli K-12, where it is exposed on the cell surface as judged by ELISA. We have localized the gene within the cloned DNA by transposon mutagenesis and deletion analysis followed by analysis of whole cells and minicells to identify the plasmid-encoded proteins. The DNA region encoding the protein seems to be conserved between El Tor and Classical strains as judged by Southern DNA hybridization.

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

A precisely ordered crystalline array is found on the surface of the bacterium Caulobacter crescentus CB15. Using an immunological assay, we identified recombinant bacteriophage clones expressing the predominant protein of this structure from a lambda 1059 library of C. crescentus CB15 DNA. A single 4.4-kilobase HindIII fragment encoded a polypeptide whose antigenic determinants, molecular weight, and peculiar solubilization properties were identical with those of the authentic predominant polypeptide (130K) of the surface array. The 130K protein was produced as a discrete product as a result of gene transcription initiated from a lambda promoter; several experiments suggested that the Caulobacter promoter for this gene is not efficiently recognized by the Escherichia coli transcription machinery. Genomic Southern analysis revealed a single copy of the 130K protein gene per genome. The 130K protein gene was hybridized with DNA of two closely related laboratory strains of C. crescentus which have lost their ability to produce a surface array. One of these strains, CB2, possesses an homologous copy of the 130K gene, whereas DNA from the other strain, CB13B1a, showed a lesser degree of hybridization to the 130K gene probe; genomic fragments which did hybridize were of different sizes in CB13 as compared with those of CB15. These findings are discussed in relation to studies of the surface array function and its role in cellular morphogenesis in this stalk-forming bacterium.

Sensitivity and specificity of monoclonal antibodies directed against antigenic determinants of Treponema pallidum Nichols in the diagnosis of syphilis

Murine anti-Treponema pallidum monoclonal antibodies were employed in studies on sensitivity and specificity of binding to examine their potential for use in the detection of low numbers of pathogenic treponemes present in various body fluids. Monoclonal antibodies were used as a primary antibody source in a solid-phase immunoblot assay system. All monoclonal antibodies assayed were capable of detecting ca. 1.0 X 10(3) to 2.5 X 10(3) treponemes. Of 13 monoclonal antibodies examined, 3 were able to detect 10(3) virulent treponemes, and 1 of these antibodies was able to reveal the presence of as few as 500 organisms. Western blot analyses showed that all anti-T. pallidum monoclonal antibodies exhibiting high sensitivities for the detection of T. pallidum cells were directed against an abundant, 47,000-dalton surface-exposed antigen of the organism (S. A. Jones, K. S. Marchitto, J. N. Miller, and M. V. Norgard, Abstr. Annu. Meet. Am. Soc. Microbiol. 1984, B173, p. 46; K. S. Marchitto, S. A. Jones, and M. V. Norgard, Abstr. Annu. Meet. Am. Soc. Microbiol. 1984, B182, p. 48). Differences in binding properties of the various monoclonal antibodies were most likely a reflection of differential binding affinities or their specificities for different epitopes on the 47,000-dalton surface antigen. With two possible exceptions, the monoclonal antibodies tested reacted specifically with T. pallidum, either purified or found within a high-contaminating tissue background, and not with Treponema phagedenis biotype Reiter, Haemophilus ducreyi, Neisseria gonorrhoeae, herpes simplex virus type 2, or normal rabbit testicular tissue. The high sensitivity and specificity exhibited by these anti-T. pallidum monoclonal antibodies make them excellent candidates for employment in new syphilis or other treponemal diagnostic tests designed to detect very low numbers of pathogenic treponemes in lesion exudates or other body fluids.

Filter radioimmunoassay, a method for large-scale serotyping of Neisseria meningitidis

A simple and rapid filter radioimmunoassay method can be used to serotype meningococcal strains on a large scale. The technique consists of simultaneous inoculation of 96 strains on nitrocellulose filters. The resulting colonies can be processed in situ, by extraction and fixation, incubation with antibodies and 125I-labeled protein A, and, finally, autoradiography. Processing many filters simultaneously, one person can serotype thousands of meningococci in a week. Multiple filters with identical strain patterns can be stored after the fixation step for future screening. The use of monoclonal antibodies is essential; polyclonal antisera, even after extensive absorption, were not specific in this assay. When results from filter radioimmunoassay and Ouchterlony microprecipitation were compared for the serotyping of 201 Neisseria meningitidis strains for serotypes 2a and 2b, filter radioimmunoassay was sufficiently sensitive and specific to be useful in mass screening.

Characterization of Neisseria gonorrhoeae protein II phase variation by use of monoclonal antibodies

The protein II (P.II) outer membrane proteins of Neisseria gonorrhoeae, which have been implicated in gonococcal pathogenesis, have been previously shown to undergo a type of phase variation in which expression of any of several different forms of the proteins may be switched on or off. We identified six electrophoretically distinct forms of P.II proteins (designated P.IIa through P.IIf) within strain FA1090, and we isolated colonial variants of FA1090 that expressed only one of the six different P.II protein forms. Two monoclonal antibodies that bound specifically and differentially to P.II proteins were produced. One antibody bound to proteins P.IIb and P.IId and was bactericidal for all colonial variants expressing P.IIb. The second antibody bound to P.IIa and was bactericidal for colonial variants expressing P.IIa. P.II protein profiles of survivors of antibody killing indicated that multiple P.II protein species may be expressed on a single bacterium and that P.II protein switching in the gonococcus is nonrandom.

Cloning and expression of Legionella pneumophila antigens in Escherichia coli

To isolate and characterize Legionella pneumophila antigens, we constructed a genomic library of L. pneumophila serogroup 1 (strain 130b). L, pneumophila DNA fragments (2.5 to 7.5 megadaltons) obtained by partial digestion with Sau 3A endonuclease and size fractionation on a sucrose density gradient were inserted into the dephosphorylated BamHI site of vector pBR322; CaCl2-treated Escherichia coli cells of strain HB101 were transformed with hybrid plasmids. To detect expression of antigens, 2,559 ampicillin-resistant transformants were transferred to nitrocellulose paper, lysed in situ, and screened by enzyme immunoassay (EIA) with E. coli-absorbed rabbit anti-L. pneumophila sera. A total of 77 (3%) of the colonies were reactive by EIA; 31 (1.2%) were strongly reactive, and 6 were strongly reactive by EIA without colony lysis. Analysis of 29 stable, strongly reactive clones by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electroblotting showed antigenic bands in 18 clones by EIA with E. coli-absorbed antisera. Absorption of antisera with heat- and Formalin-killed L. pneumophila antigen eliminated or diminished the reactivity of the antigenic bands in representative clones. These studies confirm that several L. pneumophila antigens can be cloned and expressed in E. coli.

Monoclonal antibodies reactive with all strains of Haemophilus ducreyi

We have isolated plasma cell hybridomas which secrete monoclonal antibodies directed against Haemophilus ducreyi. Two of these monoclonal antibodies recognize all strains of H. ducreyi tested to date and are capable of detecting the presence of H. ducreyi in skin lesions produced by this pathogen in experimental animals. These monoclonal antibodies which react with apparently all strains of H. ducreyi have the potential to be developed into a rapid immunodiagnostic test for chancroid.

Monoclonal antibody that recognizes an outer membrane antigen common to the pathogenic Neisseria species but not to most nonpathogenic Neisseria species

A hybridoma derived from a mouse immunized with gonococcal outer membranes produced an antibody, designated H.8, that bound to all strains of Neisseria gonorrhoeae and N. meningitidis tested, and to N. lactamica and N. cinerea, but only rarely to other nonpathogenic Neisseria species. Studies with the gonococcal strain used in production of the antibody showed that the antibody bound to a surface-exposed, protease-sensitive, and heat-modifiable outer membrane antigen that we believe is distinct from previously described gonococcal outer membrane proteins.

Expression of a Trypanosoma brucei brucei variant antigen in Escherichia coli

A cDNA library derived from antigenically homogeneous bloodstream stage Trypanosoma brucei brucei was screened with an antiserum directed against the variant surface antigen (VSA) using an enzyme-linked filter immunoassay. Several recombinant clones were detected and the clone giving the most intense reaction was further analyzed. It contained a VSA-specific cDNA insert and synthesized a protein of the expected molecular weight bearing VSA determinants. The nucleotide sequence of the insert was determined and shown to have the unusual codon bias characteristic of T. brucei VSAs, frequently employing codons specifying tRNAs rare in Escherichia coli. These results indicate that a codon bias very different from that of E. coli does not preclude the expression of a cloned sequence to detectable levels in this heterologous host.