Transformation ofStreptococcus sanguisChallis with a plasmid ofStreptococcus pneumoniae

Analysis of factors affecting surface expression and immunogenicity of recombinant proteins expressed by gram-positive commensal vectors

Several key protein structural attributes were altered in an effort to optimize expression and immunogenicity of a foreign protein (M protein from Streptococcus pyogenes) exposed on the surface of Streptococcus gordonii commensal bacterial vectors: (i) a shorter N-terminal region, (ii) the addition of a 94-amino-acid spacer, and (iii) the addition of extra C-repeat regions (CRR) from the M6 protein. A decrease in the amount of cell surface M6 was observed upon deletion of 10 or more amino acid residues at the N terminus. On the other hand, reactivity of monoclonal antibody to surface M6 increased with the addition of the spacer adjacent to the proline- and glycine-rich region, and an increase in epitope dosage was obtained by adding another CRR immediately downstream of the original CRR. The results obtained should facilitate the design of improved vaccine candidates using this antigen delivery technology.

Construction of new unencapsulated (rough) strains of Streptococcus pneumoniae

To construct rough strains of Streptococcus pneumoniae in which the capsule locus was completely deleted, a genetic cassette to be used as a donor DNA in transformation was developed. The cassette contained an aphIII gene, conferring kanamycin resistance, flanked by segments of dexB and aliA. Since, in all strains of S. pneumoniae the capsule locus is between dexB and aliA, the DNA segments of these two genes allow insertion of a 1354-bp DNA fragment containing aphIII into the pneumococcal chromosome, determining the deletion of the whole capsule locus. The capsule locus was deleted from the classic type 2 and type 3 Avery's strains, from R6 (whose complete genome sequence is released) and Rx1 (the two most commonly used transformation recipients), from a type 3 clinical strain and type 19F clinical isolate G54 (whose draft genome sequence is annotated). The effect of capsule removal was tested in 4 isogenic pairs. In unencapsulated strains, growth rate increased up to 56% and transformation frequency increased up to 1075-fold. A correlation was observed between the increase in growth rate and an increase in transformation frequency.

Cleavage and purification of intein fusion proteins using the Streptococcus gordonii spex system

A gram-positive bacterial expression vector using Streptococcus gordonii has been developed for expression and secretion, or surface anchoring of heterologous proteins. This system, termed Surface Protein Expression system or SPEX, has been used to express a variety of surface anchored and secreted proteins. In this study, the Mycobacterium xenopi (Mxe) GyrA intein and chitin binding domain from Bacillus circulans chitinase Al were used in conjunction with SPEX to express a fusion protein to facilitate secretion, cleavage, and purification. Streptococcus gordonii was transformed to express a secreted fusion protein consisting of a target protein with a C-terminal intein and chitin-binding domain. Two target proteins, the C-repeat region of the Streptococcus pyogenes M6 protein (M6) and the nuclease A (NucA) enzyme of Staphylococcus aureus, were expressed and tested for intein cleavage. The secreted fusion proteins were purified from culture medium by binding to chitin beads and subjected to reaction conditions to induce intein self-cleavage to release the target protein. The M6 and NucA fusion proteins were shown to bind chitin beads and elute under cleavage reaction conditions. In addition, NucA demonstrated enzyme activity both before and after intein cleavage.

Expression and purification of histidine-tagged proteins from the gram-positive Streptococcus gordonii SPEX system

Streptococcus gordonii (S. gordonii) has been used as a gram-positive bacterial expression vector for secreted or surface-anchored recombinant proteins. Fusion of the gram-positive bacterial N-terminal signal sequence to the target protein is all that is required for efficient export. This system is termed SPEX for Surface Protein EXpression and has been used to express proteins for a variety of uses. In this study, the SPEX system has been further developed by the construction of vectors that express polyhistidine-tagged fusion proteins. SPEX vectors were constructed with an N-terminal or C-terminal histidine tag. The C-repeat region (CRR) from Streptococcus pyogenes M6 protein and the Staphylococcus aureus nuclease A (NucA) enzyme were tested for expression. The fusion proteins were purified using metal affinity chromatography (MAC). Results show that the fusion proteins were expressed and secreted from S. gordonii with the His tag at either the N- or C-terminal position and could be purified using MAC. The M6 fusions retained immunoreactivity after expression and purification as determined by immunoblots and ELISA analyses. In addition, NucA fusions retained functional activity after MAC purification. The M6-His and NucA-His fusions were purified approximately 15- and 10-fold respectively with approximately 30% recovery of protein using MAC. This study shows that the polyhistidine tag in either the N- or C-terminal position is a viable way to purify secreted heterologous proteins from the supernatant of recombinant S. gordonii cultures. This study further illustrates the value of the SPEX system for secreted expression and purification of proteins.

Characterization of cryptic plasmids pDP1 and pSMB1 of Streptococcus pneumoniae

Cryptic plasmids pDP1 and pSMB1 from clinical strains of Streptococcus pneumoniae isolated 74 years apart were found to be essentially identical in their nucleotide sequence. pDP1, 3161 bp, contains five codirectional ORFs and presents all the general features of plasmids replicating by the rolling circle mechanism. The rep gene, 963 bp, is highly homologous to the rep gene of other streptococcal plasmids of the pC194 family.

SPEX, a system for the expression of recombinant proteins from gram-positive bacterial vectors

Using a conserved pathway for surface protein extrusion, a system has been developed for the expression and secretion of proteins from gram-positive bacteria. As proof-of-concept, the Streptococcus gordonii Challis strain has been engineered to express a series of recombinant proteins fused to the conserved region of the M6 protein of Streptococcus pyogenes. In the prototype surface protein expression system, the recombinant M6 protein is anchored to the surface of S. gordonii cells expressing it. In order to overexpress the protein and easily purify it away from the bacteria, the protein was modified to enable it to be secreted into the medium. To accomplish this, a stop codon was introduced into the gene just prior to the anchor region using site-directed mutagenesis. Using enzyme-linked immunosorbent assays, it was possible to quantitate the amount of protein expressed using this system. With little or no optimization, 3 mg of protein per liter of culture was expressed and secreted into the medium of a bacterial culture grown to an OD600 equal to 1.0. This system should be broadly applicable for the expression and secretion of a variety of proteins (antigens, hormones, and enzymes) directly into the medium.

Competence for genetic transformation in encapsulated strains of Streptococcus pneumoniae: two allelic variants of the peptide pheromone

The nucleotide sequence of comC, the gene encoding the 17-residue competence-stimulating peptide (CSP) of Streptococcus pneumoniae (L. S. Havarstein, G. Coomaraswamy, and D. A. Morrison, Proc. Natl. Acad. Sci. USA 92:11140-11144, 1995) was determined with 42 encapsulated strains of different serotypes. A new allele, comC2, was found in 13 strains, including the type 3 Avery strain, A66, while all others carried a gene (now termed comC1) identical to that originally described for strain Rx1. The predicted mature product of comC2 is also a heptadecapeptide but differs from that of comC1 at eight residues. Both CSP-1 and CSP-2 synthetic peptides were used to induce competence in the 42 strains; 48% of the strains became competent after the addition of the synthetic peptide, whereas none were transformable without the added peptides.

Electrotransformation of Streptococcus agalactiae with plasmid DNA

A protocol for efficient electrotransformation of Streptococcus agalactiae (group B streptococcus) Lancefield's strain O90R (NTCT 9993) (an unencapsulated derivative of type Ia strain O90) was developed. The Escherichia coli-Streptococcus shuttle vector pDP28 (7.8 kb) carrying the ermB gene for resistance to erythromycin was used as donor DNA. Frozen 'electrocompetent' cells were prepared by repeated washes in 10% glycerol. A 50-microliters aliquot containing about 5 x 10(9) colony forming units of bacteria was subjected to the electric pulse. Optimal conditions for electrotransformation were determined using different media, harvesting cells at different points of the growth curve, and using different field strengths. The dose-response curve for transformation of S. agalactiae with pDP28 showed one-hit kinetics as donor DNA varied between 0.01 and 3 micrograms. The efficiency of electrotransformation for this range of amounts of donor DNA was 1.2 x 10(4) cfu micrograms-1. The transformation frequencies obtained with this electroporation protocol are high enough to allow both subcloning and shotgun cloning of streptococcal DNA in S. agalactiae.

Expression of M6 protein gene of Streptococcus pyogenes in Streptococcus gordonii after chromosomal integration and transcriptional fusion

The M6 protein of Streptococcus pyogenes was expressed on the cell surface and secreted in Streptococcus gordonii Challis (formerly Streptococcus sanguis) after chromosomal integration of a promoterless M6 protein gene (emm-6.1). The ermC gene, conferring resistance to erythromycin, was cloned downstream of emm-6.1, within the same ClaI fragment. The initiation codon of emm-6.1 was 19 bp downstream of a ClaI site, so that ClaI cleavage would leave the gene promoterless. The ClaI fragment containing the promoterless emm-6.1 and ermC was ligated in vitro with a ClaI digest of S. gordonii chromosomal DNA. Random chromosomal integration of the heterologous DNA was obtained by using the ligation mixture to transform the naturally competent S. gordonii Challis. Twenty-eight percent of transformants selected for erythromycin resistance also expressed M6. Among the best M6 producers, 10 clones were selected for the stability of their phenotype. Nine of the 10 clones were shown to harbour one intact copy of the emm-6.1/ermC ClaI fragment integrated into the chromosome. These strains both expressed M6 protein on the surface and secreted different amounts of the molecule, since in each case the protein was produced after a transcriptional fusion of emm-6.1 with a different chromosomal promoter. A S. gordonii strain expressing large amounts of surface M6 protein, as judged by immunofluorescence and Western blot, was compared to the M- parental strain in a standard opsonophagocytosis assay. Of the isogenic pair, M6+ S. gordonii survived better in human blood and was phagocytosed at a slower rate.

Delivery and expression of a heterologous antigen on the surface of streptococci

We have developed a system in which a foreign antigen replaces nearly all of the surface-exposed region of the fibrillar M protein from Streptococcus pyogenes and is fused to the C-terminal attachment motif of the M molecule. The fusion protein is thus expressed on the surface of Streptococcus gordonii, a commensal organism of the oral cavity. The antigen chosen to be expressed within the context of the M6 molecule was the E7 protein (98 amino acids) of human papillomavirus type 16. Stable recombinant streptococci were obtained by integrating genetic constructs into the chromosome, exploiting in vivo homologous recombination. The M6-E7 fusion protein expressed on the S. gordonii surface was shown to be immunogenic in mice. This is the first step in the construction of recombinant live vaccines in which nonpathogenic streptococci as well as other gram-positive bacteria may be used as vectors to deliver heterologous antigens to the immune system.

Method and parameters for genetic transformation of Streptococcus sanguis Challis

A simple procedure for genetic transformation of Streptococcus sanguis Challis was developed and standardized. During the exponential phase of growth, cells became competent while growing as diplococci in broth containing 10% foetal calf serum. High levels of competence were maintained by the cultures for 60 min. Competent cells could be stored frozen without loss of competence for at least three years. Using total chromosomal DNA as donor, the dose-response curve for transformation of a point mutation (streptomycin resistance) showed one-hit kinetics, as the DNA concentration varied from 0.000001 to 10 micrograms/ml. At 10 micrograms/ml, more than 2.2% of the colony-forming units were transformed to streptomycin resistance, while transforming activity remained detectable with 1 pg of DNA/ml. Optimal time of exposure of competent cells to transforming DNA was 30 min. The transformation reaction was inhibited at 0 and 4 degrees C, whereas it occurred efficiently both at 25 and 37 degrees C.

Host-vector system for integration of recombinant DNA into chromosomes of transformable and nontransformable streptococci

We describe a genetic system in which transformation of Streptococcus pneumoniae and Streptococcus sanguis was used to insert recombinant DNA into the conjugative chromosomal element omega (cat tetM) 6001 (omega 6001). The element containing the recombinant DNA was then transferred by conjugation to the chromosome of transformable and nontransformable streptococci. When Escherichia coli plasmid pDP36 was used as donor in transformation, it was capable of inserting 5.9 kilobases of heterologous DNA into the chromosome of competent streptococcal strains carrying omega 6001; the transformants were scored for erythromycin resistance. Genetic analysis showed that in a fraction of the erythromycin-resistant transformants the integration via flanking homology of the heterologous DNA caused inactivation of the tetM gene of omega 6001. By analyzing the stability of the resistance markers, we found that stable integration of heterologous DNA was achieved only in the erythromycin-resistant, tetracycline-sensitive transformants. It was possible to detect conjugal transfer of the heterologous sequences from stable transformants to strains of S. pneumoniae, S. sanguis, Streptococcus pyogenes, and Streptococcus faecalis. The omega 6001-pDP36 host-vector system opens new possibilities for gene transfer in streptococci. By this method cloned streptococcal DNA (possibly mutagenized in vitro) can be returned to the original host, greatly facilitating complementation tests and fine physiological studies.