Surface protein-CAT reporter fusions demonstrate differential gene expression in the vir regulon of Streptococcus pyogenes

Regulation of virulence by environmental signals in group A streptococci: influence of osmolarity, temperature, gas exchange, and iron limitation on emm transcription

Transcription of the gene encoding the antiphagocytic M protein (emm) of the group A streptococci has been shown to be regulated by CO2 (M. G. Caparon, R. T. Geist, J. Perez-Casal, and J. R. Scott, J. Bacteriol. 174:5693-5701, 1992). We tested the influence of additional environmental growth conditions on emm expression. Increased osmolarity, low temperature, growth with free exchange of gasses, or the restricted availability of iron resulted in decreased transcription from the emm promoter.

Specific binding of the activator Mga to promoter sequences of the emm and scpA genes in the group A streptococcus

Transcription of the surface-associated virulence factors of the group A streptococcus (GAS) Streptococcus pyogenes, M protein (emm) and the C5a peptidase (scpA), is activated by a protein called Mga (formerly Mry or VirR). To determine whether Mga binds directly to the promoters of the genes it regulates, a protein resulting from the fusion of Mga to the C-terminal end of maltose-binding protein was purified from Escherichia coli. Specific binding to the promoter regions of the scpA and emm alleles of the type M6 GAS strain JRS4 was demonstrated by electrophoresis of the DNA-protein complex. Competition studies showed that the region upstream of scpA bound MBP-Mga with a slightly higher affinity than did the region upstream of emm. DNase I protection experiments identified a single 45-bp binding site immediately upstream of and overlapping the -35 region of both promoters. Sequences homologous to the protected regions were found in the promoters of many emm, scp, and emm-like genes from strains of different serotypes of GAS, and a consensus Mga binding site was deduced.

Characterization of a gene coding for a type IIo bacterial IgG-binding protein

Two antigenic classes of non-immune IgG-binding proteins can be expressed by group A streptococci. One antigenic group of proteins is recognized by an antibody prepared against the product of a cloned fcrA gene (anti-FcRA). In this study, the immunogen used to prepare the antibody that defines the second antigenic class was shown to be the product of the emm-like (emmL) gene of M serotype 55 group A isolate, A928. The emmL55 gene expressed in E. coli produced an M(r) approximately 58,000 molecule which bound human IgG1, IgG2, IgG3 and IgG4, as well as horse, rabbit and pig IgG in a non-immune fashion. These properties are characteristic of the previously described type IIo IgG-binding protein isolated from this strain. In addition, the recombinant protein was reactive with human serum albumin and fibrinogen. The emmL 55 gene sequence was analysed and found to have the organization and sequence characteristics of a typical class I emm-like gene.

DNA sequence of the serum opacity factor of group A streptococci: identification of a fibronectin-binding repeat domain

The serum opacity factor (SOF) is a group A streptococcal protein that induces opacity of mammalian serum. The serum opacity factor 22 gene (sof22) from an M type 22 strain was cloned from an EMBL4 library by screening for plaques exhibiting serum opacity activity. DNA sequencing yielded an open reading frame of 3,075 bp. Its deduced amino acid sequence predicts a protein of 1,025 residues with a molecular weight of 112,735, a size that approximates that of the SOF22 protein isolated from both the original streptococcal strain and Escherichia coli harboring the cloned sof22 gene. The molecule is composed of three domains: an N-terminal domain responsible for the opacity reaction (opacity domain), a repeat domain with fibronectin-binding (Fn-binding) activity, and a C-terminal cell attachment domain. The C-terminal end of SOF22 is characterized by a hexameric LPXTGX motif, an adjacent hydrophobic region, and a charged C terminus, which are the hallmarks of cell-bound surface proteins found on nearly all gram-positive bacteria. Immediately upstream of this cell anchor region, SOF22 contains four tandem repeat sequence blocks, flanked by prolinerich segments. The repeats share up to 50% identity with a repeated motif found in other group A streptococcal Fn-binding proteins and exhibit Fn-binding activity, as shown by subcloning experiments. According to deletion analysis, the opacity domain is confined to the region N terminal to the repeat segment. Thus, SOF22 is unique among the known Fn-binding proteins from gram-positive bacteria in containing an independent module with a defined function in its N-terminal portion. Southern blot analysis with a probe from this N-terminal region indicates that the opacity domain of SOF varies extensively among different SOF-producing M types.

The group A streptococcal virR49 gene controls expression of four structural vir regulon genes

Within a genomic locus termed the vir regulon, virR genes of opacity factor-nonproducing (OF-) group A streptococci (GAS) are known to control the expression of the genes encoding M protein (emm) and C5a peptidase (scpA) and of virR itself. Within the corresponding genomic locus, opacity factor-producing (OF+) GAS harbor additional emm-related genes encoding immunoglobulin G- and immunoglobulin A-binding proteins (fcrA and enn, respectively). The virR gene region of the OF+ GAS M-type 49 strain CS101 was amplified by PCR, and 2,650 bp were directly sequenced. An open reading frame of 1,599 bp exhibited 76% overall homology to published virR sequences. By utilizing mRNA analysis, the 5' ends of two specific transcripts were mapped 370 and 174 bp upstream of the start codon of this open reading frame. The deduced sequences of the corresponding promoters and their locations differed from those of previously reported virR promoters. Transcripts from wild-type fcrA49, emm49, enn49, and scpA49 genes located downstream of virR49 were characterized as being monocistronic. The transcripts were quantified and mapped for their 5' ends. Subsequently, the virR49 gene was inactivated by specific insertion of a nonreplicative pSF152 vector containing recombinant virR49 sequences. The RNA from the resulting vir-mut strain did not contain transcripts of virR49, fcrA49, emm49, or enn49 and contained reduced amounts of the scpA49 transcript when compared with wild-type RNA. The mRNA control from the streptokinase gene was demonstrated not to be affected. When strain vir-mut was rotated in human blood, it was found to be fully sensitive to phagocytosis by human leukocytes. Thus, the present study provides evidence that virR genes in OF+ GAS could be involved in the control of up to five vir regulon genes, and their unaffected regulatory activity is associated with features postulated as crucial for GAS virulence.

Molecular characterization of the cfb gene encoding group B streptococcal CAMP-factor

An internal fragment of the cfb gene from group B streptococcal (GBS) strain R268 was amplified by polymerase chain reaction (PCR) using degenerate primers with sequences derived from the CAMP-factor amino acid (aa) sequence of GBS strain NCTC8181 [Rühlmann et al. (1988) FEBS Lett 235:262-266]. After cloning and sequencing this fragment, the remainder of cfb and the adjacent 5' and 3' sequences were amplified by inverted PCR of genomic DNA and directly sequenced from the PCR product. Within the 1560 bp sequenced, a complete cfb gene deviating in two deduced aa residues from the published sequence was identified. In addition, the cfbR268 sequence contained a 29-aa leader peptide. Using primers directed to the 5' and 3' ends of cfb for PCR, a cfb gene of uniform size could be detected in 19 clinical GBS isolates including three phenotypically CAMP-negative strains. Utilizing Northern blot analysis and primer extension assays, the cfbR268 promoter was located and the length of the cfb transcript was assessed at about 1100 bp. In a parallel experiment, no cfb transcript could be detected from the CAMP-negative GBS strain 74-360. The complete cfbR268 gene and different portions of its 5' and 3' ends were cloned into the plasmid pJLA602 and expressed in E. coli DH5 alpha. The recombinant peptides could be detected by Western immunoblots with polyclonal antiserum. Only the full-sized recombinant CAMP-factor was found to exert co-hemolytic activity in a sheep-blood agar assay. This co-hemolytic activity could be inhibited by anti-CAMP antiserum.

Streptococcus pyogenes type M12 protein shows selective binding to some human immunoglobulin G3 myeloma proteins

Purified, recombinant M12 protein from Streptococcus pyogenes CS24 has recently been demonstrated to bind human immunoglobulin G3 (IgG3). The binding site for IgG has been localized to an internal peptide encoded by a PvuII fragment of the gene emm12. We have investigated the ability of an isolated recombinant M12 protein consisting of the peptide encoded by the PvuII fragment to bind various monoclonal human IgG3 myeloma proteins representing a number of both Caucasian and Oriental IgG3 Gm(allotypic) phenotypes. Of nine Caucasian IgG3 myeloma proteins, only two bound strongly to the recombinant M12 protein in enzyme-linked immunosorbent assays. The allotypic phenotypes of the reactive proteins were IgG3m(b+)(g-) and IgG3m(b-)(g+). No binding was seen for seven IgG3 myeloma proteins of Oriental origin with G3m(st+)(u-)(b+)(g-), G3m(st-)(u+)(b+)(g-), G3m(st-)(u+)(b-)(g+), and G3m(st-)(u-)(b-)(g+) phenotypes. The binding of human IgG3 to M12 protein seems to be related to features other than its Gm allotypic markers. Selective reactivity of IgG3 myeloma proteins with M12 protein may provide another way to subclassify human IgG3 molecules. The biological significance of the selective reactivity is not known.

M12 protein from Streptococcus pyogenes is a receptor for immunoglobulin G3 and human albumin

We previously showed that M12 protein from opacity factor-negative Streptococcus pyogenes (group A streptococci) CS24 is responsible for immunoglobulin G3 (IgG3) binding activity. Here, we report that this M protein binds human serum albumin (HSA). Deletion analysis showed that the C repeats are sufficient for binding HSA, although upstream regions may be required for optimal binding. Like protein G, IgG3 and HSA bind to independent domains in the M protein. Experiments showed that bound IgG3 did not inhibit HSA binding to the M protein. The interaction between M12 protein and HSA is specific. M12 protein does not bind chicken egg and bovine serum albumins. Alignments of C1 and C2 repeats of M12 protein to sequences at the carboxy termini of other M proteins and Ig receptors revealed highly homologous sequences in the FcRV, M5, M6, ML2.1, and M57 proteins, suggesting that all could bind HSA. As predicted from the alignment, M5 protein and M6+ streptococci bound HSA, whereas an isogenic M6- mutant did not bind HSA. Furthermore, M2 protein from an opacity factor-positive strain also bound HSA.

Streptococcus pyogenes type IIa IgG Fc receptor expression is co-ordinately regulated with M protein and streptococcal C5a peptidase

Streptococcus pyogenes is an important agent of human disease which expresses a variety of proteins and polysaccharides on its surface. Surface molecules M protein and streptococcal C5a peptidase (SCPA) are virulence factors which undergo concurrent phase variation and are under the co-ordinate control of the virR locus. Most opacity factor-positive (OF+) strains of S. pyogenes also express IgG Fc receptor proteins on their surface. These studies were initiated to determine whether the type IIa Fc receptor on the surface of S. pyogenes phase-varies with members of this regulatory circuit. Several methods were applied to M+ and M- variant strains to evaluate this question. (i) Immunoblot assays quantified Fc receptors on whole cells by using human IgG myeloma protein and receptor-specific antibody. M+ strains bound IgG and antibody specific for Fc protein, whereas M- strains did not. (ii) Enzyme-linked immunosorbent assays quantified Fc receptor antigen expression and showed that M+ strains produce more Fc receptor protein than their M- derivatives. (iii) Quantitative RNA dot blots showed that the message for the Fc receptor gene (fcrA) was reduced in M- strains. RNA from M+ strains hybridized to the fcrA probe at a greater dilution than that from their M- counterparts. (iv) Northern hybridization showed that the fcrA transcript is 1200 nucleotides in size and distinct from transcripts for M and SCPA proteins. These data are evidence for the co-ordinate transcriptional control of the Fc receptor, M protein, and SCPA and show that these proteins co-ordinately phase-vary within the same regulatory circuit.

A group A streptococcal Enn protein potentially resulting from intergenomic recombination exhibits atypical immunoglobulin-binding characteristics

The gene encoding the Enn protein (enn) of the M untypeable group A streptococcal (GAS) strain 64/14 was amplified by polymerase chain reaction, cloned into the expression vector pJLA602 and expressed in Escherichia coli DH5 alpha. Unlike other GAS-Enn proteins, which exhibit IgA-binding activity, the recombinant Enn enn64/14 protein reacted preferentially with human IgG3. The 1050 bp open reading frame comprising the enn64/14 gene was completely sequenced. The region of the gene encoding the signal peptide and the C-terminus exhibited > 95% homology to corresponding sections of other enn genes. The region of enn64/14 encoding the N-terminus of the mature Enn protein was found to be highly homologous to the corresponding section of the gene encoding the M-like protein of GAS serotype M9 (emmL9). The recombinant protein encoded by emmL9 was found to react with all four human IgG subclasses. About 30% of the 1152 bp open reading frame of emmL9 encoding the N-terminus was found to display > 90% homology to the corresponding section of enn64/14 but was < 50% homologous in the remainder of the gene sequence. The functional analysis of the subcloned N-terminal section of emmL9 demonstrated a polypeptide exhibiting selective binding to human IgG3. These findings suggested that enn64/14 was a hybrid gene formed by recombination of an enn gene and an emmL9 gene. The putative recombinational event could have involved a set of flanking 7 bp direct repeats. Since enn64/14 and emmL9 are genes from different phylogenetic lineages of GAS, this report provides evidence that intergenomic recombinations between different types of GAS genes can occur and could lead to hybrid proteins with unique Ig-binding characteristics.

Analysis of Streptococcus pyogenes promoters by using novel Tn916-based shuttle vectors for the construction of transcriptional fusions to chloramphenicol acetyltransferase

We have developed a series of shuttle vectors based on the conjugative transposon Tn916 that have been designed for the analysis of transcriptional regulation in Streptococcus pyogenes and other gram-positive bacteria. Designated the pVIT vectors (vectors for integration into Tn916), the vectors are small, stable plasmids in Escherichia coli to facilitate the fusion of promoters from cloned S. pyogenes genes to a promoterless gene which encodes chloramphenicol acetyltransferase. The vectors each contain one or more small regions of Tn916 to direct the integration of the transcriptional fusion into the transposon via homologous recombination following transformation of S. pyogenes or other suitable gram-positive hosts. Integration can be monitored by the inactivation or replacement of an antibiotic resistance determinant in modified derivatives of Tn916. Promoter activity can then be quantitated by the determination of chloramphenicol acetyltransferase-specific activity. In addition, since integration is into loci that do not disrupt the conjugative transpositional functions of Tn916, the vectors are useful for analysis of regulation in strains that are difficult or impossible to transform and can be introduced into these strains by conjugation following transformation of an intermediate host. The promoters for the genes which encode both the M protein and protein F of S. pyogenes were active in pVIT vectors, as was the region which controls transcription of mry, a trans-acting positive regulator of M protein expression. However, neither of the two characterized promoters for mry demonstrated activity when independently analyzed in pVIT-generated partial diploid strains, suggesting that regulation of mry is more complex than predicted by current models. The broad host range of Tn916 should make the pVIT vectors useful for analysis of regulation in numerous other bacterial species.

VirR and Mry are homologous trans-acting regulators of M protein and C5a peptidase expression in group A streptococci

Transcription of the group A streptococcal M12 protein gene (emm12) and the C5a peptidase gene (scpA), which encodes an inhibitor of complement-mediated chemotaxis, was previously shown to depend on a third genetic locus, designated virR. A 1.6 kb region of DNA which is 200 bp upstream of emm12 and is thought to contain the virR locus, was sequenced. An open reading frame which overlaps deletion mutations that define virR was identified. The sequence of the encoded VirR protein, which was deduced to contain 499 amino acids, is characteristic of cytoplasmic proteins. Comparison of the VirR protein to a variety of DNA binding proteins, such as lambda Cro, revealed a DNA binding motif. VirR was also compared to the M6 positive regulator, mry, and found to be 98% homologous. The predicted virR promoter is preceded by two sets of inverted repeats, in contrast to mry which is preceded by one repeat. Introduction of virR on the shuttle vector pAM401 into a strain of group A Streptococcus with a deletion in the chromosomal virR gene demonstrated that the VirR protein activated transcription of both emm12 and scpA genes in trans. Analysis of RNA by Northern blot using virR-specific probes identified two virR transcripts, a 1.6 kb transcript which corresponds to the predicted size of the gene, and a second transcript, 3.5 kb, which also overlaps virR. These results demonstrate that virR and mry are structurally and functionally very similar and show that the former is a trans activator of both M protein and C5a peptidase synthesis.

Adherence and fibronectin binding are environmentally regulated in the group A streptococci

The ability of the pathogenic Gram-positive bacterium Streptococcus pyogenes (group A streptococcus) to bind fibronectin and adhere to respiratory epithelial cells is dependent on a surface protein called protein F. In this study, we have examined the regulation of expression of protein F and have shown that it is environmentally regulated in response to alterations in atmosphere. In six recent clinical isolates expression of protein F was repressed during growth under reduced concentrations of O2. Expression in an anaerobic environment was induced by both superoxide-generating and redox-altering reagents. However, regulation did not involve mry, a gene that controls expression of several streptococcal surface proteins. Protein F was constitutively expressed in one of two laboratory-passaged strains analysed, and in a complementation analysis using an allele of the gene that encodes protein F (prtF) cloned from a regulated strain and expressed in a constitutive strain, the constitutive phenotype was shown to be dominant in trans. Regulation, as monitored by fusion of prtF to a promoterless chloramphenicol acetyltransferase gene, involved transcriptional control. Environmentally induced alterations in protein F expression affected the ability of the bacterium to adhere to epithelial cells, which suggests that the ability to regulate expression of protein F may be important during infection.

Signal transduction and virulence regulation in human and animal pathogens

Pathogens have developed many strategies for survival in animals and humans which possess very effective defense mechanisms. Although there are many different ways, in which pathogenic bacteria solved the problem to overcome the host defense, some common features of virulence mechanisms can be detected even in phylogenetically very distant bacteria (Finlay and Falkow (1989) Microb. Rev. 6, 1375-1383). One important feature is that the regulation of expression of virulence factors and the exact timing of their expression is very important for many of the pathogenic bacteria, as most of them have to encounter different growth situations during an infection cycle, which require a fast adaptation to the new situation by the expression of different factors. This review gives an overview about the mechanisms used by pathogenic bacteria to accomplish the difficult task of regulation of their virulence potential in response to environmental changes. In addition, the relationship of these virulence regulatory systems with other signal transduction mechanisms not involved in pathogenicity is discussed.

Positive transcriptional control of mry regulates virulence in the group A streptococcus

Transcription of the antiphagocytic M protein in the group A streptococcus (Streptococcus pyogenes) is environmentally regulated in response to CO2 and requires Mry, a trans-acting positive regulatory protein. We have examined the role of Mry in environmental regulation by analysing the factors that regulate expression of the gene that encodes Mry (mry). By employing a strategy that utilizes integrational plasmids, it was found that expression of mry requires the participation of DNA sequences that extend 473 base pairs upstream of the Mry coding region. Transcription of mry, as analysed in S1 nuclease protection assays, is initiated from two separate promoters located within this extended regulatory region. Construction and analysis of transcriptional fusions between the mry promoters and a promoterless chloramphenicol acetyltransferase gene demonstrated that mry is autoregulated and environmentally regulated in response to the level of CO2. These data suggest a model for the regulation of virulence in S. pyogenes where positive transcriptional control of mry in response to environmental stimuli regulates the expression of the M protein.

Three different types of organization of the vir regulon in group A streptococci

The DNA of group A streptococci (GAS) encodes several important virulence factors such as the antiphagocytic M protein, the Ig-Fc-binding M-related proteins (FcrA-like and EnnX-like) and the complement factor-inactivating C5a peptidase. The corresponding genes emm, fcrA, ennX, and scpA, respectively, were assumed to be located close together in the GAS genome. Additionally, emm and scpA have been found to be under the positive, coordinate control of the virR locus, which led to the designation "vir regulon" for the corresponding genomic segment. In order to map the vir regulons of many GAS serotypes and to analyse any correlation between the organization of vir regulons and circumscribed heterogeneities within the emm, virR, and scpA genes, an approach using several distinct sets of polymerase chain reaction (PCR) experiments was chosen. By examination of the genomic DNA of 42 GAS isolates from 36 different M serotypes three patterns of vir regulon topography were found. The first, designated "large vir regulon" (LVR), consists of virR--fcrA(-like)--emm--ennX(-like)--scpA. The second, designated "small vir regulon" (SVR), contains virR--emm--scpA, and the last, designated "unusual vir regulon" (UVR), resembles SVR but contains additional heterogeneous sequences between emm and scpA. The patterns correlate with heterogeneities at the 3' ends of the virR and scpA genes, with the M classification system and the occurrence of specific non-coding intervening sequences within the vir regulons. The potential impact of these patterns on models to account for generation of vir regulons is discussed.

Ubiquitous occurrence of virR and scpA genes in group A streptococci

Until now a few serotypes of M-class I group A streptococci (GAS) have been shown to encode VirR, a positive regulatory factor for the coordinate expression of the M protein (emm) and C5a peptidase (scpA) genes. The polymerase chain reaction technique has been applied to the genomic template of 36 GAS serotypes to demonstrate the general presence of VirR (virR) genes and scpA in GAS of both M classes. A virR gene region conserved in size was demonstrated for every strain investigated. Differences between virR genes from GAS of the two M classes were mainly confined to the 3' end of the gene and a region upstream of the gene's promoter. Every M-class II strain and some M-class I isolates were shown to possess a scpA gene of 4.6 kb, the rest of the M-class I GAS harbors a 3.5-kb scpA gene. The additional segment of 1.1 kb in the large-size scpA genes was located within a region of direct repeats at the 3' end of the gene. Among the serotypes encoding a large-size scpA gene a minority exhibits additional sequence variation downstream of the region of direct repeats.