Complementation of divergent mga genes in group A Streptococcus

Characterization of the Group A Streptococcus Mga virulence regulator reveals a role for the C-terminal region in oligomerization and transcriptional activation

The Group A Streptococcus (GAS) is a strict human pathogen that causes a broad spectrum of illnesses. One of the key regulators of virulence in GAS is the transcriptional activator Mga, which co-ordinates the early stages of infection. Although the targets of Mga have been well characterized, basic biochemical analyses have been limited due to difficulties in obtaining purified protein. In this study, high-level purification of soluble Mga was achieved, enabling the first detailed characterization of the protein. Fluorescence titrations coupled with filter-binding assays indicate that Mga binds cognate DNA with nanomolar affinity. Gel filtration analyses, analytical ultracentrifugation and co-immunoprecipitation experiments demonstrate that Mga forms oligomers in solution.Moreover, the ability of the protein to oligomerize in solution was found to correlate with transcriptional activation; DNA binding appears to be necessary but insufficient for full activity. Truncation analyses reveal that the uncharacterized C-terminal region of Mga, possessing similarity to phosphotransferase system EIIB proteins, plays a critical role in oligomerization and in vivo activity. Mga from a divergent serotype was found to behave similarly, suggesting that this study describes a general mechanism for Mga regulation of target virulence genes within GAS and provides insight into related regulators in other Gram-positive pathogens.

Defining the Mga regulon: comparative transcriptome analysis reveals both direct and indirect regulation by Mga in the group A streptococcus

The regulator Mga in the group A streptococcus (GAS) is known to directly activate several virulence genes important for colonization and immune evasion. Transcriptome analysis comparing two mga-1 serotypes (M1 SF370, M6 JRS4) and one mga-2 serotype (M4 GA40634) against their isogenic mga-inactivated strains uncovered a broader Mga regulon profile containing both activated and repressed genes with predicted functions primarily related to sugar metabolism. This was reflected in the altered abilities of M1 and M4 Mga mutants to grow in chemically defined media with a single sugar source compared with their wild-type counterparts. Although the M1 and M4 Mga profiles were similar, the M6 JRS4 was clearly distinct, even from other M6 strains. Real-time RT-PCR and Northern blots confirmed that established core Mga regulon genes directly activated by Mga (emm, scpA, sof, fba) exhibited the highest activation levels across all strains tested. Spy2036 encoding a cytosolic hypothetical protein was highly activated in all three serotypes and was called gene regulated by Mga (grm). Mga bound directly to Pgrm, which overlaps the Mga-regulated Psof in OF+ strains, suggesting that grm is part of the core Mga regulon and Mga is able to activate divergently transcribed genes from a single site. Furthermore, Mga activated speB when detectable in the wild-type strain, although direct binding of Mga to PspeB could not be demonstrated. Thus, Mga is able to both directly and indirectly regulate genes shown to be important for virulence and the metabolic homeostasis of GAS.

Domains required for transcriptional activation show conservation in the mga family of virulence gene regulators

Mga, or the multigene regulator of the group A streptococcus (GAS) (Streptococcus pyogenes), is a transcriptional regulator of virulence genes important for colonization and immune evasion. All serotypes of the GAS possess one of two divergent mga alleles (mga-1 or mga-2), and orthologues of Mga have also been identified in other pathogenic streptococci. To date, the only functional motifs established within Mga are two amino-terminal DNA-binding domains (HTH-3 and HTH-4). To uncover novel domains, a random mutagenesis screen using an M6 Mga (mga-1) was undertaken to find mutations leading to a defect in transcriptional activation of the Mga-regulated emm gene. In addition to mutations in the established DNA-binding domains, the screen also revealed mutations in a region conserved among several Mga orthologues. Alanine scanning helped resolve the boundaries of this conserved Mga domain (CMD-1) spanning from residues 10 to 15 of the protein, with the two flanking amino acid residues likely involved in protein stability. Transcriptional reporter analyses demonstrated the importance of CMD-1 for activation of Pemm and autoactivation of Pmga in the serotype M6 Mga. Mutational analyses showed that both CMD-1 and HTH-4 are also necessary for activation of the promoter target Pmrp in a divergent serotype M4 Mga (mga-2), suggesting a conserved functionality. However, in contrast to M6, the M4 Mga mutants did not show a defect in autoregulation. Mutation of similar conserved residues in the Mga-like regulator DmgB from S. dysgalactiae subsp. dysgalactiae showed that CMD-1 and HTH-4 are critical for transcriptional activation in this orthologue, implying that a common mechanism of virulence gene activation may exist for members of the Mga family of regulators.

Identification of residues responsible for the defective virulence gene regulator Mga produced by a natural mutant of Streptococcus pyogenes

Mga is a transcriptional regulator in the pathogen Streptococcus pyogenes that positively activates several important virulence genes involved in colonization and immune evasion in the human host. A naturally occurring mutant of Mga that is defective in its ability to activate transcription has been identified in the serotype M50 strain B514-Sm. Sequence alignment of the defective M50 Mga with the fully functional Mga from serotypes M4 and M49 revealed only three amino acid changes that might result in a defective protein. Electrophoretic mobility shift assays using purified M50 and M4 maltose binding protein-Mga found that both exhibited DNA-binding activity towards regulated promoters. Thus, the significance of each residue for the functionality of M50 Mga was explored through introduction of "gain-of-function" mutations based on M4 Mga. Transcriptional studies of the mutant alleles under both constitutive (PrpsL) and autoactivated (Pmga4) promoters illustrated that an arginine-to-methionine change at position 461 of M50 Mga protein fully restored activation of downstream genes. Western blot analyses of steady-state Mga levels suggest that the M461 residue may play a role in overall conformation and protein stability of Mga. However, despite the conservation of the M461 protein among all other Mga proteins, it does not appear to be necessary for activity in a divergent M6 Mga. These studies highlight the potential differences that exist between divergent Mga proteins in this important human pathogen.

Transcriptional activation of sclA by Mga requires a distal binding site in Streptococcus pyogenes

Streptococcus pyogenes (the group A streptococcus [GAS]) is a medically significant pathogen of humans, causing a range of diseases from pharyngitis to necrotizing fasciitis. Several important GAS virulence genes are under the control of a pleiotropic regulator called Mga, or the multiple gene regulator of GAS, including the gene encoding the streptococcal collagen-like protein, or sclA. Analysis of the genome sequence upstream of sclA revealed two potential Mga-binding sites with homology to the published Mga-binding element, which were called PsclA-I (distal) and PsclA-II (proximal) based on their location relative to a predicted start of transcription. Primer extension was used to confirm that the Mga-dependent transcriptional start site for sclA was located adjacent to the proximal PsclA-II binding site. By using overlapping PsclA promoter probes and purified Mga-His fusion protein, it was shown by electrophoretic mobility shift assays that, unlike other Mga-regulated promoters, Mga binds only to a distal DNA-binding site (PsclA-I). Binding of Mga to PsclA-I could be competed with cold probes corresponding to known Mga-regulated promoters (Pemm, PscpA, and Pmga) but not with a nonspecific probe or the proximal PsclA-II fragment. With the use of a plasmid-based green fluorescent protein transcriptional reporter system, the full-length PsclA was not sufficient to reproduce normal Mga-regulated activation. However, studies using a single-copy gusA transcriptional reporter system integrated at the native sclA chromosomal locus clearly demonstrated that the distal PsclA-I binding site is required for Mga regulation. Therefore, PsclA represents a new class of Mga-regulated promoters that requires a single distal binding site for activation.

Expression of the secondary sigma factor sigmaX in Streptococcus pyogenes is restricted at two levels

Secondary RNA polymerase sigma factors in many bacteria are responsible for regulating a vast range of processes including virulence. A protein (sigma(X)) in the gram-positive human pathogen Streptococcus pyogenes (the group A Streptococcus or GAS) was recently shown to function in vitro as a secondary sigma factor. We report here the isolation of a mutant in which both sigX genes are inactivated, show that sigma(X) functions in GAS cells, and show that the amount of sigma(X) is controlled at two levels. Primer extension analysis indicates that sigX transcription is low in GAS cells grown in Todd-Hewitt yeast broth, and immunoblot assays with a sigma(X)-specific polyclonal antibody demonstrate that the protein does not accumulate in these cells. To increase the level of sigX transcription in GAS, we constructed a strain that constitutively expresses the sigX gene from a heterologous promoter. Expression of sigX from this promoter led to transcription of the sigma(X)-dependent cinA promoter in GAS cells. We found that expression of the sigX gene in a clpP mutant strain resulted in greater accumulation of sigma(X) protein, which resulted in higher levels of transcription from the sigma(X)-dependent promoters cinA, smf, and cglA. In addition, a clpP mutant containing sigX only at its wild-type loci on the chromosome generated more transcription from the sigma(X)-dependent cinA promoter than did the wild-type parental strain. Therefore, sigma(X) activity in GAS is limited by low-level transcription of the sigX structural genes and by clpP, which appears to negatively regulate sigma(X) accumulation.

Two DNA-binding domains of Mga are required for virulence gene activation in the group A streptococcus

Mga is a DNA-binding protein that activates expression of several important virulence genes in the group A streptococcus (GAS), including those encoding M protein (emm), C5a peptidase (scpA) and Mga (mga). To determine the functionality of four potential helix-turn-helix DNA-binding motifs (HTH1-HTH4) identified within the amino-terminus of Mga, alanine substitutions were introduced within each domain in a MBP-Mga fusion allele and purified proteins were assayed for binding to Mga-specific promoter fragments (Pmga, PscpA and Pemm) in vitro. Although HTH-1 and HTH-2 mutations showed wild type DNA-binding activity, an altered HTH-3 domain resulted in reduced binding to the three promoters and an HTH-4 mutant was devoid of detectable binding activity. Plasmid-encoded expression of the HTH-3 and HTH-4 alleles from a constitutive promoter (Pspac) in the mga-deleted GAS strain JRS519 demonstrated that Mga-regulated emm expression correlated directly to the DNA-binding activity observed for each mutant protein in vitro. Single-copy expression of HTH-3 and HTH-4 from their native Pmga resulted in a dramatic reduction in autoregulated mga expression in both mutant strains. Thus, Mga appears to contain two DNA-binding domains (HTH-3 and HTH-4) that are required for direct activation of the Mga virulence regulon in vivo.

Restoration of Mga function to a Streptococcus pyogenes strain (M Type 50) that is virulent in mice

The Mga protein in B514Sm, a Streptococcus pyogenes strain isolated as a mouse pathogen, contains amino acid substitutions at conserved sites that render the protein defective. Replacement of mga50 with the functional homolog mga4.1 restored full expression of Mga-regulated proteins. Restoration of Mga function did not affect fibrinogen binding, nor did it affect virulence in several mouse models of group A streptococcus infection.

Identification of a novel glycoprotein-binding activity in Streptococcus pyogenes regulated by the mga gene

The interaction between Streptococcus pyogenes and the host cell surface is not completely understood. Characterization of the adhesion mechanisms of the bacterium to the host cell surface is needed in order to develop new vaccines and anti-adhesion drugs. The presence of glycoprotein-binding activities among streptococcal strains was investigated. An activity binding to thyroglobulin, fetuin, asialofetuin and mucin but not non-glycosylated proteins was found to be present in the majority of the S. pyogenes strains studied. Cross-inhibition experiments suggested that the glycoproteins share a common structure recognized by the bacteria. The glycoprotein-binding activity was found to be proteinaceous, tightly attached to the bacterial surface and it also mediated the adherence of bacteria to solid surfaces coated with glycoproteins. The activity was found by transposon mutagenesis and complementation to be regulated by the multiple-gene regulator Mga, which has been implicated as a regulator of S. pyogenes virulence factors.

Attenuated expression of the mga virulence regulon in an M serotype 50 mouse-virulent group A streptococcal strain

The attenuated expression of virulence genes found in a group A streptococcal strain that is naturally pathogenic for mice was postulated to result from a defect in the strain's multigene regulator, Mga. The sequence of the mga gene reveals three amino acid changes in the gene product that might affect protein function. The defect in the mga gene was complemented by providing either the closely similar mga4 allele or a more divergent mga1 allele in trans. Complementation increased the amount of emm50 transcript and the quantity of surface-extractable M protein, restoring virulence function.

Regulation of mga transcription in the group A streptococcus: specific binding of mga within its own promoter and evidence for a negative regulator

Transcription of mga, encoding the multiple virulence gene regulator of the group A streptococcus, is positively autoregulated. This regulation requires a DNA region (Pmga) that contains both a promoter proximal to mga (P2) and a promoter located further upstream (P1). To determine if Mga has a direct role in this process, its ability to bind to specific sequences within Pmga was tested. A purified fusion of Mga to the C-terminal end of maltose-binding protein (MBP-Mga), encoded by malE-mga, was shown previously to bind to the promoter regions of Mga-regulated genes, including scpA and emm. We report here that MBP-Mga can function in vivo to regulate emm and mga. Electrophoretic mobility shift assays and DNase I footprinting were used to demonstrate specific binding of MBP-Mga to two ca. 59-bp binding sites in Pmga centered around bases -108 and -180 from the major P2 start of transcription. Mga binding sites from Pemm and PscpA were shown to compete for binding at the two Pmga sites, suggesting that the same domain of Mga interacts at all of these promoter targets. Deletion of the distal Pmga binding site (site I) in vivo resulted in loss of Mga-dependent transcription from the P2 start. However, the same lesion resulted in an increase in P1 transcription that was independent of Mga. This suggests the existence of a repressor of mga transcription with a binding site overlapping those of Mga.

Expression of Two Different Antiphagocytic M Proteins by Streptococcus pyogenes of the OF+ Lineage

All clinical isolates of Streptococcus pyogenes (group A streptococcus) share the ability to resist phagocytosis and grow in human blood. In many strains, this property is due to the expression of a single antiphagocytic M protein, while other strains express more than one M-like molecule, of which the role in phagocytosis resistance is unclear. In particular, all S. pyogenes strains of the OF+ lineage, representing approximately half of all isolates, express two M-like proteins, Mrp and Emm, which are immunologically unrelated. These two proteins bind different ligands that have been implicated in phagocytosis resistance: Mrp binds fibrinogen and Emm binds the complement inhibitor C4BP. Using a clinical isolate of the common serotype 22, we created mutants affected in the mrp and emm genes and characterized them in phagocytosis experiments and by electron microscopy. A double mutant mrp−emm− showed strongly decreased resistance to phagocytosis, while mrp− and emm− single mutants grew well in blood. However, optimal growth required the expression of both Mrp and Emm. Experiments in which coagulation was inhibited using the specific thrombin inhibitor, hirudin, rather than heparin, indicated that Emm is more important than Mrp for resistance to phagocytosis. Tuftlike surface structures typical for S. pyogenes were still present in the mrp−emm− double mutant, but not in a mutant affected in the regulatory gene mga, indicating that the presence of these surface structures is not directly correlated to phagocytosis resistance. Our data imply that OF+ strains of S. pyogenes express two antiphagocytic M proteins with different ligand-binding properties.

Role of mga in growth phase regulation of virulence genes of the group A streptococcus

To determine whether growth phase affects the expression of mga and other virulence-associated genes in the group A streptococcus (GAS), total RNA was isolated from the serotype M6 GAS strain JRS4 at different phases of growth and transcript levels were quantitated by hybridization with radiolabeled DNA probes. Expression of mga (which encodes a multiple gene regulator) and the Mga-regulated genes emm (which encodes M protein) and scpA (which encodes a complement C5a peptidase) was found to be maximal in exponential phase and shut off as the bacteria entered stationary phase, while the housekeeping genes recA and rpsL showed constant transcript levels over the same period of growth. Expression of mga from a foreign phage promoter in a mga-deleted GAS strain (JRS519) altered the wild-type growth phase-dependent transcription profile seen for emm and scpA, as well as for mga. Therefore, the temporal control of mga expression requires its upstream promoter region, and the subsequent growth phase regulation of emm and scpA is Mga dependent. A number of putative virulence genes in JRS4 were shown not to require Mga for their expression, although several exhibited growth phase-dependent regulation that was similar to mga, i.e., slo (which encodes streptolysin O) and plr (encoding the plasmin receptor/glyceraldehyde-3-phosphate dehydrogenase). Still others showed a markedly different pattern of expression (the genes for the superantigen toxins MF and SpeC). These results suggest the existence of complex levels of global regulation sensitive to growth phase that directly control the expression of virulence genes and mga in GAS.