Coregulation of type 12 M protein and streptococcal C5a peptidase genes in group A streptococci: evidence for a virulence regulon controlled by the virR locus

Abstract

Group A streptococci express at least two surface-associated virulence factors, the antiphagocytic M protein and the antichemotactic streptococcal C5a peptidase (SCP). Preliminary evidence suggested that the biosynthesis of these two proteins is coordinately controlled and subject to simultaneous phase variation. To explore this possibility further, a series of phase-switching and phase-locked M-variants were assayed for SCP by enzyme-linked immunosorbent assay inhibition and for SCP-specific mRNA by dot blot hybridization. All Mcultures produced diminished amounts of SCP antigen and specific mRNA, whereas revertants produced quantities equivalent to those of the wild-type M' culture. A phase-locked strain that harbors a deletion in a region upstream of the M12 and SCP genes, termed the virR locus, failed to produce SCP antigen or SCP-specific transcripts. The SCP-specific transcript produced by M' bacteria was shown by Northern (RNA) blot hybridization to be 4 kilobases in size, distinguishing it from the transcript which encodes M protein. These data demonstrate that phase switching of both SCP and M12 proteins is at the transcriptional level and that expression is under the control of the upstream virR locus. We propose that the genetic determinants of these proteins and of colony morphology comprise a virulence regulon.The survival and multiplication of bacterial pathogens in a susceptible host depend on their abilities to acquire appropriate nutrients, interact with tissue receptors, and resist host defenses. A recent review (10) noted that the expression of multiple virulence determinants by gram-negative pathogens is a finely tuned process which is responsive to environmental changes and the general physiological state of the bacterial cell. It is now apparent that these virulence genes are globally controlled by a master gene which couples external stimuli to response pathways (10).The avoidance of human immunological defenses by group A streptococci is dependent on the following surface components: a diffuse hyaluronic capsule which mimics the ground substance of animal tissues (25); immunoglobulin G Fc receptors, which may disrupt recognition by antibodies (6b); a C5a peptidase which destroys chemotactic signals (23); and M protein, which interferes with the deposition of C3b opsonin to prevent phagocytic uptake (8, 9). M protein, thought to be the key determinant of virulence, has been the subject of intensive investigation. This dimeric coiled-coil molecule protrudes from the cell wall, where it both blocks the deposition of C3b opsonin and limits the interaction of bound C3b with receptors on polymorphonuclear leukocytes (8, 9). As a result, M+ streptococci are resistant to phagocytosis in the absence of type-specific M protein antibody and complement.Group A streptococcal cultures have long been recognized as genetically unstable (6,17,20). The expression of M protein on their surfaces, colony morphology, and virulence were reported to vary dramatically both in laboratory cultures (6, 17) and in st...

Identification of a gene that positively regulates expression of listeriolysin, the major virulence factor of listeria monocytogenes.

“…Similarly, morphological changes observed with group A streptococci grown under laboratory conditions are associated with a loss of virulence. These stably locked avirulentphase mutants have all been shown to harbor small deletions within the virR locus (25). The data presented here add L. monocytogenes to this list of pathogenic bacteria exhibiting lowered virulence due to a deletion within a locus regulatory for a virulence factor.…”

Mry, a trans-acting positive regulator of the M protein gene of Streptococcus pyogenes with similarity to the receptor proteins of two-component regulatory systems

“…Because M12 strains with reduced production both of M12 and of the CSa peptidase contained deletions upstream of emml2, Simpson et al (33) have named this region vir. Our sequence analysis indicates that this vir region lies within the mry gene, so it seemed possible that the Mry protein is needed for the production of other potential virulence factors.…”

“…These deletions appear to render the strains M12 negative. Because they also reduce production of C5a peptidase, these deletions have been named virR (33,34). We wished to clarify the relationship between the virR mutants and mry-l and to gain a better understanding of the mode of action of the mry-1 insertion mutation.…”

Generation and Surface Localization of Intact M Protein inStreptococcus pyogenesAre Dependent onsagA

“…A major component of this global regulatory circuit found in all GAS strains is the multiple gene regulator Mga, which activates expression of many virulence genes in an environmentally controlled fashion. These include emm (8,10) and scpA (10,50), and in strains where they are present, the emm-like genes (26, 44) sic (1), scl, and sof (37). Mga, which is also autoregulated, controls gene expression by direct interaction with the promoter region of the genes it regulates (36).…”