Regulation of mga Transcription in the Group A Streptococcus: Specific Binding of Mga within Its Own Promoter and Evidence for a Negative Regulator

The group A streptococcus (GAS), Streptococcus pyogenes, is a human pathogen that elicits a wide array of suppurative diseases of the skin and throat, including pharyngitis and impetigo. In some cases, GAS infections can lead to serious nonsuppurative sequelae such as rheumatic fever and glomerulonephritis (4). Serious invasive streptococcal diseases, such as necrotizing fasciitis, myositis, and streptococcal toxic shock syndrome, are characterized by infections within normally sterile sites of the human body and can often lead to death in typically healthy individuals (2). The ability of GAS to adapt and grow within the different niches of the human host that it encounters during the course of these infections suggests an environmentally responsive control of its virulence determinants. Mga is a multiple-gene regulator of GAS that responds to environmental signals by activating transcription of several virulence genes. These include the genes encoding the antiphagocytic M protein (emm), C5a peptidase (scpA), and in strains where these are present, M-like proteins (e.g., fcrA, enn, and sph) and serum opacity factor (sof) (5, 8, 10, 17, 24, 28). Mga functions as a DNA-binding protein that interacts directly with sequences in the promoters of two Mga-regulated genes, emm and scpA (22). Binding of Mga occurs at a 45-bp site within each of the two promoters centered 52 bp upstream of the start of transcription, which may allow Mga to interact with RNA polymerase and stimulate transcription initiation (22). Mga is also required for its own positive autoregulation, and this requires 473 bp of DNA located directly upstream of its coding sequence (25). This extended Pmga region contains two distinct promoters: a distal start of transcription, P1, and a start site located proximal to mga, P2 (12, 25). Studies on an M type 6 GAS strain have determined that the P2 promoter represents the major start site for mga transcription. It exhibits about 10-fold more activity than the P1 promoter (25). The use of dual promoters (P1 and P2) within Pmga has been demonstrated for other GAS serotypes as well (e.g., M types 12 and 49) and most likely represents a general mechanism of mga expression (3, 27). Currently, very little is known about the specifics of mga regulation and the role of Mga in this process. Many bacterial transcription factors regulate their own expression by binding directly to their own promoters (14, 19). To aid in the identification of other Mga-regulated promoters in GAS, a consensus Mga binding sequence was derived from Pemm and PscpA binding studies (22). Inspection of the Pmga region revealed no significant sequence homology with the consensus Mga binding element found upstream of Pemm and PscpA. This suggests that Mga does not bind directly to Pmga as it does to the other promoters. However, the Mga protein possesses two distinct N-terminal domains that show predicted helix-turn-helix DNA-binding motifs (26). Therefore, it is possible that Mga binding to Pmga involves different residues of the protein than those that bind to Pemm and PscpA. To investigate a possible direct role of Mga in autoregulation of mga expression, the ability of Mga to bind specifically within the Pmga regulatory region was investigated.