Characterization of the TrxSR Two-Component Signal Transduction System of Streptococcus pyogenes and its Role in Virulence Regulation

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The Gram-positive group A streptococcus (GAS) is a strict human pathogen, which causes a wide variety of infections, ranging in severity from minor to life threatening. In order to cause such a diverse array of diseases, GAS utilizes two-component signal transduction systems (TCS) to coordinately regulate sets of virulence genes in response to changing host conditions. The present study investigates the role of the TrxSR TCS in the regulation of virulence of the GAS. Using an insertional inactivation mutation in TrxR in serotype M1 MGAS5005, transcriptome studies established that TrxR activates transcription of Mga-regulated virulence genes, a separate non-TCS regulatory pathway controlling factors important for immune evasion and colonization. Transcriptional reporter fusions of Pmga to firefly luciferase revealed that the TrxR regulation occurs through the Pmga promoter. Additionally, electrophoretic mobility shift assays using purified His-MBP tagged TrxR established specific binding of TrxR to Pmga, although the interaction appeared to be transient. To determine the importance of signal transduction for TrxR-mediated regulation of the Mga regulon and virulence, an in vitro reconstitution assay was performed with purified TrxR and TrxS. Using both wild type and mutated forms of the TrxSR proteins, we demonstrated that TrxSR is a functional two-component phosphorelay system. Interestingly, phosphorylation of TrxR did not appear to be critical for DNA binding and regulation, since a TrxR D55A mutation did not change the expression of TrxR regulated genes in GAS based on EMSA and qPCR. In order to investigate whether there is a functional conservation of TrxR's involvement in GAS virulence regulation, mutations were made in serotype M4 and M49 strains representing either throat only or generalist strains. We have determined that TrxR regulates mga and Mga-regulated genes (emm, arp) in the M4 and M49 backgrounds, suggesting conservation of TrxR's role in virulence regulation. Overall, TrxSR represents a functional TCS that appears to directly regulate the Mga virulence regulon independent of phosphorelay. Furthermore, the functional conservation of TrxR regulation of Mga in other serotypes suggests a conserved role for its involvement in virulence regulation in GAS.