Determining the Roles of a Conserved α-Helix in a Global Virulence Regulator from Staphylococcus aureus

SarA, a pleiotropic transcription regulator, is encoded by Staphylococcus aureus, a pathogenic bacterium. The expression of many virulence and non-virulence genes in S. aureus is modulated by this regulator. Structural studies have shown it to be a winged-helix DNA-binding protein carrying two monomers. Each SarA monomer is composed of five alpha-helices (alpha 1-alpha 5), three beta-strands (beta 1-beta 3) and multiple loops. The putative DNA binding region of SarA is constituted with alpha 3, alpha 4, beta 2, and beta 3, whereas, its dimerization seems to occur using alpha 1, alpha 2, and alpha 5. Interestingly, many SarA-like proteins are dimeric and use three or more helices for their dimerization. To clearly understand the roles of helix alpha 1 in the dimerization, we have constructed and purified a SarA mutant (Delta alpha 1) that lacks helix alpha 1. Our in-depth studies with Delta alpha 1 indicate that the helix alpha 1 is critical for preserving the structure, DNA binding activity and thermodynamic stability of SarA. However, the helix has little affected its dimerization ability. Possible reasons for such anomaly have been discussed at length.