Structure of Rot, a global regulator of virulence genes in Staphylococcus aureus

SarS and Rot are necessary for the repression of lukED and lukSF-PV in Staphylococcus aureus

IMPORTANCE

The leukocidins play an important role in disarming the host immune system and promoting infection. While both SarS and Rot have been established as repressors of leukocidins, the importance of each repressor in infection is unclear. Here, we demonstrate that repression by SarS and Rot is not additive and show that in addition to upregulating expression of each other, they are also able to bind concurrently to the leukocidin promoters. These findings suggest that both repressors are necessary for maximal repression of lukED and lukSF-PV and illuminate another complex relationship among Staphylococcus aureus virulence regulators.

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 α-helices (α1-α5), three β-strands (β1-β3) and multiple loops. The putative DNA binding region of SarA is constituted with α3, α4, β2, and β3, whereas, its dimerization seems to occur using α1, α2, and α5. Interestingly, many SarA-like proteins are dimeric and use three or more helices for their dimerization. To clearly understand the roles of helix α1 in the dimerization, we have constructed and purified a SarA mutant (Δα1) that lacks helix α1. Our in-depth studies with Δα1 indicate that the helix α1 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.

Virulence Factor Targeting of the Bacterial Pathogen Staphylococcus aureus for Vaccine and Therapeutics

BACKGROUND

Staphylococcus aureus is a major bacterial pathogen capable of causing a range of infections in humans from gastrointestinal disease, skin and soft tissue infections, to severe outcomes such as sepsis. Staphylococcal infections in humans can be frequent and recurring, with treatments becoming less effective due to the growing persistence of antibiotic resistant S. aureus strains. Due to the prevalence of antibiotic resistance, and the current limitations on antibiotic development, an active and highly promising avenue of research has been to develop strategies to specifically inhibit the activity of virulence factors produced S. aureus as an alternative means to treat disease.

OBJECTIVE

In this review we specifically highlight several major virulence factors produced by S. aureus for which recent advances in antivirulence approaches may hold promise as an alternative means to treating diseases caused by this pathogen. Strategies to inhibit virulence factors can range from small molecule inhibitors, to antibodies, to mutant and toxoid forms of the virulence proteins.

CONCLUSION

The major prevalence of antibiotic resistant strains of S. aureus combined with the lack of new antibiotic discoveries highlight the need for vigorous research into alternative strategies to combat diseases caused by this highly successful pathogen. Current efforts to develop specific antivirulence strategies, vaccine approaches, and alternative therapies for treating severe disease caused by S. aureus have the potential to stem the tide against the limitations that we face in the post-antibiotic era.

A Surfactant-Induced Functional Modulation of a Global Virulence Regulator from Staphylococcus aureus

Triton X-100 (TX-100), a useful non-ionic surfactant, reduced the methicillin resistance in Staphylococcus aureus significantly. Many S. aureus proteins were expressed in the presence of TX-100. SarA, one of the TX-100-induced proteins, acts as a global virulence regulator in S. aureus. To understand the effects of TX-100 on the structure, and function of SarA, a recombinant S. aureus SarA (rSarA) and its derivative (C9W) have been investigated in the presence of varying concentrations of this surfactant using various probes. Our data have revealed that both rSarA and C9W bind to the cognate DNA with nearly similar affinity in the absence of TX-100. Interestingly, their DNA binding activities have been significantly increased in the presence of pre-micellar concentration of TX-100. The increase of TX-100 concentrations to micellar or post-micellar concentration did not greatly enhance their activities further. TX-100 molecules have altered the secondary and tertiary structures of both proteins to some extents. Size of the rSarA-TX-100 complex appears to be intermediate to those of rSarA and TX-100. Additional analyses show a relatively moderate interaction between C9W and TX-100. Binding of TX-100 to C9W has, however, occurred by a cooperative pathway particularly at micellar and higher concentrations of this surfactant. Taken together, TX-100-induced structural alteration of rSarA and C9W might be responsible for their increased DNA binding activity. As TX-100 has stabilized the somewhat weaker SarA-DNA complex effectively, it could be used to study its structure in the future.

Crystallographic studies of SarV, a global regulator from Staphylococcus aureus

SarV, a member of the SarA protein family, is a global transcriptional regulator which has been reported to be involved in the regulation of autolysis in Staphylococcus aureus. In this study, SarV from S. aureus was successfully cloned, expressed, purified and crystallized. X-ray diffraction data were collected to 2.10 Å resolution. The crystals of SarV belonged to the monoclinic space group P21, with unit-cell parameters a = 36.40, b = 119.64, c = 66.80 Å, α = γ = 90, β = 98.75°. The Matthews coefficient and the solvent content were estimated to be 2.57 Å(3) Da(-1) and 52%, respectively, suggesting the presence of four molecules in the asymmetric unit. The results of size-exclusion chromatography (SEC) indicated that S. aureus SarV exists as a homodimer in solution. Unfortunately, the structure cannot be solved by molecular replacement because of the low sequence identity of S. aureus SarV to known structures. Further phase determination by selenomethionine single-wavelength anomalous dispersion (SAD) and the heavy-atom method is in progress.

Structure of the DNA-binding domain of the response regulator SaeR fromStaphylococcus aureus

The SaeR/S two-component regulatory system is essential for controlling the expression of many virulence factors inStaphylococcus aureus. SaeR, a member of the OmpR/PhoB family, is a response regulator with an N-terminal regulatory domain and a C-terminal DNA-binding domain. In order to elucidate how SaeR binds to the promoter regions of target genes, the crystal structure of the DNA-binding domain of SaeR (SaeRDBD) was solved at 2.5 Å resolution. The structure reveals that SaeRDBDexists as a monomer and has the canonical winged helix–turn–helix module. EMSA experiments suggested that full-length SaeR can bind to the P1 promoter and that the binding affinity is higher than that of its C-terminal DNA-binding domain. Five key residues on the winged helix–turn–helix module were verified to be important for binding to the P1 promoterin vitroand for the physiological function of SaeRin vivo.

Chemical and thermal unfolding of a global staphylococcal virulence regulator with a flexible C-terminal end

SarA, a Staphylococcus aureus-specific dimeric protein, modulates the expression of numerous proteins including various virulence factors. Interestingly, S. aureus synthesizes multiple SarA paralogs seemingly for optimizing the expression of its virulence factors. To understand the domain structure/flexibility and the folding/unfolding mechanism of the SarA protein family, we have studied a recombinant SarA (designated rSarA) using various in vitro probes. Limited proteolysis of rSarA and the subsequent analysis of the resulting protein fragments suggested it to be a single-domain protein with a long, flexible C-terminal end. rSarA was unfolded by different mechanisms in the presence of different chemical and physical denaturants. While urea-induced unfolding of rSarA occurred successively via the formation of a dimeric and a monomeric intermediate, GdnCl-induced unfolding of this protein proceeded through the production of two dimeric intermediates. The surface hydrophobicity and the structures of the intermediates were not identical and also differed significantly from those of native rSarA. Of the intermediates, the GdnCl-generated intermediates not only possessed a molten globule-like structure but also exhibited resistance to dissociation during their unfolding. Compared to the native rSarA, the intermediate that was originated at lower GdnCl concentration carried a compact shape, whereas, other intermediates owned a swelled shape. The chemical-induced unfolding, unlike thermal unfolding of rSarA, was completely reversible in nature.

Structure-based functional characterization of repressor of toxin (Rot), a central regulator of Staphylococcus aureus virulence

Staphylococcus aureus is responsible for a large number of diverse infections worldwide. In order to support its pathogenic lifestyle, S. aureus has to regulate the expression of virulence factors in a coordinated fashion. One of the central regulators of the S. aureus virulence regulatory networks is the transcription factor repressor of toxin (Rot). Rot plays a key role in regulating S. aureus virulence through activation or repression of promoters that control expression of a large number of critical virulence factors. However, the mechanism by which Rot mediates gene regulation has remained elusive. Here, we have determined the crystal structure of Rot and used this information to probe the contribution made by specific residues to Rot function. Rot was found to form a dimer, with each monomer harboring a winged helix-turn-helix (WHTH) DNA-binding motif. Despite an overall acidic pI, the asymmetric electrostatic charge profile suggests that Rot can orient the WHTH domain to bind DNA. Structure-based site-directed mutagenesis studies demonstrated that R(91), at the tip of the wing, plays an important role in DNA binding, likely through interaction with the minor groove. We also found that Y(66), predicted to bind within the major groove, contributes to Rot interaction with target promoters. Evaluation of Rot binding to different activated and repressed promoters revealed that certain mutations on Rot exhibit promoter-specific effects, suggesting for the first time that Rot differentially interacts with target promoters. This work provides insight into a precise mechanism by which Rot controls virulence factor regulation in S. aureus.