Design and development of a novel genetic probe for the analysis of repressor-operator interactions

Corynebacterium diphtheriae: Diphtheria Toxin, the tox Operon, and Its Regulation by Fe2+ Activation of apo-DtxR

Diphtheria is one of the most well studied of all the bacterial infectious diseases. These milestone studies of toxigenic Corynebacterium diphtheriae along with its primary virulence determinant, diphtheria toxin, have established the paradigm for the study of other related bacterial protein toxins. This review highlights those studies that have contributed to our current understanding of the structure-function relationships of diphtheria toxin, the molecular mechanism of its entry into the eukaryotic cell cytosol, the regulation of diphtheria tox expression by holo-DtxR, and the molecular basis of transition metal ion activation of apo-DtxR itself. These seminal studies have laid the foundation for the protein engineering of diphtheria toxin and the development of highly potent eukaryotic cell-surface receptor-targeted fusion protein toxins for the treatment of human diseases that range from T cell malignancies to steroid-resistant graft-versus-host disease to metastatic melanoma. This deeper scientific understanding of diphtheria toxin and the regulation of its expression have metamorphosed the third-most-potent bacterial toxin known into a life-saving targeted protein therapeutic, thereby at least partially fulfilling Paul Erlich's concept of a magic bullet-"a chemical that binds to and specifically kills microbes or tumor cells."

The SH3-like domain switches its interaction partners to modulate the repression activity of mycobacterial iron-dependent transcription regulator in response to metal ion fluctuations

Iron-dependent regulator (IdeR), a metal ion-activated pleiotropic transcription factor, plays a critical role in maintaining the intracellular iron homeostasis in Mycobacteria, which is important for the normal growth of the cells. This study was initially performed in an attempt to elucidate all potential interactions between the various domains of IdeR that occur in living mycobacterial cells. This led to a hitherto unidentified self-association for the SH3-like domain of IdeR. Further studies demonstrate that the SH3-like domain interacts with different partners in the dimeric forms of IdeR depending on the levels of metal ions in the environment: it undergoes inter-subunit self-association in the metal-free DNA-non-binding form, but interacts with the N-terminal domain in the metal-bound DNA-binding form in an intra-subunit manner to finely modulate the transcription repression activity of IdeR. Our more detailed mapping studies reveal that the SH3-like domain uses an overlapping surface to participate in these two interactions, which therefore occur in a mutually exclusive fashion. This novel mechanism would allow an effective and cooperative interconversion between the two functional forms of IdeR. Our data also demonstrate that a disturbance of the interactions involving the SH3-like domain impairs the transcription repression activity of IdeR and delays the growth of mycobacterial cells.

Genetic and biophysical studies of diphtheria toxin repressor (DtxR) and the hyperactive mutant DtxR(E175K) support a multistep model of activation

The diphtheria toxin repressor (DtxR) from Corynebacterium diphtheriae is the prototypic member of a superfamily of transition metal ion-activated transcriptional regulators that have been isolated from Gram-positive prokaryotes. Upon binding divalent transition metal ions, the N-terminal domain of DtxR undergoes a dynamic structural organization leading to homodimerization and target DNA binding. We have used site-directed mutagenesis and NMR analysis to probe the mechanism by which apo-DtxR transits from an inactive to a fully active repressor upon metal ion binding. We demonstrate that the ancillary metal-binding site mutant DtxR(H79A) requires higher concentrations of metal ions for activation both in vivo and in vitro, providing a functional correlation to the proposed cooperativity between ancillary and primary binding sites. We also demonstrate that the C-terminal src homology 3 (SH3)-like domain of DtxR functions to modulate repressor activity by (i) binding to the polyprolyl tether region between the N- and C-terminal domains, and (ii) destabilizing the ancillary binding site, leading to full inactivation of the repressor. Finally, we show by NMR analysis that the hyperactive phenotype of DtxR(E175K) results from the stabilization of a structural intermediate in the activation process. Taken together, the data presented support a multistep model for the activation of apo-DtxR by transition metal ions.

The src homology 3-like domain of the diphtheria toxin repressor (DtxR) modulates repressor activation through interaction with the ancillary metal ion-binding site

The diphtheria toxin repressor (DtxR) is a transition metal ion-activated repressor that acts as a global regulatory element in the control of iron-sensitive genes in Corynebacterium diphtheriae. We recently described (L. Sun, J. C. vanderSpek, and J. R. Murphy, Proc. Natl. Acad. Sci. USA 95:14985-14990, 1998) the isolation and in vivo characterization of a hyperactive mutant of DtxR, DtxR(E175K), that appeared to be constitutively active. We demonstrate here that while DtxR(E175K) remains active in vivo in the presence of 300 micro M 2,2'dipyridyl, the purified repressor is, in fact, dependent upon low levels of transition metal ion to transit from the inactive apo form to the active metal ion-bound form of the repressor. Binding studies using 8-anilino-1-naphthalenesulfonic acid suggest that the E175K mutation stabilizes an intermediate of the molten-globule form of the repressor, increasing exposure of hydrophobic residues to solvent. We demonstrate that the hyperactive DtxR(E175K) phenotype is dependent upon an intact ancillary metal ion-binding site (site 1) of the repressor. These observations support the hypothesis that metal ion binding in the ancillary site facilitates the conversion of the inactive apo-repressor to its active, operator-binding conformation. Furthermore, these results support the hypothesis that the C-terminal src homology 3-like domain of DtxR plays an active role in the modulation of repressor activity.