The last two decades have seen a worldwide resurgence in infections caused by Treponema pallidum subsp. pallidum, the syphilis spirochete. The syphilis spirochete’s well-recognized capacity for early dissemination and immune evasion has earned it the designation ‘the stealth pathogen’. Despite the many hurdles to studying syphilis pathogenesis, most notably the inability to culture and to genetically manipulate T. pallidum, in recent years, considerable progress has been made in elucidating the structural, physiologic, and regulatory facets of stealth pathogenicity. In this Review, we integrate this eclectic body of information to garner fresh insights into the highly successful parasitic lifestyles of the syphilis spirochete and related pathogenic treponemes.

In Borrelia burgdorferi (Bb), the Lyme disease spirochete, the alternative σ factor σ54 (RpoN) directly activates transcription of another alternative σ factor, σS (RpoS) which, in turn, controls the expression of virulence-associated membrane lipoproteins. As is customary in σ54-dependent gene control, a putative NtrC-like enhancer-binding protein, Rrp2, is required to activate the RpoN-RpoS pathway. However, recently it was found that rpoS transcription in Bb also requires another regulator, BosR, which was previously designated as a Fur or PerR homolog. Given this unexpected requirement for a second activator to promote σ54-dependent gene transcription, and the fact that regulatory mechanisms among similar species of pathogenic bacteria can be strain-specific, we sought to confirm the regulatory role of BosR in a second virulent strain (strain 297) of Bb. Indeed, BosR displayed the same influence over lipoprotein expression and mammalian infectivity for strain Bb 297 that were previously noted for Bb strain B31. We subsequently found that recombinant BosR (rBosR) bound to the rpoS gene at three distinct sites, and that binding occurred despite the absence of consensus Fur or Per boxes. This led to the identification of a novel direct repeat sequence (TAAATTAAAT) critical for rBosR binding in vitro. Mutations in the repeat sequence markedly inhibited or abolished rBosR binding. Taken together, our studies provide new mechanistic insights into how BosR likely acts directly on rpoS as a positive transcriptional activator. Additional novelty is engendered by the facts that, although BosR is a Fur or PerR homolog and it contains zinc (like Fur and PerR), it has other unique features that clearly set it apart from these other regulators. Our findings also have broader implications regarding a previously unappreciated layer of control that can be involved in σ54–dependent gene regulation in bacteria.

This paper compares the biophysical and mechanistic properties of a typical type I dehydroquinase (DHQase), from the biosynthetic shikimate pathway of Escherichia coli, and a typical type II DHQase, from the quinate pathway of Aspergillus nidulans. C.d. shows that the two proteins have different secondary-structure compositions; the type I enzyme contains approx. 50% alpha-helix while the type II enzyme contains approx. 75% alpha-helix. The stability of the two types of DHQase was compared by denaturant-induced unfolding, as monitored by c.d., and by differential scanning calorimetry. The type II enzyme unfolds at concentrations of denaturant 4-fold greater than the type I and through a series of discrete transitions, while the type I enzyme unfolds in a single transition. These differences in conformational stability were also evident from the calorimetric experiments which show that type I DHQase unfolds as a single co-operative dimer at 57 degrees C whereas the type II enzyme unfolds above 82 degrees C and through a series of transitions suggesting higher orders of structure than that seen for the type I enzyme. Sedimentation and Mr analysis of both proteins by analytical ultracentrifugation is consistent with the unfolding data. The type I DHQase exists predominantly as a dimer with Mr = 46,000 +/- 2000 (a weighted average affected by the presence of monomer) and has a sedimentation coefficient s0(20,w) = 4.12 (+/- 0.08) S whereas the type II enzyme is a dodecamer, weight-average Mr = 190,000 +/- 10,000 and has a sedimentation coefficient, s0(20,w) = 9.96 (+/- 0.21) S. Although both enzymes have reactive histidine residues in the active site and can be inactivated by diethyl pyrocarbonate, the possibility that these structurally dissimilar enzymes catalyse the same dehydration reaction by the same catalytic mechanism is deemed unlikely by three criteria: (1) they have very different pH/log kcat. profiles and pH optima; (2) imine intermediates, which are known to play a central role in the mechanism of type I enzymes, could not be detected (by borohydride reduction) in the type II enzyme; (3) unlike Schiff's base-forming type I enzymes, there are no conserved lysine residues in type II amino acid sequences.

Background:The TP0796 lipoprotein of Treponema pallidum belongs to the poorly characterized ApbE superfamily. Results: TP0796 hydrolyzed FAD into FMN and AMP, consistent with the general enzymatic mechanism of an FAD pyrophosphatase. Conclusion: This novel metal-dependent enzyme probably plays an essential role in flavin homeostasis in T. pallidum. Significance: This is the first description of a metal-dependent FAD pyrophosphatase in bacteria.

We previously demonstrated that Treponema pallidum TroA is a periplasmic metal-binding protein (MBP) with a distinctive alpha-helical backbone. To better understand the mechanisms of metal binding and release by TroA, we determined the crystal structure of the apoprotein at a resolution of 2.5 Å and compared it to that of the Zn(II)-bound form (Protein Data Bank accession code 1toa). apo-TroA shows a conformation even more closed than that of its Zn(II)-bound counterpart due to a 4°tilt of the C-terminal domain (residues 190 through 308) about an axis parallel to the poorly flexible backbone helix. This domain tilting pushes two loops (residues 248 through 253 and 277 through 286) towards the metal-binding site by more than 1 Å, resulting in an unfavorable interaction of I251 with D66. To avoid this contact, D66 shifts towards H68, one of the four Zn ( Originally reported to be an outer membrane protein of the syphilis spirochete Treponema pallidum (3, 4), TroA has been shown by genetic, ultrastructural, and X-ray crystallographic studies to actually be the binding component of an ATP-binding cassette (ABC) transport system which shuttles Zn(II) and possibly other transition metals (e.g., manganese) across the T. pallidum cytoplasmic membrane (1, 11-13, 18, 21; K. R. O. Hazlett, F. Rusnak, and J. D. Radolf, unpublished observations). TroA is 1 of more than 40 members of a newly defined cluster of metal-binding proteins within the extended family of bacterial solute-binding receptors (SBRs) (8,23).Members of the SBR family have N-and C-terminal domains separated by a crevice which serves as the ligand-binding site (22). The domains of the receptors for nonmetal ligands such as sulfate, phosphate, amino acids, oligopeptides, monosaccharides, and oligosaccharides are hinged by two or three ␤ strands that function according to a so-called "Venus flytrap" mechanism. According to this model, a hinge-bending motion traps the ligand within the cleft between the two domains. Conversely, upon release of the ligand, the receptor adopts an open conformation, in which the two domains of the apoprotein are further apart and the binding site is more solvent accessible (22). Unlike with the nonmetal SBRs, the two domains of the MBPs are linked by an extended and presumably poorly flexible ␣-helix which interacts along its entire length with both domains (17, 18). Because of this structural difference, we hypothesized that ligand binding and release by MBPs would proceed via mechanisms different from those by which non-metal-binding SBRs proceed. We have obtained evidence for this hypothesis by determining the crystal structure of apo-TroA and comparing it to that of the Zn(II)-bound form of the protein.Preparation and crystallization of apo-TroA. A recombinant DNA construct encoding TroA with an N-terminal His 6 tag but without its leader peptide (residues 23 through 308) 6 was previously described (11). Protein was overexpressed in Escherichia coli DH5␣ and purified from cell lysates by use of a Ni-nitrilotriacetic acid agaros...