Helicobacter pylori AmiF formamidase that hydrolyzes formamide to produce formic acid and ammonia belongs to a member of the nitrilase superfamily. The crystal structure of AmiF was solved to 1.75 Å resolution using single-wavelength anomalous dispersion methods. The structure consists of a homohexamer related by 3-fold symmetry in which each subunit has an ␣-␤-␤-␣ four-layer architecture characteristic of the nitrilase superfamily. One exterior ␣ layer faces the solvent, whereas the other one associates with that of the neighbor subunit, forming a tight ␣-␤-␤-␣-␣-␤-␤-␣ dimer. The apo and liganded crystal structures of an inactive mutant C166S were also determined to 2.50 and 2.30 Å , respectively. These structures reveal a small formamide-binding pocket that includes

Cytolethal distending toxin (CDT) produced by Campylobacter jejuni comprises a heterotrimeric complex formed by CdtA, CdtB, and CdtC. Among these toxin subunits, CdtA and CdtC function as essential proteins that mediate toxin binding to cytoplasmic membranes followed by delivery of CdtB into the nucleus. The binding of CdtA/CdtC to the cell surface is mediated by cholesterol, a major component in lipid rafts. Although the putative cholesterol recognition/interaction amino acid consensus (CRAC) domain of CDT has been reported from several bacterial pathogens, the protein regions contributing to CDT binding to cholesterol in C. jejuni remain unclear. Here, we selected a potential CRAC-like region present in the CdtC from C. jejuni for analysis. Molecular modeling showed that the predicted functional domain had the shape of a hydrophobic groove, facilitating cholesterol localization to this domain. Mutation of a tyrosine residue in the CRAC-like region decreased direct binding of CdtC to cholesterol rather than toxin intermolecular interactions and led to impaired CDT intoxication. These results provide a molecular link between C. jejuni CdtC and membrane-lipid rafts through the CRAC-like region, which contributes to toxin recognition and interaction with cholesterol.

An experimental investigation of the Hall effect and the resistivity of germanium alloys at temperatures from room temperature down to the liquid helium temperature range is reported. Germanium samples with different kinds of impurity and different concentrations were used. At higher temperatures, satisfactory agreement between theory and experiment was found. The activation energy of the impurity states was found to be of the order of 10 millivolts. The scattering of the carriers in the conduction and filled bands consists mainly of lattice scattering and ionized impurity scattering. At low temperatures, however, anomalies in the Hall curves were observed. The Hall curve for every sample measured went through a maximum as the temperature was lowered. At the same time the resistivity approached a saturation value. The Hall curves of low resistivity samples finally became flat at very low temperatures. These anomalies are explained on the assumption of small but finite mobility of carriers in the impurity states. This conduction in the impurity states assumes importance at low temperatures when the concentration of carriers in the conduction band becomes very low. When this simultaneous conduction in the conduction band and the impurity band is considered, the behaviors in the Hall and the resistivity curves are satisfactorily explained. The mobility in the impurity band is found to increase with the impurity concentration, but is rather temperature-independent.

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SummaryHelicobacter pylori inhabits the gastric mucosa where it senses and responds to various stresses via a twocomponent systems (TCSs) that enable its persistent colonization. The aim of this study was to investigate whether any of the three paired TCSs (ArsRS, FleRS and CrdRS) in H. pylori respond to nitrosative stress. The results showed that the expression of crdS was significantly increased upon exposure to nitric oxide (NO). crdS-knockout (ΔcrdS) and crdR/crdS-knockout (ΔcrdRS) H. pylori, but not arsS-knockout (ΔarsS) or fleS-knockout (ΔfleS) H. pylori, showed a significant loss of viability upon exposure to NO compared with wild-type strain. Knockin crdS (ΔcrdS-in) significantly restored viability in the presence of NO. Global transcriptional profiling analysis of wild-type and ΔcrdS H. pylori in the presence or absence of NO showed that 101 genes were differentially expressed, including copper resistance determinant A (crdA), transport, binding and envelope proteins. The CrdR binding motifs were investigated by competitive electrophoretic mobility shift assay, which revealed that the two AC-rich regions in the crdA promoter region are required for binding. These results demonstrate that CrdR-crdA interaction enables H. pylori to survive under nitrosative stress.