Structure of Arc repressor in solution: evidence for a family of beta-sheet DNA-binding proteins

The Arc repressor, which is involved in the switch between lysis and lysogeny of Salmonella bacteriophage P22, does not belong to any of the known classes of DNA-binding proteins. Mutagenesis studies show that the DNA-binding region is located in the 15 N-terminal amino-acid residues. We have now determined the three-dimensional structure of the Arc dimer from an extensive set of interproton-distance data obtained from 1H NMR spectroscopy. A priori, intra- and inter-monomer nuclear Overhauser effects (NOEs) cannot be distinguished for a symmetric dimer. But by using the homology with the Escherichia coli Met repressor we could interpret the NOEs unambiguously in an iterative structure refinement procedure. The final structure satisfies a large set of NOE constraints (1,352 for the dimer). It shows a strongly intertwined dimer, in which residues 8-14 of different monomers form an antiparallel beta-sheet. A model for the Arc repressor-operator complex can account for all available biochemical and genetic data. In this model two Arc dimers bind with their beta-sheet regions in successive major grooves on one side of the DNA helix, similar to the Met repressor interaction. Thus, Arc and Met repressors are members of the same family of proteins, which contain an antiparallel beta-sheet as the DNA-binding motif.     

Microbial cycling of volatile organic sulfur compounds

Microbial cycling of volatile organic sulfur compounds (VOSCs), especially dimethyl sulfide (DMS) and methanethiol (MT), is intensively studied because these compounds play an important role in the processes of global warming, acid precipitation, and the global sulfur cycle. VOSC concentrations in freshwater sediments are low due to the balance between the formation and degradation of these compounds. These reactions occur for the greater part at the oxic/anoxic interphase of sediment and water column. In contrast to marine ecosystems, where dimethylsulfoniopropionate is the main precursor of MT and DMS, in freshwater ecosystems, VOSCs are formed mainly by methylation of sulfide and to a lesser extent from the degradation of S-containing amino acids. One of the major routes for DMS and MT formation through sulfide methylation is anaerobic O-demethylation of methoxylated aromatic compounds. Inhibition studies have revealed that the major part of the endogenously produced MT and DMS is degraded anaerobically by methanogens. The major bacterial groups involved in formation and consumption of VOSCs are described.     

Mutations in a human homologue of Drosophila crumbs cause retinitis pigmentosa (RP12).

Retinitis pigmentosa (RP) comprises a clinically and genetically heterogeneous group of diseases that afflicts approximately 1.5 million people worldwide. Affected individuals suffer from a progressive degeneration of the photoreceptors, eventually resulting in severe visual impairment. To isolate candidate genes for chorioretinal diseases, we cloned cDNAs specifically or preferentially expressed in the human retina and the retinal pigment epithelium (RPE) through a novel suppression subtractive hybridization (SSH) method. One of these cDNAs (RET3C11) mapped to chromosome 1q31-q32.1, a region harbouring a gene involved in a severe form of autosomal recessive RP characterized by a typical preservation of the para-arteriolar RPE (RP12; ref. 3). The full-length cDNA encodes an extracellular protein with 19 EGF-like domains, 3 laminin A G-like domains and a C-type lectin domain. This protein is homologous to the Drosophila melanogaster protein crumbs (CRB), and denoted CRB1 (crumbs homologue 1). In ten unrelated RP patients with preserved para-arteriolar RPE, we identified a homozygous AluY insertion disrupting the ORF, five homozygous missense mutations and four compound heterozygous mutations in CRB1. The similarity to CRB suggests a role for CRB1 in cell-cell interaction and possibly in the maintenance of cell polarity in the retina. The distinct RPE abnormalities observed in RP12 patients suggest that CRB1 mutations trigger a novel mechanism of photoreceptor degeneration.     

Photon emission in tumor biology.

Photon emission from mammalian cells has been subject of study for many years. Growing research activity is directed on the photon emission within the field of tumor biology. These studies, applying high-sensitivity photon counting methods, have paid attention to several aspects, including photon emission from serum of tumor-bearing animals, photon emission of tumors and of isolated tumor cells. In addition, research activity is increased with respect to the photon emission induced by white light from cultured tumor cells. In this review we report on the different aspects of spontaneous and induced photon emission of tumor cells as compared to normal cells. Throughout these studies the question of a functional biological role of this spontaneous and light-induced photon emission has been raised and some different points of view will be discussed.     

Isolation of a candidate gene for Norrie disease by positional cloning.

The gene for Norrie disease, an X-linked disorder characterized by progressive atrophy of the eyes, mental disturbances and deafness, has been mapped to chromosome Xp11.4 close to DXS7 and the monoamine oxidase (MAO) genes. By subcloning a YAC with a 640 kilobases (kb) insert which spans the DXS7-MAOB interval we have generated a cosmid contig which extends 250 kb beyond the MAOB gene. With one of these cosmids, microdeletions were detected in several patients with Norrie disease. Screening of cDNA libraries has enabled us to isolate and sequence a likely candidate gene for Norrie disease which is expressed in retina, choroid and fetal brain. No homologous sequences were found in DNA and protein databases indicating that this cDNA is part of a gene encoding a 'pioneer' protein.     

Epithelial antimicrobial peptides: innate local host response elements

Multicellular organisms have to survive in an environment laden with numerous microorganisms, which represent a potential hazard to life. Different strategies have been developed to ward off infections by preventing microorganisms from entering surfaces and by preventing the attack of microorganisms that have already entered the epithelia. Therefore, it is not surprising that epithelia are equipped with various antimicrobial substances that act rapidly to kill a broad range of microorganisms. This review summarizes our present knowledge about epithelial peptide antibiotics produced in plants, invertebrates, and vertebrates including humans. There is now strong evidence that in addition to constitutively secreted peptide antibiotics, others are induced upon contact with microorganisms or by proinflammatory cytokines. beta-Defensins represent one family of vertebrate antimicrobial peptides, members of which are inducible and have recently been identified in humans. The defensin-characteristic local expression pattern may indicate that specialized surfaces express a characteristic surface antimicrobial peptide pattern that might define the characteristic microflora as well as the density of microorganisms present on the surface.