MRN and the race to the break

In all living cells, DNA is constantly threatened by both endogenous and exogenous agents. In order to protect genetic information, all cells have developed a sophisticated network of proteins, which constantly monitor genomic integrity. This network, termed the DNA damage response, senses and signals the presence of DNA damage to effect numerous biological responses, including DNA repair, transient cell cycle arrests (“checkpoints”) and apoptosis. The MRN complex (MRX in yeast), composed of Mre11, Rad50 and Nbs1 (Xrs2), is a key component of the immediate early response to DNA damage, involved in a cross-talk between the repair and checkpoint machinery. Using its ability to bind DNA ends, it is ideally placed to sense and signal the presence of double strand breaks and plays an important role in DNA repair and cellular survival. Here, we summarise recent observation on MRN structure, function, regulation and emerging mechanisms by which the MRN nano-machinery protects genomic integrity. Finally, we discuss the biological significance of the unique MRN structure and summarise the emerging sequence of early events of the response to double strand breaks orchestrated by the MRN complex.     

Nitric Oxide Production in Striatum and Pallidum of Cirrhotic Rats

Ammonium and manganese are neurotoxic agents related to brain metabolic disturbances observed after prolonged liver damage. The aim of this study was to assess the production of nitric oxide (NO) in the brain of cirrhotic rats exposed to manganese. We induced cirrhosis by bile duct ligation for 4 weeks in rats. From brain, striatum and globus pallidus were dissected out, and NO synthase activity and the content of nitrites plus nitrates (NOx) were determined. In pallidum we found a diminished constitutive NO synthase activity from cirrhotic rats, independently of manganese exposure. This result was confirmed by low levels of NOx in the same brain area (P<0.05, two-way ANOVA). This finding was not related to protein expression of NO synthase since no differences were observed in immunoblot signals between cirrhotic and sham-operated animals. Results from present study suggest that the production of NO is reduced in basal ganglia during cirrhosis.     

A class III histidine kinase acts as a novel virulence factor in Botrytis cinerea

Filamentous ascomycetes contain large numbers of histidine kinases (HK) that belong to eleven classes. Members of class III from different species were previously shown to be involved in osmoregulation and resistance to dicarboximide and phenylpyrrole fungicides. We have inactivated the gene encoding the single group III HK, BOS1, in the economically important plant pathogen Botrytis cinerea. BOS1 inactivation had pleiotropic effects on the fungus. Besides the expected osmosensitivity and resistance to fungicides, null mutants presented additional characteristics indicating that BOS1 is necessary for normal macroconidiation and full virulence. On standard culture media, null mutants very rarely formed conidiophores and those few conidiophores failed to produce conidia. This defect could be partially restored with 1 M sorbitol, suggesting that another BOS1-independent signal cascade may be involved in macroconidiation. The mutants were not found to be hypersensitive to various oxidative stresses but were more resistant to menadione. Finally, pathogenicity tests showed that bos1-null mutants were significantly reduced in the ability to infect host plants. Appressorium morphogenesis was not altered; however, in planta growth was severely reduced. To our knowledge, this is the first class III HK characterized as a pathogenicity factor in a plant-pathogenic ascomycete.     

Chromosomal inversion discovered in C3H/HeJ mice

Mice of the inbred mouse strain C3H/HeJ have been shown to be homozygous for a chromosomal inversion on Chromosome (Chr) 6. The inversion encompasses about 20% of the chromosome from approximately 73 Mb to approximately 116 Mb. The importance of this finding is that linkage crosses using C3H/HeJ will show no recombination in this region of Chr 6. The inversion has no apparent effect on the phenotype of C3H/HeJ mice and its presence should not affect biological studies; however, use of C3H/HeJ mice for genetic analysis of Chr 6 should be avoided or the results interpreted with the inversion in mind. The inversion has been named In(6)1J (inversion Chr 6, Jackson 1).     

Functional significance of factor H binding to Neisseria meningitidis

Neisseria meningitidis is an important cause of septicemia and meningitis. To cause disease, the bacterium must successfully survive in the bloodstream where it has to avoid being killed by host innate immune mechanisms, particularly the complement system. A number of pathogenic microbes bind factor H (fH), the negative regulator of the alternative pathway of complement activation, to promote their survival in vivo. In this study, we show that N. meningitidis binds fH to its surface. Binding to serogroups A, B, and C N. meningitidis strains was detected by FACS and Far Western blot analysis, and occurred in the absence of other serum factors such as C3b. Unlike Neisseria gonorrhoeae, binding of fH to N. meningitidis was independent of sialic acid on the bacterium, either as a component of its LPS or its capsule. Characterization of the major fH binding partner demonstrated that it is a 33-kDa protein; examination of insertion mutants showed that porins A and B, outer membrane porins expressed by N. meningitidis, do not contribute significantly to fH binding. We examined the physiological consequences of fH bound to the bacterial surface. We found that fH retains its activity as a cofactor of factor I when bound to the bacterium and contributes to the ability of N. meningitidis to avoid complement-mediated killing in the presence of human serum. Therefore, the recruitment of fH provides another mechanism by which this important human pathogen evades host innate immunity.     

Hox genes in sea spiders (Pycnogonida) and the homology of arthropod head segments

The pycnogonids (or sea spiders) are an enigmatic group of arthropods, classified in recent phylogenies as a sister-group of either euchelicerates (horseshoe crabs and arachnids), or all other extant arthropods. Because of their bizarre morpho-anatomy, homologies with other arthropod taxa have been difficult to assess. We review the main morphology-based hypotheses of correspondence between anterior segments of pycnogonids, arachnids and mandibulates. In an attempt to provide new relevant data to these controversial issues, we performed a PCR survey of Hox genes in two pycnogonid species, Endeis spinosa and Nymphon gracile, from which we could recover nine and six Hox genes, respectively. Phylogenetic analyses allowed to identify their orthology relationships. The Deformed gene from E. spinosa and the abdominal-A gene from N. gracile exhibit unusual sequence divergence in their homeodomains, which, in the latter case, may be correlated with the extreme reduction of the posterior region in pycnogonids. Expression patterns of two Hox genes (labial and Deformed) in the E. spinosa protonymphon larva are discussed. The anterior boundaries of their expression domains favour homology between sea spider chelifores, euchelicerates chelicerae and mandibulate (first) antennae, in contradistinction with previously proposed alternative schemes such as the protocerebral identity of sea spider chelifores or the absence of a deutocerebrum in chelicerates. In addition, while anatomical and embryological evidences suggest the possibility that the ovigers of sea spiders could be a duplicated pair of pedipalps, the Hox data support them as modified anterior walking legs, consistent with the classical views.Supplementary material is available for this article at and is accessible for authorized users.Guest editors Jean Deutsch and Gerhard Scholtz     

Making water flow: a comparison of the hydrodynamic characteristics of 12 different benthic biological flumes

Flume tanks are becoming increasingly important research tools in aquatic ecology, to link biological to hydrodynamical processes. There is no such thing as a “standard flume tank”, and no flume tank is suitable for every type of research question. A series of experiments has been carried out to characterise and compare the hydrodynamic characteristics of 12 different flume tanks that are designed specifically for biological research. These facilities are part of the EU network BioFlow. The flumes could be divided into four basic design types: straight, racetrack, annular and field flumes. In each facility, two vertical velocity profiles were measured: one at 0.05 m s⁻¹ and one at 0.25 m s⁻¹. In those flumes equipped with Acoustic Doppler Velocimeters (ADV), time series were also recorded for each velocity at two heights above the bottom: 0.05 m and 20% of the water depth. From these measurements turbulence characteristics, such as TKE and Reynolds stress, were derived, and autocorrelation spectra of the horizontal along-stream velocity component were plotted. The flume measurements were compared to two sets of velocity profiles measured in the field.Despite the fact that some flumes were relatively small, turbulence was fully developed in all channels. Straight and racetrack flumes generally produced boundary layers with a clearly definable logarithmic layer, similar to measurements in the field taken under steady flow conditions. The two annular flumes produced relatively thin boundary layers, presumably due to secondary flows developing in the curved channels. The profiles in the field flumes also differed considerably from the expected log profile. This may either have been due the construction of the flume, or due to unsteady conditions during measurement. Constraints imposed by the different flume designs on the suitability for different types of boundary layer research, as well as scaling issues are discussed.     

The reb1-1 mutation of Arabidopsis. Effect on the structure and localization of galactose-containing cell wall polysaccharides

The Arabidopsis (Arabidopsis thaliana) root epidermal bulger1-1 (reb1-1) mutant (allelic to root hair defective1 [rhd1]) is characterized by a reduced root elongation rate and by bulging of trichoblast cells. The REB1/RHD1 gene belongs to a family of UDP-D-Glucose 4-epimerases involved in the synthesis of D-Galactose (Gal). Our previous study showed that certain arabinogalactan protein epitopes were not expressed in bulging trichoblasts of the mutant. In this study, using a combination of microscopical and biochemical methods, we have investigated the occurrence and the structure of three major Gal-containing polysaccharides, namely, xyloglucan (XyG), rhamnogalacturonan (RG)-I, and RG-II in the mutant root cell walls. Our immunocytochemical data show that swollen trichoblasts were not stained with the monoclonal antibody CCRC-M1 specific for alpha-L-Fucp-(1-->2)-beta-D-Galp side chains of XyG, whereas they were stained with anti-XyG antibodies specific for XyG backbone. In addition, analysis of a hemicellulosic fraction from roots demonstrates the presence of two structurally different XyGs in reb1-1. One is structurally similar to wild-type XyG and the other is devoid of fuco-galactosylated side chains and has the characteristic of being insoluble. Similar to anti-XyG antibodies, anti-bupleuran 2IIC, a polyclonal antibody specific for galactosyl epitopes associated with pectins, stained all root epidermal cells of both wild type and reb1-1. Similarly, anti-RG-II antibodies also stained swollen trichoblasts in the mutant. In addition, structural analysis of pectic polymers revealed no change in the galactosylation of RG-I and RG-II isolated from reb1-1 root cells. These findings demonstrate that the reb1-1 mutation affects XyG structure, but not that of pectic polysaccharides, thus lending support to the hypothesis that biosynthesis of Gal as well as galactosylation of complex polysaccharides is regulated at the polymer level.