1,721,349 research outputs found
Some moduli stacks of symplectic bundles on a curve are rational (Pre-published version)
No full textLet C be a smooth projective curve of genus g ≥ 2 over a field k. Given a line bundle L on C, let Sympl2n,L be the moduli stack of vector bundles E of rank 2n on C endowed with a nowhere degenerate symplectic form b : E ⊗ E −→ L up to scalars. We prove that this stack is birational to BGm × As for some s if deg(E) = n · deg(L) is odd and C admits a rational point P ∈ C(k) as well as a line bundle ξ of degree 0 with ξ⊗2 ∼= OC . It follows that the corresponding coarse moduli scheme of Ramanathan-stable symplectic bundles is rational in this case.Ye
Moduli spaces of connections on a Riemann surface.
Full text linkProceedings of the U.S.-India workshop in Teichmüller theory and moduli problems,Harish-Chandra Research Institute, Allahabad, January 2006Let E be a holomorphic vector bundle over a compact connected Riemann surface X. The vector bundle E admits a holomorphic projective connection if and only if for every holomorphic direct summand F of E of positive rank, the equality degree(E)=rank(E) = degree(F)=rank(F) holds. Fix a point x0 in X. There is a logarithmic connection on E, singular over x0 with residue ¡d n IdEx0 if and only if the
equality degree(E)=rank(E) = degree(F)=rank(F) holds. Fix an integer n ¸ 2, and also ¯x an integer d coprime to n. Let M(n; d) denote the moduli space of logarithmic
SL(n;C){connections on X singular of x0 with residue ¡ d n
Id. The isomorphism class of the variety M(n; d) determines the isomorphism class of the Riemann surface X.Depto. de Álgebra, Geometría y TopologíaFac. de Ciencias MatemáticasTRUEpu
Theta Bundle, Quillen Connection and the Hodge Theoretic Projective Structure
No full textThere are two canonical projective structures on any compact Riemann surface of genus at least two: one
coming from the uniformization theorem, and the other from Hodge theory. They produce two (different) families
of projective structures over the moduli space \M_g of compact Riemann surfaces. A recent work of Biswas,
Favale, Pirola, and Torelli shows that families of projective structures over M_g admit an
equivalent characterization in terms of complex connections on the dual L of
the determinant of the Hodge line bundle
over M_g; the same work gave the connection on L corresponding to the projective
structures coming from uniformization. Here we construct the
connection on L corresponding to the family of Hodge theoretic projective structures. This
connection is described in three different
ways: Firstly as the connection induced on L by the Chern connection of the L^2-metric
on the Hodge bundle, secondly as an appropriate root of the Quillen metric induced by the (square of the)
Theta line bundle on the universal family of abelian varieties, endowed with the natural Hermitian metric
given by the polarization, and finally as Quillen connection gotten using the Arakelov metric on the
universal curve, modified by Faltings' delta invariant
Atiyah sequences, connections and characteristic forms for principal bundles over groupoids and stacks
Full text linkWe construct connections and characteristic forms for principal bundles over groupoids and stacks in the differentiable, holomorphic and algebraic category using Atiyah exact sequences associated with transversal tangential distributions.
Nous construisons les connexions et formes caractéristiques pour les fibrés principaux sur
les groupoïdes et les champs dans la catégorie différentiable, holomorphe et algébrique à
l’aide des suites d’Atiyah associées aux distributions transversales tangentielles
Torelli Theorem for the Deligne–Hitchin Moduli Space
Full text linkFix integers g ≥ 3 and r ≥ 2, with r ≥ 3 if g = 3. Given a compact connected Riemann surface X of genus g, let MDH(X) denote the corresponding SL(r, C) Deligne-Hitchin moduli space. We prove that the complex analytic space MDH(X) determines (up to an isomorphism) the unordered pair {X, overline{X}} , where {overline{X}} is the Riemann surface defined by the opposite almost complex structure on X
Group Compactifications and Moduli Spaces
Full text linkWe give a summary of joint work with Michael Thaddeus that realizes toroidal compactifications of split reductive groups as moduli spaces of framed bundles on chains of rational curves. We include an extension of this work that covers Artin stacks with good moduli spaces. We discuss, for complex groups, the symplectic counterpart of these compactifications, and conclude with some open problems about the moduli problems concerned
Electron Transport Chain Is Biochemically Linked to Pilus Assembly Required for Polymicrobial Interactions and Biofilm Formation in the Gram-Positive Actinobacterium <i>Actinomyces oris</i>
Full text linkABSTRACT
The Gram-positive actinobacteria
Actinomyces
spp. are key colonizers in the development of oral biofilms due to the inherent ability of
Actinomyces
to adhere to receptor polysaccharides on the surface of oral streptococci and host cells. This receptor-dependent bacterial interaction, or coaggregation, requires a unique sortase-catalyzed pilus consisting of the pilus shaft FimA and the coaggregation factor CafA forming the pilus tip. While the essential role of the sortase machine SrtC2 in pilus assembly, biofilm formation, and coaggregation has been established, little is known about
trans
-acting factors contributing to these processes. We report here a large-scale Tn
5
transposon screen for mutants defective in
Actinomyces oris
coaggregation with
Streptococcus oralis
. We obtained 33 independent clones, 13 of which completely failed to aggregate with
S. oralis
, and the remainder of which exhibited a range of phenotypes from severely to weakly defective coaggregation. The former had Tn
5
insertions in
fimA
,
cafA
, or
srtC2
, as expected; the latter were mapped to genes coding for uncharacterized proteins and various
nuo
genes encoding the NADH dehydrogenase subunits. Electron microscopy and biochemical analyses of mutants with nonpolar deletions of
nuo
genes and
ubiE
, a menaquinone C-methyltransferase-encoding gene downstream of the
nuo
locus, confirmed the pilus and coaggregation defects. Both
nuoA
and
ubiE
mutants were defective in oxidation of MdbA, the major oxidoreductase required for oxidative folding of pilus proteins. Furthermore, supplementation of the
ubiE
mutant with exogenous menaquinone-4 rescued the cell growth and pilus defects. Altogether, we propose that the
A. oris
electron transport chain is biochemically linked to pilus assembly via oxidative protein folding.
IMPORTANCE
The Gram-positive actinobacterium
A. oris
expresses adhesive pili, or fimbriae, that are essential to biofilm formation and
Actinomyces
interactions with other bacteria, termed coaggregation. While the critical role of the conserved sortase machine in pilus assembly and the disulfide bond-forming catalyst MdbA in oxidative folding of pilins has been established, little is known about other
trans
-acting factors involved in these processes. Using a Tn
5
transposon screen for mutants defective in coaggregation with
Streptococcus oralis
, we found that genetic disruption of the NADH dehydrogenase and menaquinone biosynthesis detrimentally alters pilus assembly. Further biochemical characterizations determined that menaquinone is important for reactivation of MdbA. This study supports the notion that the electron transport chain is biochemically linked to pilus assembly in
A. oris
via oxidative folding of pilin precursors.
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Lectin-like molecules of Lactobacillus rhamnosus GG inhibit pathogenic Escherichia coli and Salmonella biofilm formation
Full text linkObjectives : Increased antibiotic resistance has catalyzed the research on new antibacterial molecules and alternative strategies, such as the application of beneficial bacteria. Since lectin molecules have unique sugar-recognizing capacities, and pathogens are often decorated with sugars that affect their survival and infectivity, we explored whether lectins from the probiotic strain Lactobacillus rhamnosus GG have antipathogenic properties.
Methods : The genome sequence of L. rhamnosus GG was screened for the presence of lectin-like proteins. Two genes, LGG_RS02780 and LGG_RS02750, encoding for polypeptides with an N-terminal conserved L-type lectin domain were detected and designated Llp1 (lectinlike protein 1) and Llp2. The capacity of Llp1 and Llp2 to inhibit biofilm formation of various pathogens was investigated. Sugar specificity was determined by Sepharose beads assays and glycan array screening.
Results : The isolated lectin domains of Llp1 and Llp2 possess pronounced inhibitory activity against biofilm formation by various pathogens, including clinical Salmonella species and uropathogenic E. coli, with Llp2 being more active than Llp1. In addition, sugar binding assays with Llp1 and Llp2 indicate specificity for complex glycans. Both proteins are also involved in the adhesion capacity of L. rhamnosus GG to gastrointestinal and vaginal epithelial cells.
Conclusions : Lectins isolated from or expressed by beneficial lactobacilli could be considered promising bio-active ingredients for improved prophylaxis of urogenital and gastrointestinal infections
Group B streptococcal infection and activation of human astrocytes.
Full text linkBACKGROUND:Streptococcus agalactiae (Group B Streptococcus, GBS) is the leading cause of life-threatening meningitis in human newborns in industrialized countries. Meningitis results from neonatal infection that occurs when GBS leaves the bloodstream (bacteremia), crosses the blood-brain barrier (BBB), and enters the central nervous system (CNS), where the bacteria contact the meninges. Although GBS is known to invade the BBB, subsequent interaction with astrocytes that physically associate with brain endothelium has not been well studied. METHODOLOGY/PRINCIPAL FINDINGS:We hypothesize that human astrocytes play a unique role in GBS infection and contribute to the development of meningitis. To address this, we used a well- characterized human fetal astrocyte cell line, SVG-A, and examined GBS infection in vitro. We observed that all GBS strains of representative clinically dominant serotypes (Ia, Ib, III, and V) were able to adhere to and invade astrocytes. Cellular invasion was dependent on host actin cytoskeleton rearrangements, and was specific to GBS as Streptococcus gordonii failed to enter astrocytes. Analysis of isogenic mutant GBS strains deficient in various cell surface organelles showed that anchored LTA, serine-rich repeat protein (Srr1) and fibronectin binding (SfbA) proteins all contribute to host cell internalization. Wild-type GBS also displayed an ability to persist and survive within an intracellular compartment for at least 12 h following invasion. Moreover, GBS infection resulted in increased astrocyte transcription of interleukin (IL)-1β, IL-6 and VEGF. CONCLUSIONS/SIGNIFICANCE:This study has further characterized the interaction of GBS with human astrocytes, and has identified the importance of specific virulence factors in these interactions. Understanding the role of astrocytes during GBS infection will provide important information regarding BBB disruption and the development of neonatal meningitis
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