4,568 research outputs found
The Alignment between Satellites and Central Galaxies: Theory vs. Observations
Recent studies have shown that the distribution of satellite galaxies is
preferentially aligned with the major axis of their central galaxy. The
strength of this alignment has been found to depend strongly on the colours of
the satellite and central galaxies, and only weakly on the mass of the halo in
which the galaxies reside. In this paper we study whether these alignment
signals, and their dependence on galaxy and halo properties, can be reproduced
in a hierarchical structure formation model of a CDM concordance
cosmology. To that extent we use a large -body simulation which we populate
with galaxies following a semi-analytical model for galaxy formation. We find
that if the orientation of the central galaxy is perfectly aligned with that of
its dark matter halo, then the predicted central-satellite alignment signal is
much stronger than observed. If, however, the minor axis of a central galaxy is
perfectly aligned with the angular momentum vector of its dark matter halo, we
can accurately reproduce the observed alignment strength as function of halo
mass and galaxy color. Although this suggests that the orientation of central
galaxies is governed by the angular momentum of their dark matter haloes, we
emphasize that any other scenario in which the minor axes of central galaxy and
halo are misaligned by (on average) will match the data
equally well. Finally, we show that dependence of the alignment strength on the
color of the central galaxy is most likely an artefact due to interlopers in
the group catalogue. The dependence on the color of the satellite galaxies, on
the other hand, is real and owes to the fact that red satellites are associated
with subhaloes that were more massive at their time of accretion.Comment: 13 Pages, 10 Figures, one figure replaced. added in discussion about
comparison with others results, Updated version to match accepted version to
MNRA
Acetyl-Phosphate Is Not a Global Regulatory Bridge between Virulence and Central Metabolism in Borrelia burgdorferi
In B. burgdorferi, the Rrp2-RpoN-RpoS signaling cascade is a distinctive system that coordinates the expression of virulence factors required for successful transition between its arthropod vector and mammalian hosts. Rrp2 (BB0763), an RpoN specific response regulator, is essential to activate this regulatory pathway. Previous investigations have attempted to identify the phosphate donor of Rrp2, including the cognate histidine kinase, Hk2 (BB0764), non-cognate histidine kinases such as Hk1, CheA1, and CheA2, and small molecular weight P-donors such as carbamoyl-phosphate and acetyl-phosphate (AcP). In a report by Xu et al., exogenous sodium acetate led to increased expression of RpoS and OspC and it was hypothesized this effect was due to increased levels of AcP via the enzyme AckA (BB0622). Genome analyses identified only one pathway that could generate AcP in B. burgdorferi: the acetate/mevalonate pathway that synthesizes the lipid, undecaprenyl phosphate (C55-P, lipid I), which is essential for cell wall biogenesis. To assess the role of AcP in Rrp2-dependent regulation of RpoS and OspC, we used a unique selection strategy to generate mutants that lacked ackA (bb0622: acetate to AcP) or pta (bb0589: AcP to acetyl-CoA). These mutants have an absolute requirement for mevalonate and demonstrate that ackA and pta are required for cell viability. When the ΔackA or Δpta mutant was exposed to conditions (i.e., increased temperature or cell density) that up-regulate the expression of RpoS and OspC, normal induction of those proteins was observed. In addition, adding 20mM acetate or 20mM benzoate to the growth media of B. burgdorferi strain B31 ΔackA induced the expression of RpoS and OspC. These data suggest that AcP (generated by AckA) is not directly involved in modulating the Rrp2-RpoN-RpoS regulatory pathway and that exogenous acetate or benzoate are triggering an acid stress response in B. burgdorferi
The sigma factor σ54 is required for the long-term survival of Leptospira biflexa in water
Leptospira spp. comprise both pathogenic and free-living saprophytic species. Little is known about the environmental adaptation and survival mechanisms of Leptospira. Alternative sigma factor, σ54 (RpoN) is known to play an important role in environmental and host adaptation in many bacteria. In this study, we constructed an rpoN mutant by allele exchange, and the complemented strain in saprophytic L. biflexa. Transcriptome analysis revealed that expression of several genes involved in nitrogen uptake and metabolism, including amtB1, glnB-amtB2, ntrX and narK, were controlled by σ54 . While wild-type L. biflexa could not grow under nitrogen-limiting conditions but was able to survive under such conditions and recover rapidly, the rpoN mutant was not. The rpoN mutant also had dramatically reduced ability to survive long-term in water. σ54 appears to regulate expression of amtB1, glnK-amtB2, ntrX and narK in an indirect manner. However, we identified a novel nitrogen-related gene, LEPBI_I1011, whose expression was directly under the control of σ54 (herein renamed as rcfA for RpoN-controlled factor A). Taken together, our data reveal that the σ54 regulatory network plays an important role in the long-term environmental survival of Leptospira spp
LtpA, a CdnL-type CarD regulator, is important for the enzootic cycle of the Lyme disease pathogen
Little is known about how Borrelia burgdorferi, the Lyme disease pathogen, adapts and survives in the tick vector. We previously identified a bacterial CarD N-terminal-like (CdnL) protein, LtpA (BB0355), in B. burgdorferi that is preferably expressed at lower temperatures, which is a surrogate condition mimicking the tick portion of the enzootic cycle of B. burgdorferi. CdnL-family proteins, an emerging class of bacterial RNAP-interacting transcription factors, are essential for the viability of Mycobacterium tuberculosis and Myxococcus xanthus. Previous attempts to inactivate ltpA in B. burgdorferi have not been successful. In this study, we report the construction of a ltpA mutant in the infectious strain of B. burgdorferi, strain B31-5A4NP1. Unlike CdnL in M. tuberculosis and M. xanthus, LtpA is dispensable for the viability of B. burgdorferi. However, the ltpA mutant exhibits a reduced growth rate and a cold-sensitive phenotype. We demonstrate that LtpA positively regulates 16S rRNA expression, which contributes to the growth defects in the ltpA mutant. The ltpA mutant remains capable of infecting mice, albeit with delayed infection. Additionally, the ltpA mutant produces markedly reduced spirochetal loads in ticks and was not able to infect mice via tick infection. Overall, LtpA represents a novel regulator in the CdnL family that has an important role in the enzootic cycle of B. burgdorferi
Behaviour of the energy gap near a commensurate-incommensurate transition in double layer quantum Hall systems at nu=1
The charged excitations in the system of the title are vortex-antivortex
pairs in the spin-texture described in the theory by Yang et al which, in the
commensurate phase, are bound together by a ``string''. It is shown that their
excitation energy drops as the string lengthens as the parallel magnetic field
approaches the critical value, then goes up again in the incommensurate phase.
This produces a sharp downward cusp at the critical point. An alternative
description based on the role of disorder in the tunnelling and which appears
not to produce a minimum in the excitation energy is also discussed. It is
suggested that a similar transition could also occur in compressible
Fermi-liquid-like states.Comment: latex file, 17 page
- …