885 research outputs found
Intestinal spirochaetes of the genus Brachyspira share a partially conserved 26 kilobase genomic region with Enterococcus faecalis and Escherichia coli
Anaerobic intestinal spirochaetes of the genus Brachyspira include both pathogenic and commensal species. The two best-studied members are the pathogenic species B. hyodysenteriae (the aetiological agent of swine dysentery) and B. pilosicoli (a cause of intestinal spirochaetosis in humans and other species). Analysis of near-complete genome sequences of these two species identiļ¬ ed a highly conserved 26 kilobase (kb) region that was shared, against a background of otherwise very little sequence conservation between the two species. PCR ampliļ¬cation was used to identify sets of contiguous genes from this region in the related Brachyspira species B. intermedia, B. innocens, B. murdochii, B. alvinipulli, and B. aalborgi, and demonstrated the presence of at least part of this region in species from throughout the genus. Comparative genomic analysis with other sequenced bacterial species revealed that none of the completely sequenced spirochaete species from different genera contained this conserved cluster of coding sequences. In contrast, Enterococcus faecalis and Escherichia coli contained high gene cluster conservation across the 26 kb region, against an expected background of little sequence conservation between these phylogenetically distinct species. The conserved region in B. hyodysenteriae contained ļ¬ve genes predicted to be associated with amino acid transport and metabolism, four with energy production and conversion, two with nucleotide transport and metabolism, one with ion transport and metabolism, and four with poorly characterised or uncertain function, including an ankyrin repeat unit at the 5ā end. The most likely explanation for the presence of this 26 kb region in the Brachyspira species and in two unrelated enteric bacterial species is that the region has been involved in horizontal gene transfer
Change in the distribution of a member of the strand line community: the seaweed fly (Diptera: Coelopidae)
1. Coastal organisms are predicted to be particularly susceptible to the impact of global warming. In this study the distribution and relative abundance of two coastal invertebrates, Coelopa frigida (Fabricius) and C. pilipes are investigated. 2. Coelopa pilipes has a more southerly distribution than C. frigida , and prefers a warmer climate. Coelopa pilipes is less resistant to sub-zero temperatures than C. frigida and its northerly distribution is probably limited by cold winter days. 3. The most recent distribution map of C. frigida and C. pilipes in northern Europe was published a decade ago and showed the northerly extent of the distribution of C. pilipes reaching the north coast of mainland Scotland but its complete absence from the Western and Northern Isles. 4. C. pilipes has now spread throughout the Western Isles and the Orkney Islands but is still absent from Shetland. There has also been an increase in the relative frequency of C. pilipes at sites harbouring coelopids on the British mainland. A similar pattern of distribution change along the west coast of Sweden is reported. 5. It is proposed that these changes have occurred primarily as a result of global warming and in particular due to the recent increase in winter temperatures. A number of other indirect effects may have also contributed to these changes, including a probable change in macroalgae distribution. The implications of these changes for the wrack bed ecosystem and at higher trophic levels are considered
Local Dynamics and Strong Correlation Physics I: 1D and 2D Half-filled Hubbard Models
We report on a non-perturbative approach to the 1D and 2D Hubbard models that
is capable of recovering both strong and weak-coupling limits. We first show
that even when the on-site Coulomb repulsion, U, is much smaller than the
bandwith, the Mott-Hubbard gap never closes at half-filling in both 1D and 2D.
Consequently, the Hubbard model at half-filling is always in the
strong-coupling non-perturbative regime. For both large and small U, we find
that the population of nearest-neighbour singlet states approaches a value of
order unity as as would be expected for antiferromagnetic order. We
also find that the double occupancy is a smooth monotonic function of U and
approaches the anticipated non-interacting limit and large U limits. Finally,
in our results for the heat capacity in 1D differ by no more than 1% from the
Bethe ansatz predictions. In addition, we find that in 2D, the heat capacity vs
T for different values of U exhibits a universal crossing point at two
characteristic temperatures as is seen experimentally in a wide range of
strongly-correlated systems such as , , and . The
success of this method in recovering well-established results that stem
fundamentally from the Coulomb interaction suggests that local dynamics are at
the heart of the physics of strongly correlated systems.Comment: 10 pages, 16 figures included in text, Final version for publication
with a reference added and minor corrections. Phys. Rev. B, in pres
Asymptotic Expansions for Stationary Distributions of Perturbed Semi-Markov Processes
New algorithms for computing of asymptotic expansions for stationary
distributions of nonlinearly perturbed semi-Markov processes are presented. The
algorithms are based on special techniques of sequential phase space reduction,
which can be applied to processes with asymptotically coupled and uncoupled
finite phase spaces.Comment: 83 page
Is weak temperature dependence of electron dephasing possible?
The first-principle theory of electron dephasing by disorder-induced two
state fluctuators is developed. There exist two mechanisms of dephasing. First,
dephasing occurs due to direct transitions between the defect levels caused by
inelastic electron-defect scattering. The second mechanism is due to violation
of the time reversal symmetry caused by time-dependent fluctuations of the
scattering potential. These fluctuations originate from an interaction between
the dynamic defects and conduction electrons forming a thermal bath. The first
contribution to the dephasing rate saturates as temperature decreases. The
second contribution does not saturate, although its temperature dependence is
rather weak, . The quantitative estimates based on the
experimental data show that these mechanisms considered can explain the weak
temperature dependence of the dephasing rate in some temperature interval.
However, below some temperature dependent on the model of dynamic defects the
dephasing rate tends rapidly to zero. The relation to earlier studies of the
dephasing caused by the dynamical defects is discussed.Comment: 14 pages, 6 figures, submitted to PR
A glassy contribution to the heat capacity of hcp He solids
We model the low-temperature specific heat of solid He in the hexagonal
closed packed structure by invoking two-level tunneling states in addition to
the usual phonon contribution of a Debye crystal for temperatures far below the
Debye temperature, . By introducing a cutoff energy in the
two-level tunneling density of states, we can describe the excess specific heat
observed in solid hcp He, as well as the low-temperature linear term in the
specific heat. Agreement is found with recent measurements of the temperature
behavior of both specific heat and pressure. These results suggest the presence
of a very small fraction, at the parts-per-million (ppm) level, of two-level
tunneling systems in solid He, irrespective of the existence of
supersolidity.Comment: 11 pages, 4 figure
Rings and rigidity transitions in network glasses
Three elastic phases of covalent networks, (I) floppy, (II) isostatically
rigid and (III) stressed-rigid have now been identified in glasses at specific
degrees of cross-linking (or chemical composition) both in theory and
experiments. Here we use size-increasing cluster combinatorics and constraint
counting algorithms to study analytically possible consequences of
self-organization. In the presence of small rings that can be locally I, II or
III, we obtain two transitions instead of the previously reported single
percolative transition at the mean coordination number , one from a
floppy to an isostatic rigid phase, and a second one from an isostatic to a
stressed rigid phase. The width of the intermediate phase and the
order of the phase transitions depend on the nature of medium range order
(relative ring fractions). We compare the results to the Group IV
chalcogenides, such as Ge-Se and Si-Se, for which evidence of an intermediate
phase has been obtained, and for which estimates of ring fractions can be made
from structures of high T crystalline phases.Comment: 29 pages, revtex, 7 eps figure
Collective dynamics of internal states in a Bose gas
Theory for the Rabi and internal Josephson effects in an interacting Bose gas
in the cold collision regime is presented. By using microscopic transport
equation for the density matrix the problem is mapped onto a problem of
precession of two coupled classical spins. In the absence of an external
excitation field our results agree with the theory for the density induced
frequency shifts in atomic clocks. In the presence of the external field, the
internal Josephson effect takes place in a condensed Bose gas as well as in a
non-condensed gas. The crossover from Rabi oscillations to the Josephson
oscillations as a function of interaction strength is studied in detail.Comment: 18 pages, 2 figure
Antiferromagnetic and van Hove Scenarios for the Cuprates: Taking the Best of Both Worlds
A theory for the high temperature superconductors is proposed. Holes are
spin-1/2, charge e, quasiparticles strongly dressed by spin fluctuations. Based
on their dispersion, it is claimed that the experimentally observed van Hove
singularities of the cuprates are likely originated by antiferromagnetic (AF)
correlations. From the two carriers problem in the 2D t-J model, an effective
Hamiltonian for holes is defined with %no free parameters. This effective model
has superconductivity in the channel, a critical
temperature at the optimal hole density, ,
and a quasiparticle lifetime linearly dependent with energy. Other experimental
results are also reproduced by the theory.Comment: 12 pages, 4 figures (on request), RevTeX (version 3.0), preprint
NHMF
- ā¦