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排序方式: 共有362条查询结果,搜索用时 9 毫秒
51.
Livia R. Tanjung Gabrielle Whittle Bruce E. Shaw Garry A. Bloomfield Margaret E. Katz Brian F. Cheetham 《Anaerobe》2009,15(5):219-224
The Gram-negative anaerobic pathogen Dichelobacter nodosus is the principal causative agent of footrot in sheep. The intA, intB and intC elements are mobile genetic elements which integrate into two tRNA genes downstream from csrA (formerly glpA) and pnpA in the D. nodosus chromosome. CsrA homologues act as global repressors of virulence in several bacterial pathogens, as does polynucleotide phosphorylase, the product of pnpA. We have proposed a model in which virulence in D. nodosus is controlled in part by the integration of genetic elements downstream from csrA and pnpA, altering the expression of these putative global regulators of virulence. We describe here a novel integrated genetic element, the intD element, which is 32 kb in size and contains an integrase gene, intD, several genes related to genes on other integrated elements of D. nodosus, a type IV secretion system and a putative mobilisation region, suggesting that the intD element has a role in the transfer of other genetic elements. Most of the D. nodosus strains examined which contained the intD gene were benign, with intD integrated next to pnpA, supporting our previous observation that virulent strains of D. nodosus have the intA element next to pnpA. 相似文献
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54.
M. van der Gaag A. J. M. Debets H. D. Osiewacz R. F. Hoekstra 《Molecular & general genetics : MGG》1998,258(5):521-529
A natural population of recently isolated Podospora anserina strains was screened for homologues of the linear longevity-inducing plasmid pAL2-1. Of the 78 wild-type isolates, 14 hybridised
with a pAL2-1 specific probe, half of which contained a single plasmid and the other half multiple plasmid copies (plasmid
family). All strains except one plasmid-containing strain, senesced normally. However, no inserted plasmid sequences were
detected in the mitochondrial DNA, as was the case for the longevity-inducing pAL2-1 plasmid. Occasional loss of plasmids
and of repeated plasmid sequences occurred during sexual transfer. Plasmid transmission was equally efficient for mono- and
dikaryotic spores and was independent of the genetic background of the strains. Furthermore, horizontal transfer experiments
showed that the linear plasmid could easily infect plasmid-free strains. Horizontal transfer was even observed between strains
showing a clear vegetative incompatibility response (barrage). The linear plasmids are inherited maternally; however, paternal
transmission was observed in crosses between confronted vegetative-incompatible strains. Paternal transmission of the plasmid
was never observed using isolated spermatia for fertilisation, showing that mitochondrial plasmids can only gain access to
maternal sexual reproductive structures following horizontal transfer. These findings have implications for both the function
of vegetative incompatibility in fungi and for the mechanism of maintenance of linear plasmids.
Received: 13 November 1997 / Accepted: 17 February 1998 相似文献
55.
56.
A. Schmidt J. Engelage H.-J. Bischof 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1999,185(1):69-79
Responses of neurons of the optic tectum, the prominent, highly laminated mesencephalic station of the tectofugal visual
pathway in birds, to computer-generated and other visual stimuli were examined in zebra finches. Our study shows that the
contralateral retina projects to the tectum in topographic order. The representation of the visual field is tilted against
the horizon by 22°. The representation of the contralateral hemifield extends to the ipsilateral side by 15°. Most neurons
have receptive fields with excitatory centres of different shapes and inhibitory surround. A new type of neuronal receptive
field is described which has an excitatory centre and a surround which is movement sensitive and preferably excited by very
small spots. The first type of neurons is mostly located in upper tectal layers, the latter only in deeper layers. Excitatory
centre sizes increase with depth, and there is a tendency of smaller receptive fields in the foveal region. The representation
of the frontal visual field does not show specializations which could be expected if it were used for fixation of grain during
pecking. Our results are in accordance with previous behavioural experiments.
Accepted: 30 April 1999 相似文献
57.
Kinematic data on primate head and neck posture were collected by filming 29 primate species during locomotion. These were used to test whether head and neck posture are significant influences on basicranial flexion and whether the Frankfurt plane can legitimately be employed in paleoanthropological studies. Three kinematic measurements were recorded as angles relative to the gravity vector, the inclination of the orbital plane, the inclination of the neck, and the inclination of the Frankfurt plane. A fourth kinematic measurement was calculated as the angle between the neck and the orbital plane (the head-neck angle [HNA]). The functional relationships of basicranial flexion were examined by calculating the correlations and partial correlations between HNA and craniometric measurements representing basicranial flexion, orbital kyphosis, and relative brain size (Ross and Ravosa [1993] Am. J. Phys. Anthropol. 91:305–324). Significant partial correlations were observed between relative brain size and basicranial flexion and between HNA and orbital kyphosis. This indicates that brain size, rather than head and neck posture, is the primary influence on flexion, while the degree of orbital kyphosis may act to reorient the visual field in response to variation in head and neck posture. Regarding registration planes, the Frankfurt plane was found to be horizontal in humans but inclined in all nonhuman primates. In contrast, nearly all primates (including humans) oriented their orbits such that they faced anteriorly and slightly inferiorly. These results suggest that for certain functional craniometric studies, the orbital plane may be a more suitable registration plane than Frankfurt “Horizontal.” Am J Phys Anthropol 108:205–222, 1999. © 1999 Wiley-Liss, Inc. 相似文献
58.
Elisabetta Catalani Silvia Tomassini Massimo Dal Monte Luigi Bosco Giovanni Casini 《Cell and tissue research》2009,336(3):423-438
Fibroblast growth factors (FGFs) exert basic functions both during embryonic development and in the adult. The expression
of FGFs and their receptors has been reported in mammalian retinas, although information on the organization of the FGF system
is still incomplete. Here, we report a detailed double-label immunohistochemical investigation of the localization patterns
of FGF1 and its receptors FGFR1 and FGFR2 in adult and early postnatal mouse retinas. In adult retinas, FGF1 is localized
to ganglion cells, horizontal cells, and photoreceptor inner and outer segments. FGFR1 is found in ganglion cells and Müller
cells, whereas FGFR2 is primarily located in ganglion cells, the nuclei of Müller cells, and glycine-containing amacrine cells.
During postnatal development, the patterns of FGF1, FGFR1, and FGFR2 immunostaining are similar to those in the adult, although
transient FGF1-expressing cells have been detected in the proximal inner nuclear layer before eye opening. These patterns
are consistent with a major involvement of FGF1, FGFR1, and FGFR2 in ganglion cell maturation (during development) and survival
(in the adult). Moreover, FGF1 may affect amacrine cell development, whereas Müller cells appear to be regulated via both
FGFR1 and FGFR2 throughout postnatal life. In immature retinas, large numbers of amacrine cells, including those containing
calbindin and glycine, display both FGF1 and FGFR2 immunoreactivities in their nuclei, suggesting an action of FGF1 on FGFR2
leading to the maturation of these amacrine cells during a restricted period of postnatal development.
This work was supported by funding from the Italian Ministry of Education. 相似文献
59.
Tristram O. West Gregg Marland Nagendra Singh Budhendra L. Bhaduri Adam B. Roddy 《Biogeochemistry》2009,94(1):29-41
Carbon dioxide is taken up by agricultural crops and released soon after during the consumption of agricultural commodities.
The global net impact of this process on carbon flux to the atmosphere is negligible, but impact on the spatial distribution
of carbon dioxide uptake and release across regions and continents is significant. To estimate the consumption and release
of carbon by humans over the landscape, we developed a carbon budget for humans in the United States. The budget was derived
from food commodity intake data for the US and from algorithms representing the metabolic processing of carbon by humans.
Data on consumption, respiration, and waste of carbon by humans were distributed over the US using geospatial population data
with a resolution of ~450 × 450 m. The average adult in the US contains about 21 kg C and consumes about 67 kg C year−1 which is balanced by the annual release of about 59 kg C as expired CO2, 7 kg C as feces and urine, and less than 1 kg C as flatus, sweat, and aromatic compounds. In 2000, an estimated 17.2 Tg C
were consumed by the US population and 15.2 Tg C were expired to the atmosphere as CO2. Historically, carbon stock in the US human population has increased between 1790 and 2006 from 0.06 Tg to 5.37 Tg. Displacement
and release of total harvested carbon per capita in the US is nearly 12% of per capita fossil fuel emissions. Humans are using,
storing, and transporting carbon about the Earth’s surface. Inclusion of these carbon dynamics in regional carbon budgets
can improve our understanding of carbon sources and sinks. 相似文献
60.
Zygomorphic flowers are usually more complex than actinomorphic flowers and are more likely to be visited by specialized pollinators.
Complex zygomorphic flowers tend to be oriented horizontally. It is hypothesized that a horizontal flower orientation ensures
effective pollen transfer by facilitating pollinator recognition (the recognition-facilitation hypothesis) and/or pollinator
landing (the landing-control hypothesis). To examine these two hypotheses, we altered the angle of Commelina communis flowers and examined the efficiency of pollen transfer, as well as the behavior of their visitors. We exposed unmanipulated
(horizontal-), upward-, and downward-facing flowers to syrphid flies (mostly Episyrphus balteatus), which are natural visitors to C. communis. The frequency of pollinator approaches and landings, as well as the amount of pollen deposited by E. balteatus, decreased for the downward-facing flowers, supporting both hypotheses. The upward-facing flowers received the same numbers
of approaches and landings as the unmanipulated flowers, but experienced more illegitimate landings. In addition, the visitors
failed to touch the stigmas or anthers on the upward-facing flowers, leading to reduced pollen export and receipt, and supporting
the landing-control hypothesis. Collectively, our data suggested that the horizontal orientation of zygomorphic flowers enhances
pollen transfer by both facilitating pollinator recognition and controlling pollinator landing position. These findings suggest
that zygomorphic flowers which deviate from a horizontal orientation may have lower fitness because of decreased pollen transfer. 相似文献