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101.
Although bacterial species display wide variation in their overall GC contents, the genes within a particular species' genome
are relatively similar in base composition. As a result, sequences that are novel to a bacterial genome—i.e., DNA introduced
through recent horizontal transfer—often bear unusual sequence characteristics and can be distinguished from ancestral DNA.
At the time of introgression, horizontally transferred genes reflect the base composition of the donor genome; but, over time,
these sequences will ameliorate to reflect the DNA composition of the new genome because the introgressed genes are subject
to the same mutational processes affecting all genes in the recipient genome. This process of amelioration is evident in a
large group of genes involved in host-cell invasion by enteric bacteria and can be modeled to predict the amount of time required
after transfer for foreign DNA to resemble native DNA. Furthermore, models of amelioration can be used to estimate the time
of introgression of foreign genes in a chromosome. Applying this approach to a 1.43-megabase continuous sequence, we have
calculated that the entire Escherichia coli chromosome contains more than 600 kb of horizontally transferred, protein-coding DNA. Estimates of amelioration times indicate
that this DNA has accumulated at a rate of 31 kb per million years, which is on the order of the amount of variant DNA introduced
by point mutations. This rate predicts that the E. coli and Salmonella enterica lineages have each gained and lost more than 3 megabases of novel DNA since their divergence.
Received: 7 July 1996 / Accepted: 27 September 1996 相似文献
102.
Fran?ois Guerrero Henri Burnet 《European journal of applied physiology and occupational physiology》1995,71(1):87-94
Carotid blood flow was measured in rats by implanted transit-time ultrasonic flowprobes during hyperbaric experiments at up to 70 bar (7 MPa) using an helium-oxygen hyperoxic (partial pressure of O2 = 400 mbar) mixture. Before the hyperbaric experiment, an intracerebroventricular injection of phosphate saline buffered solution (PBS) or aminophylline, an adenosine receptor blocker, in PBS was given. Throughout the hyperbaric experiment carotid blood flow increased with ambient pressure in both PBS, i.e. control, and aminophylline treated rats. The increase in carotid blood flow was significantly attenuated in aminophylline treated rats. Additional experiments showed that the increased carotid blood flow was independent of hyperoxia as well as of temperature. The hypothesis that the hyperbaric dependent increase in carotid blood flow was mediated by brain adenosine receptors and its implication regarding a cerebral vasodilatation are discussed. 相似文献
103.
H. Nomura H. Suzuki 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1995,176(1):11-15
To elucidate the role of the water receptor in the frog (Rana catesbeiana), reflex activities elicited by its excitation were studied. Application of tap water to the oral mucosa depressed the rhythmical movement of gorge (buccal) respiration, accompanied by an elevation of the inner pressure of the oral cavity (buccal pressure). Tonic reflex discharges were elicited in the nerves innervating the submental and submaxillary muscles, which close the nostrils, the pterygoid and the profound portion of the major masseter muscles, which produce a strong bite, and the geniohyoid and hyoglossus muscles, which elevate buccal pressure. These muscles, except for the pterygoid, also participate in the rhythmical movement of gorge respiration as expiratory muscles. Rhythmical movements in the minor masseter and sternohyoid muscles, which act as inspiratory muscles in gorge respiration, were depressed by the water stimulation of the oral mucosa. These findings indicate that the water receptor plays a role in the interruption of gorge respiratory movements, accompanied by an elevation of buccal pressure. 相似文献
104.
电刺激兔肾脏传入神经对血压,心率及加压素释放的影响 总被引:1,自引:0,他引:1
本工作以兔为实验对象,观察电刺激肾脏传入神经(ARN)对血压、心率、颈交感神经放电、以及加压素(AVP)合成和释放的影响,并对ARN进入中枢的通路作了观察。结果显示,电刺激ARN可以引起血压下降、心率减慢、颈交感神经放电抑制等反应,ARN的兴奋还可使下丘脑的视上核、室旁核中的AVP含量增加,垂体中AVP含量下降,血浆AVP水平升高。硝普钠的降压实验和静脉注射AVP受体阻断剂AVPa的实验均证实了A 相似文献
105.
Yasuo Ezaki 《Ecological Research》1995,10(3):359-368
Settlement of male great reed warblers in the breeding ground was highly asynchronous at Lake Biwa in Japan. It took over
1 month from the appearance of the first male to a saturation in number of males. In resource-defense-polygyny, males are
expected to try to defend as large an area as possible in the optimal habitat. In fact, a small number of the earliest settling
males divided up the breeding ground almost completely as territories among themselves and these were later reduced in size
with the addition of later settlers. The reduction was not due to a seasonal decline of aggressiveness on the part of the
owners but to a higher level of intrusion pressure by later arriving males. The neighbor-neighbor relationship, once established,
was rather stable. Home range overlap was small and territorial contacts were few between neighbors. Territorial boundaries
seem not to shift despite the addition of new males as long as the neighbors were the same. The stable relationships between
neighbors are expected to help the earlier resident to maintain his large and good territories throughout the breeding season
and hence polygyny is favored in this species. 相似文献
106.
Summary In the present study the binding of [3H]MK-801 to glutamatergic receptors of the NMDA type was compared in spontaneously hypertensive (SHR) and normotensive (WKY) rats in various brain structures (including nucleus tractus solitarii) by quantitative receptor autoradiography. Additionally, blood pressure changes after treatment with the NMDA antagonist MK-801 were studied in both strains. There were no differences between SHR and WKY rats either in the level of [3H]MK-801 binding or in the hypertensive reaction to MK-801. 相似文献
107.
植物生活史对策的进化 总被引:19,自引:0,他引:19
植物生活史对策的进化班勇(中国林业科学院林业研究所,北京100091)EvolutionofLifeHistoryStrategyinPlants¥BanYong(ResearchInstituteofForestry,ChineseAcademyo... 相似文献
108.
心肺压力感受器持续卸荷对前臂血流、血管阻力及心率变异性谱的影响 总被引:1,自引:0,他引:1
低压值下体负压(LBNP)可仅使心肺压力感受器卸荷。采用-2kPaLBNP实验结果表明:LBNP既不引起动脉血压变化,也不引起心率改变,但却引起基础胸阻抗(Z。)从对照的21.8±0.4升高到22.5±0.5Ω(P<0.01),前臂血管阻力(FVR)从12.3±0.9升高到19.9±1.4U(P<0.01),前臂血流(FBF)从对照时7.1±0.5降低到4.3±0.3ml·min ̄(-1)·100ml ̄(-1),心率变异性谱(HRV)未发生任何变化,即心肺压力感受器卸荷时心脏自主神经活动水平与均衡性不受影响。由于FVR和FBF的变化可间接反映外周血管交感传出活动水平,上述实验结果提示,心肺压力感受器对外周血管及心脏自主神经活动的调节可能存在机能分化现象。 相似文献
109.
BRUCE MICHAEL PAVLIK 《Plant, cell & environment》1984,7(7):531-539
Abstract Midday water potentials of blades of the dune grasses Ammophila arenaria (L.) Link and Elymus mollis Trin. ex Spreng. growing in situ declined over the summer growing period, indicating a trend of increasing water stress. An analysis of the water relations characteristics of these blades using pressure-volume techniques demonstrated that both species increased bulk osmotic pressure at full hydration () and, therefore, bulk turgor as an acclimation response. In A. arenaria, however, the increase of osmotic pressure (+ 0.35 MPa) was entirely the result of decreasing symplasmic water content. The increase of osmotic pressure (+ 0.54 MPa) observed in E. mollis blades was due to solute accumulation (72% of Δ) and to a lesser degree, decreased symplasmic water content (28% of Δ). Osmotic adjustment in E. mollis blades was accompanied by a significant decrease in tissue elasticity (max went from 12 to 19 MPa). The elastic properties of A. arenaria blades remained constant over the same period and had a maximum modulus (10 MPa) that was always less than that of E. mollis, As estimated from Höfler plots, these seasonal adjustments of osmotic pressure and differences in tissue elasticity enabled plants in situ to maintain turgor pressure in the range of 0.5–0.6 MPa at the lowest water potentials of mid-August. Laboratorygrown plants exhibited the species-specific differences in osmotic pressure, turgor pressure, and tissue elasticity observed in field plants. Although certain alterations of leaf structure were expected to coincide with the observed changes and species-specific differences in symplasmic water content and tissue elasticity, these could not be detected by measurements of specific leaf weight or the ratio of dry matter to saturated water content. 相似文献
110.
A. Georgallas D. L. Hunter T. Lookman M. J. Zuckermann D. A. Pink 《European biophysics journal : EBJ》1984,11(2):79-86
We have modelled a phospholipid bilayer as two monolayer sheets which interact with each other by a coupling which depends upon the states of the lipid hydrocarbon chains in each sheet. We make use of a model (Georgallas and Pink 1982a) and its parameters, already used to study monolayer phase changes at the LC-LE transition, in order to study the lipid main transition. Although the monolayer coexistence curve can be calculated exactly, we have made use of high-temperature series expansions to calculate the critical point of the bilayer. We also present the results of computer simulations on triangular lattices for the pressure-area isotherms. We find: (i) the interaction between the sheets of a DPPC bilayer is about 1.5–2% of the maximum interaction within the plane of each sheet; (ii) the internal lateral pressure of a DPPC bilayer is about 30.5 dyne/cm; (iii) the bilayer transition enthalpy depends sensitively upon the coupling between the sheets. Should this coupling vary from sample to sample (due, possibly, to its preparation) then very different values of transition enthalpy may be measured. (iv) We present a rough rule-of-thumb for estimating the internal lateral pressure of a bilayer from a knowledge of the corresponding monolayer pressure-area isotherms.Abbreviations LC-LE
liquid condensed — liquid expanded
- DPPC
dipalmitoylphosphatidylcholine
- Q
transition enthalpy
Work supported in part by the Natural Sciences and Engineering Research Council of Canada 相似文献