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1.
Little is known about directed motility of bacteria that move by type IV pilus-mediated (twitching) motility. Here, we found that during periodic cell reversals of Myxoccocus xanthus, type IV pili were disassembled at one pole and reassembled at the other pole. Accompanying these reversals, FrzS, a protein required for directed motility, moved in an oscillatory pattern between the cell poles. The frequency of the oscillations was controlled by the Frz chemosensory system, which is essential for directed motility. Pole-to-pole migration of FrzS appeared to involve movement along a filament running the length of the cell. FrzS dynamics may thus regulate cell polarity during directed motility. 相似文献
2.
The bacterium Myxococcus xanthus has two motility systems: S motility, which is powered by type IV pilus retraction, and A motility, which is powered by unknown mechanism(s). We found that A motility involved transient adhesion complexes that remained at fixed positions relative to the substratum as cells moved forward. Complexes assembled at leading cell poles and dispersed at the rear of the cells. When cells reversed direction, the A-motility clusters relocalized to the new leading poles together with S-motility proteins. The Frz chemosensory system coordinated the two motility systems. The dynamics of protein cluster localization suggest that intracellular motors and force transmission by dynamic focal adhesions can power bacterial motility. 相似文献
3.
Fibrillar structures, originating from the plant cell wall in the intercellular spaces of leaves of ;Red Kidney' bean, Phaseolus vulgaris L., engulfed a saprophytic bacterium, Pseudomonas putida, after its initial attachment to the host walls. Phytopathogenic bacteria, Pseudomonas phaseolicola and Pseudomonas tomato, did not adhere to the plant cell wall nor were they encapsulated. Bean lectins may be involved in the attachment and encapsulation processes. 相似文献
4.
A random-code technique has been used at Arecibo to obtain delay-Doppler radar images of the full disk of Mercury. Anomalously bright features were found at the north and south poles. The north polar feature is oblong (4 degrees by 8 degrees ) and offset from the pole. The smaller south polar feature is mostly confined to the floor of the crater Chao Meng-Fu. The polar locations and radar properties of these features indicate that they may be produced by volume scattering in ice. The images also reveal a variety of more subdued reflectivity features ranging in size from hundreds to thousands of kilometers; some of these appear to have an impact origin. 相似文献
5.
选用我国丰富的毛竹作为结构材料,对圆竹的主要物理力学指标进行了试验与分析.研究结果表明,圆竹的基本力学指标高于TC13级针叶木材,可以作为结构材料使用.设计了几种圆竹构件和结构的金属连接件,基本实现了圆竹构件加工的标准化和施工的预制化.设计了基于墙板模数的圆竹墙体单元,并对2片墙体进行了抗侧力试验,圆竹墙体的抗侧力性能与U型连接件密切相关,抗侧向能力约为同类型轻型木结构墙体的65%.进行了3个圆竹屋架的静载试验,结果表明屋架的极限承载力由变形控制,平均值为12.3 kN.根据试验结果和木结构设计规范,设计和建造了一个约50 m2的预制装配式圆竹房屋示范建筑,验证了该技术的可行性和适用性.示范建筑已使用3年多,目前状况良好. 相似文献
6.
7.
Eukaryotic chromosomes are anchored to a spindle apparatus during mitosis, but no such structure is known during chromosome segregation in bacteria. When sister chromosomes are segregated during sporulation in Bacillus subtilis, the replication origin regions migrate to opposite poles of the cell. If and how origin regions are fastened at the poles has not been determined. Here we describe a developmental protein, RacA, that acts as a bridge between the origin region and the cell poles. We propose that RacA assembles into an adhesive patch at a centromere-like element near the origin, causing chromosomes to stick at the poles. 相似文献
8.
Distribution of hydrogen in the near surface of Mars: evidence for subsurface ice deposits 总被引：1，自引：0，他引：1
Boynton WV Feldman WC Squyres SW Prettyman TH Bruckner J Evans LG Reedy RC Starr R Arnold JR Drake DM Englert PA Metzger AE Mitrofanov I Trombka JI D'Uston C Wanke H Gasnault O Hamara DK Janes DM Marcialis RL Maurice S Mikheeva I Taylor GJ Tokar R Shinohara C 《Science (New York, N.Y.)》2002,297(5578):81-85
Using the Gamma-Ray Spectrometer on the Mars Odyssey, we have identified two regions near the poles that are enriched in hydrogen. The data indicate the presence of a subsurface layer enriched in hydrogen overlain by a hydrogen-poor layer. The thickness of the upper layer decreases with decreasing distance to the pole, ranging from a column density of about 150 grams per square centimeter at -42 degrees latitude to about 40 grams per square centimeter at -77 degrees. The hydrogen-rich regions correlate with regions of predicted ice stability. We suggest that the host of the hydrogen in the subsurface layer is ice, which constitutes 35 +/- 15% of the layer by weight. 相似文献
9.
Domínguez-Escobar J Chastanet A Crevenna AH Fromion V Wedlich-Söldner R Carballido-López R 《Science (New York, N.Y.)》2011,333(6039):225-228
The peptidoglycan cell wall and the actin-like MreB cytoskeleton are major determinants of cell shape in rod-shaped bacteria. The prevailing model postulates that helical, membrane-associated MreB filaments organize elongation-specific peptidoglycan-synthesizing complexes along sidewalls. We used total internal reflection fluorescence microscopy to visualize the dynamic relation between MreB isoforms and cell wall synthesis in live Bacillus subtilis cells. During exponential growth, MreB proteins did not form helical structures. Instead, together with other morphogenetic factors, they assembled into discrete patches that moved processively along peripheral tracks perpendicular to the cell axis. Patch motility was largely powered by cell wall synthesis, and MreB polymers restricted diffusion of patch components in the membrane and oriented patch motion. 相似文献
10.
Bacterial differentiation 总被引：27，自引：0，他引：27
11.
针对一类线性系统的区域极点配置的稳定问题，以机器人手臂为研究对象，对被控对象进行相关描述并且建立运动模型。给出线性系统区域极点配置在机器人手臂的运动模型稳定中的重要结论。利用LMI求解出控制器，并例举一个具体实例通过计算机仿真证实结论的正确有效性，通过分析得出区域极点配置的控制方式较精确极点配置的控制方式在稳定性特征上能够更好的控制系统，使得系统满足静态与动态特性。 相似文献
12.
Gram-positive bacteria face unique challenges in generating biologically active conformations for their exported proteins because they lack a dedicated compartment for folding secreted polypeptides. We have discovered that protein secretion by way of the general secretory (Sec) pathway in the important human pathogen Streptococcus pyogenes proceeds through a single microdomain. Unlike other mechanisms for asymmetry involving the Sec pathway, proteins destined for secretion are targeted to a single locus distal to either cell pole that has specialized to contain the Sec translocons. This subcellular organization may represent a paradigm for secretion common to other Gram-positive pathogens with profound implications for pathogenesis. 相似文献
13.
The Infrared Thermal Mappers aboard the two Viking orbiters obtained solar reflectance and infrared emission measurements of the Martian north and south polar regions during an entire Mars year. The observations were used to determine annual radiation budgets, infer annual carbon dioxide frost budgets, and constrain spring season surface and atmospheric properties with the aid of a polar radiative model. The results provide further confirmation of the presence of permanent CO(2)frost deposits near the south pole and show that the stability of these deposits can be explained by their high reflectivities. In the north, the observed absence of solid CO(2) during summer was primarily the result of enhanced CO(2) sublimation rates due to lower frost reflectivities during spring. The results suggest that the present asymmetric behavior of CO(2)frost at the Martian poles is caused by preferential contamination of the north seasonal polar cap by atmospheric dust. 相似文献
14.
Loose M Fischer-Friedrich E Ries J Kruse K Schwille P 《Science (New York, N.Y.)》2008,320(5877):789-792
In the bacterium Escherichia coli, the Min proteins oscillate between the cell poles to select the cell center as division site. This dynamic pattern has been proposed to arise by self-organization of these proteins, and several models have suggested a reaction-diffusion type mechanism. Here, we found that the Min proteins spontaneously formed planar surface waves on a flat membrane in vitro. The formation and maintenance of these patterns, which extended for hundreds of micrometers, required adenosine 5'-triphosphate (ATP), and they persisted for hours. We present a reaction-diffusion model of the MinD and MinE dynamics that accounts for our experimental observations and also captures the in vivo oscillations. 相似文献
15.
Plague bacteria target immune cells during infection 总被引：1，自引：0，他引：1
Marketon MM DePaolo RW DeBord KL Jabri B Schneewind O 《Science (New York, N.Y.)》2005,309(5741):1739-1741
The plague is caused by the bacterium Yersinia pestis. Plague bacteria are thought to inject effector Yop proteins into host cells via the type III pathway. The identity of the host cells targeted for injection during plague infection is unknown. We found, using Yop beta-lactamase hybrids and fluorescent staining of live cells from plague-infected animals, that Y. pestis selected immune cells for injection. In vivo, dendritic cells, macrophages, and neutrophils were injected most frequently, whereas B and T lymphocytes were rarely selected. Thus, it appears that Y. pestis disables these cell populations to annihilate host immune responses during plague. 相似文献
16.
A simple system was developed for obtaining x-ray diffraction patterns from magnetically oriented solutions of macromolecular assemblies. A small permanent magnet was designed that produces a magnetic field of 16 kilogauss in a volume of 1 cubic millimeter and is mountable on most x-ray cameras. Many subcellular structures have sufficient diamagnetic anisotropy that they exhibit orientation in dilute solution when placed between the poles of the magnet. Diffraction from solutions oriented in this magnet can provide substantially more structural information than small-angle scattering from isotropic solutions. In favorable cases, such as dilute solutions of filamentous bacteriophages, it is possible to produce oriented fiber diffraction patterns from which intensities along layer lines can be measured to 7-angstrom resolution. The magnetically induced birefringence observed in solutions of other macromolecular assemblies suggests that this technique may have broad applicability to subcellular structures. 相似文献
17.
Fletcher LN Irwin PG Orton GS Teanby NA Achterberg RK Bjoraker GL Read PL Simon-Miller AA Howett C de Kok R Bowles N Calcutt SB Hesman B Flasar FM 《Science (New York, N.Y.)》2008,319(5859):79-81
Saturn's poles exhibit an unexpected symmetry in hot, cyclonic polar vortices, despite huge seasonal differences in solar flux. The cores of both vortices are depleted in phosphine gas, probably resulting from subsidence of air into the troposphere. The warm cores are present throughout the upper troposphere and stratosphere at both poles. The thermal structure associated with the marked hexagonal polar jet at 77 degrees N has been observed for the first time. Both the warm cyclonic belt at 79 degrees N and the cold anticyclonic zone at 75 degrees N exhibit the hexagonal structure. 相似文献
18.
Many pathogenic bacteria use a type III secretion nanomachine (an injectisome) to deliver virulence proteins into the cytosol of their eukaryotic host cells. Most injectisomes possess a stiff needlelike structure of a genetically defined length. We found that a minimal needle length was required for efficient functioning of the Yersinia enterocolitica injectisome. This minimal needle length correlated with the length of the major adhesin at the bacterial surface. The needle may be required for triggering type III secretion, and its length may have evolved to match specific structures at the bacterial and host cell surfaces. 相似文献
19.
Crystalline aggregates of strain MM virus occur in polymorphonuclear leukocytes in association with osmiophilic granular structures which are occasionally membrane-bound. This suggests either a phagocytosis and segregation of virus by leukocytes, analogous to the disposition of bacteria by leukocytes, or a utilization of the leukocyte by the virus as a host cell for virus replication. 相似文献
20.
迟缓爱德华氏菌的粘附特性 总被引：4，自引：0，他引：4
用绵羊红细胞对不同来源的１５株迟缓爱德华氏菌作血凝试验。结果表明，１５株ＥＴ均具有血凝性，且不能为甘露糖，蔗糖，葡萄糖所抑制。分别用甲醛，戊二醛，胰蛋白酶处理ＥＴ的模式菌株ＥＴ９９菌体，能抑制细菌血凝。提纯的ＥＴ９９菌株的鞭毛蛋白，外膜蛋白均无血凝性。同时用ＨＥｐ－２细胞分析了１５株ＥＴ的细胞粘附特性。结果表明，有１０株ＥＴ能粘附ＨＥｐ－２细胞、粘附菌数大于１０ＣＦＵ／细胞。分别用甲醛，戊二醛，胰 相似文献