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排序方式: 共有213条查询结果,搜索用时 15 毫秒
41.
We have determined the relative concentrations of ribosomes accumulated under different growth conditions for a number of translational mutants as well as for some natural isolates of Escherichia coli. The mutants are a tRNA modification mutant (miaA), a streptomycin resistant (SmR) and a streptomycin pseudodependent (SmP) mutant as well as two ribosome ambiguity (ram) mutants. The natural isolates used in this study are known to function with submaximal ribosome kinetics. The data show that for all the ribosome mutants the concentration of ribosomes relative to that in wild type bacteria increases when the growth rate decreases. A small increase is also seen in the natural isolates. In contrast, the miaA mutant shows no increase in ribosome concentration under the same slow growth conditions. The results suggest that bacteria with kinetically impaired ribosomes can to some extent increase the number of ribosomes accumulated under poor growth conditions in order to compensate for their slower function. We use this observation to explain in part how bacteria growing in natural environments can escape the strong selection for maximized growth rates and for optimized ribosomes that are characteristic of laboratory strains. 相似文献
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R Mikkola N E Saris P A Grigoriev M A Andersson M S Salkinoja-Salonen 《European journal of biochemistry》1999,263(1):112-117
The emetic toxin of Bacillus cereus, found to cause immobilization of spermatozoa and swelling of their mitochondria, was purified and its structure found to be identical to the earlier known toxin cereulide. It increased the conductance in black-lipid membranes in KCl solutions in an ionophore-like manner. It formed adducts with K+, Na+, and NH4+ but the conductance was highly selective for K+ in relation to Na+ and H+ (three orders of magnitude). The increase in the kinetics of conductance indicated a stoichiometric ratio between the cereulide and K+. Its ionophoretic properties are thus similar to those of valinomycin. In addition, its effects on rat liver mitochondria were similar: it stimulated swelling and respiration in respiring mitochondria in the presence but not in the absence of K+, it reduced the transmembrane potential under these conditions. In nonrespiring mitochondria, swelling was seen in KNO3- but not in NaNO3-containing media, less in acetate. In NaNO3 media addition of the cereulide caused a transient diffusion potential which was reduced by adding K+. It is concluded that the toxic effects of cereulide are due to it being a K+ ionophore. 相似文献
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Ewelina Trela Qiang Lan Satu-Marja Myllymki Clmentine Villeneuve Riitta Lindstrm Vinod Kumar Sara A. Wickstrm Marja L. Mikkola 《The Journal of cell biology》2021,220(8)
The mammary gland develops from the surface ectoderm during embryogenesis and proceeds through morphological phases defined as placode, hillock, bud, and bulb stages followed by branching morphogenesis. During this early morphogenesis, the mammary bud undergoes an invagination process where the thickened bud initially protrudes above the surface epithelium and then transforms to a bulb and sinks into the underlying mesenchyme. The signaling pathways regulating the early morphogenetic steps have been identified to some extent, but the underlying cellular mechanisms remain ill defined. Here, we use 3D and 4D confocal microscopy to show that the early growth of the mammary rudiment is accomplished by migration-driven cell influx, with minor contributions of cell hypertrophy and proliferation. We delineate a hitherto undescribed invagination mechanism driven by thin, elongated keratinocytes—ring cells—that form a contractile rim around the mammary bud and likely exert force via the actomyosin network. Furthermore, we show that conditional deletion of nonmuscle myosin IIA (NMIIA) impairs invagination, resulting in abnormal mammary bud shape. 相似文献
45.
Synergistic effects in semidilute mixed solutions of alginate and lactose-modified chitosan (chitlac) 总被引:1,自引:0,他引:1
Donati I Haug IJ Scarpa T Borgogna M Draget KI Skjåk-Braek G Paoletti S 《Biomacromolecules》2007,8(3):957-962
The present study specifically aimed at preparing and characterizing semidilute binary polymer mixtures of alginate and chitlac which might find an application in the field of cell encapsulation. A polyanion, alginate, and a polycation, a lactose-modified chitosan, were mixed under physiological conditions (pH 7.4 and NaCl 0.15) and at a semidilute concentration avoiding associative phase separation. The mutual solubility was found to be dependent on the charge screening effect of the added NaCl salt, being prevented below 0.05 M NaCl. A comparison with the behavior of the polyanion (alginate) under the same experimental conditions revealed that both the viscosity and the relaxation times of the binary polymer solutions are strongly affected by the presence of the polycation. In particular, the occurrence of electrostatic interactions between the two oppositely charged polysaccharides led to a synergistic effect on the zero-shear viscosity of the solution, which showed a 4.2-fold increase with respect to that of the main component of the solution, i.e., alginate. Moreover, the relaxation time, calculated as the reciprocal of the critical share rate, markedly increased upon reducing the alginate fraction in the binary polysaccharide solution. However, the formation of the soluble complexes and the synergistic effect are reduced upon increasing the concentration of the 1:1 electrolyte. By containing a gel-forming polyanion (alginate, e.g., with Ca(2+) ions) and a bioactive polycation (chitlac, bearing a beta-linked D-galactose), the present system can be regarded as a first step toward the development of biologically active scaffold from polysaccharide mixtures. 相似文献
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The journey of developing hematopoietic stem cells 总被引:6,自引:0,他引:6
Hematopoietic stem cells (HSCs) develop during embryogenesis in a complex process that involves multiple anatomical sites. Once HSC precursors have been specified from mesoderm, they have to mature into functional HSCs and undergo self-renewing divisions to generate a pool of HSCs. During this process, developing HSCs migrate through various embryonic niches, which provide signals for their establishment and the conservation of their self-renewal ability. These processes have to be recapitulated to generate HSCs from embryonic stem cells. Elucidating the interactions between developing HSCs and their niches should facilitate the generation and expansion of HSCs in vitro to exploit their clinical potential. 相似文献
48.
Lotta Mårtensson Beatriz Díez Ingvild Wartiainen Weiwen Zheng Rehab El-Shehawy Ulla Rasmussen 《Plant and Soil》2009,325(1-2):207-218
The diazotrophic communities in a rice paddy field were characterized by a molecular polyphasic approach including DNA/RNA-DGGE fingerprinting, real time RT-PCR analysis of nifH gene and the measurement of nitrogen fixation activities. The investigation was performed on a diurnal cycle and comparisons were made between bulk and rhizosphere / root soil as well as between fertilized / unfertilized soils. Real time RT-PCR showed no significant difference in the total quantity of nifH expression under the conditions investigated. The functional diversity and dynamics of the nifH gene expressing diazotroph community investigated using RT-PCR-DGGE revealed high diurnal variations, as well as variation between different soil types. Most of the sequence types recovered from the DGGE gels and clone libraries clustered within nifH Cluster I and III (65 different nifH sequences in total). Sequence types most similar to Azoarcus spp., Metylococcus spp., Rhizobium spp., Methylocystis spp., Desulfovibrio spp., Geobacter spp., Chlorobium spp., were abundant and indicate that these species may be responsible for the observed diurnal variation in the diazotrophic community structure in these rice field samples. Previously described diazotrophic cyanobacterial genera in rice fields, such as Nostoc and Cyanothece, were present in the samples but not detectable in RT-PCR assays. 相似文献
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