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21.
Identification of ancillary fim genes affecting fimA expression in Salmonella typhimurium.
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Regulation of the gene, fimA, encoding the major fimbrial subunit of S. typhimurium S6704 was examined by using a lambda fimA-lacZ lysogen. Transformation of the lambda fimA-lacZ lysogen with various derivatives of the recombinant plasmid that encodes type 1 fimbrial expression, pISF101, indicated that two regions of this plasmid alter beta-galactosidase production. One plasmid is a deletion resulting in the loss of a 28-kDa polypeptide downstream of fimA, while the other plasmid encodes a 24- and a 27-kDa polypeptide. Northern (RNA) blot analyses indicated that the steady-state fimA mRNA levels of these transformants were high. In addition, phenotypic expression of type 1 fimbriae by agar-grown cultures is observed only in those transformants bearing plasmids which show increased beta-galactosidase and fimA mRNA levels. 相似文献
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23.
Krister Brood 《Palaeogeography, Palaeoclimatology, Palaeoecology》1976,20(3):187-208
The present paper deals with the ecology of the Bryozoa in the Upper Wenlockian of Gotland. The Upper Wenlockian sedimentary rocks of Gotland, locally known as the Halla-Mulde Beds, are deposited in a shallow sea. The sedimentary rock types include well-developed reefs of “barrier” and “fringing” types and marly limestones in both forereef and backreef positions. These sedimentary rocks are rich in Bryozoa as well as in other shelly fossils. Twenty seven bryozoan species have been identified from this stratigraphic sequence. The bryozoans tend to occur in associations. These are not strictly comparable with the neontological animal communities in the Petersen sense, but should rather be regarded as “ecozones”.Twelve different faunistic associations can be identified in the investigated material. On Bryozoa alone, five major zones are discernible. 相似文献
24.
The thermal transitions of myosin and its helical fragments have been studied with pH as the observable. Heating unbuffered solutions of these proteins near their pI values causes an abrupt rise in pH at a characteristic temperature (the "melting temperature," Tm) which is due to structural changes within the protein. Since the pH shift turns out to be insensitive to the degree of protein aggregation, we have obtained acceptable melting curves even under conditions where the protein coagulates during melting. The melting profiles and Tm vlaues of myosin, myosin rod, and light meromyosin have been found to be remarkably similar (Tm equal to 40 plus or minus 1 degree, 0.5 M KCl, pH 5.9). Proton binding which occurs during melting coincides with the unfolding of a section of myosin rod. Taken in the context of other studies, the proton binding is thought to occur near the "hinge region." 相似文献
25.
The structural stabilities of all the familiar proteolytic fragments of myosin have been investigated in melting studies over the pH ranges 5.5-7.0 in 0.5 M KCl. All fragments except subfragment 2 undergo a melting transition manifested by the cooperative uptake of protons in the temperature range 34-47 degrees C, and these fragments experience an increase in transition temperature, Tm as the pH is increased. Subfragment 2 undergoes a melting transition in the 43-55 degrees C range, manifested by the dissociation of protons, and it experiences a decrease in Tm as the pH is increased. These results suggest that pH changes can modulate the relative stabilities of the light meromysin, subfragment-1, and subfragment-2 regions of the myosin molecule. 相似文献
26.
Zsuzsanna Sük?sd M. Shel Swenson J?rgen Kjems Christine E. Heitsch 《Nucleic acids research》2013,41(5):2807-2816
Recent advances in RNA structure determination include using data from high-throughput probing experiments to improve thermodynamic prediction accuracy. We evaluate the extent and nature of improvements in data-directed predictions for a diverse set of 16S/18S ribosomal sequences using a stochastic model of experimental SHAPE data. The average accuracy for 1000 data-directed predictions always improves over the original minimum free energy (MFE) structure. However, the amount of improvement varies with the sequence, exhibiting a correlation with MFE accuracy. Further analysis of this correlation shows that accurate MFE base pairs are typically preserved in a data-directed prediction, whereas inaccurate ones are not. Thus, the positive predictive value of common base pairs is consistently higher than the directed prediction accuracy. Finally, we confirm sequence dependencies in the directability of thermodynamic predictions and investigate the potential for greater accuracy improvements in the worst performing test sequence. 相似文献
27.
Extant large constrictors, pythons and boas, have a wholly allopatric distribution that has been interpreted largely in terms of vicariance in Gondwana. Here, we describe a stem pythonid based on complete skeletons from the early-middle Eocene of Messel, Germany. The new species is close in age to the divergence of Pythonidae from North American Loxocemus and corroborates a Laurasian origin and dispersal of pythons. Remarkably, it existed in sympatry with the stem boid Eoconstrictor. These occurrences demonstrate that neither dispersal limitation nor strong competitive interactions were decisive in structuring biogeographic patterns early in the history of large, hyper-macrostomatan constrictors and exemplify the synergy between phylogenomic and palaeontological approaches in reconstructing past distributions. 相似文献
28.
Nathan G. Swenson 《Ecography》2013,36(3):264-276
Tropical tree communities present one of the most challenging systems for studying the processes underlying community assembly. Most community assembly hypotheses consider the relative importance of the ecological similarity of co‐occurring species. Quantifying this similarity is a daunting and potentially impossible task in species‐rich assemblages. During the past decade tropical tree ecologists have increasingly utilized phylogenetic trees and functional traits to estimate the ecological similarity of species in order to test mechanistic community assembly hypotheses. A large amount of work has resulted with many important advances having been made along the way. That said, there are still many outstanding challenges facing those utilizing phylogenetic and functional trait approaches to study community assembly. Here I review the conceptual background, major advances and major remaining challenges in phylogenetic‐ and trait‐based approaches to community ecology with a specific focus on tropical trees. I argue that both approaches tremendously improve our understanding of tropical tree community ecology, but neither approach has fully reached its potential thus far. 相似文献
29.
Johan Zakrisson Krister Wiklund Ove Axner Magnus Andersson 《Biophysical journal》2013,104(10):2137-2148
Type 1 fimbriae mediate adhesion of uropathogenic Escherichia coli to host cells. It has been hypothesized that due to their ability to uncoil under exposure to force, fimbriae can reduce fluid shear stress on the adhesin-receptor interaction by which the bacterium adheres to the surface. In this work, we develop a model that describes how the force on the adhesin-receptor interaction of a type 1 fimbria varies as a bacterium is affected by a time-dependent fluid flow mimicking in vivo conditions. The model combines in vivo hydrodynamic conditions with previously assessed biomechanical properties of the fimbriae. Numerical methods are used to solve for the motion and adhesion force under the presence of time-dependent fluid profiles. It is found that a bacterium tethered with a type 1 pilus will experience significantly reduced shear stress for moderate to high flow velocities and that the maximum stress the adhesin will experience is limited to ∼120 pN, which is sufficient to activate the conformational change of the FimH adhesin into its stronger state but also lower than the force required for breaking it under rapid loading. Our model thus supports the assumption that the type 1 fimbria shaft and the FimH adhesin-receptor interaction are optimized to each other, and that they give piliated bacteria significant advantages in rapidly changing fluidic environments. 相似文献
30.