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101.
102.
ExbB acts as a chaperone-like protein to stabilize TonB in the cytoplasm 总被引:19,自引:5,他引:14
The TonB protein is required to transduce energy from the cytoplasmic membrane to outer membrane transport proteins of Gram-negative bacteria. Two accessory proteins, ExbB and ExbD, are required for TonB function and it has been suggested that TonB and ExbBD form a complex in the membrane. In this paper we demonstrate that there are two spatially distinct, functional interactions between ExbBD and TonB. First, there is an interaction between ExbBD and the N-terminal signal-like peptide of TonB, probabiy the formation of a stable complex in the membrane. Second, ExbB interacts with TonB in the cytoplasm. This interaction involves the domain of TonB that is normally periplasmic. Thus, this is a transient interaction which occurs during the synthesis and/or localization of TonB, implying a chaperone-like role for ExbB. The transmembrane topology of ExbB was shown to be consistent with this role. 相似文献
103.
Adnan M. M. Mjalli Kevin T. Chapman Malcolm MacCoss Nancy A. Thornberry 《Bioorganic & medicinal chemistry letters》1993,3(12):2689-2692
Phenylalkyl ketones are potent reversible inhibitors of interleukin-1β converting enzyme (ICE). The extended alkyl chain ketones AcTyrValAlaAspCO(CH2)nPh display increased potency over the simple aryl ketone. In particular, the tetrapeptide AcTyrValAlaAspCOPh has a Ki of 10μM while AcTyrValAlaAspCO(CH2)5Ph has a Ki of 18.5nM. 相似文献
104.
THE HOLY GRAIL OF THE PERFECT CHARACTER: THE CLADISTIC TREATMENT OF MORPHOMETRIC DATA 总被引:9,自引:0,他引:9
Abstract— Data scored for cladistic analyses may be quantitative or qualitative, continuous or discrete, and show overlapping or non-overlapping values between taxa. Quantitative and qualitative are modes of expression of data, while continuous or discrete refer to properties of the set of numbers that express the data; both these pairs of terms have been confused with overlapping and non-overlapping. The degree of overlap of values between taxa is often used to filter characters in cladistic analyses: if a minimum amount of overlap is exceeded, or a minimum amount of disjunction not reached, characters are rejected as "not cladistic". However, this rests on a confusion between features of taxa and features of individual organisms (attributes). Cladistic characters are features of taxa, and comprise frequency distributions of attribute values over individuals of a taxon. Cladistic characters logically cannot overlap, although taxa may have overlapping attribute values. Thus, a priori rejection of characters that have overlapping attribute values is non-sensical. Such data may still be rejected from consideration for cladistic analysis if it could be demonstrated that they contain little recoverable phylogenetic signal. Few published analyses have empirically tested this. An analysis of overlapping morphometric data from three series of Banksia suggests that, at least in these cases, they map phylogeny almost as accurately as more conventional, qualitative morphological data. While more such tests are required, morphometric data should not be rejected a priori from cladistic analyses. 相似文献
105.
Parthenogenesis has been observed in several elasmobranch species, primarily in public aquaria. The majority of cases of parthenogenesis have occurred either when females were held without males or once a male was removed from a female's habitat. Here we report a second instance of parthenogenesis in a zebra shark female that was housed with conspecific mature males. This study calls into question the conditions under which elasmobranch females undergo parthenogenesis. 相似文献
106.
Kevin M. Brindle Alexandra M. Fulton Simon-Peter Williams 《Journal of molecular recognition : JMR》1993,6(4):159-165
Models of metabolic flux regulation are frequently based on an extrapolation of the kinetic properties of enzymes measured in vitro to the intact cell. Such an extrapolation assumes a detailed knowledge of the intracellular environment of these enzymes in terms of their free substates and effectors concentrations and possible interaction with other cellular macromolecules, which may modify their kinetic properties. These is a considerable incentive, therefore, to study the properties of enzymes directly in vivo. We have been using non-invasive NMR techniques, in conjunction with molecular genetic manipulation of enzyme levels, to study the kinetic properties of individual enzymes in vivo. We have also developed a novel strategy which has allowed us to monitor, by NMR, the ligand binding properties and mobilities of enzymes in the intact cell. This technique may also allow us to measured the diffusion coefficients of these proteins in the cell. These studies should give new insight into the properties of enzymes in vivo 相似文献
107.
Lawrence Levin Edward Fielding Kevin Ackhurst 《Flexible Services and Manufacturing Journal》1993,5(2):143-159
This article focuses on the development of a computer control system for a flexible manufacturing cell. The paper outlines the various modules that are required for a computer control system and elaborates on the development of some of these modules. This is part of an ongoing research project at the University of the Witwatersrand. The aim of the project is to demonstrate the viability of flexible manufacturing systems to South African industry. Thus the main constraints of the project were cost, ease of implementation, and ease of maintainability. This necessitated the use of existing machinery and of computer systems and software that were readily available and already familiar to industry. 相似文献
108.
In the marine unicellular chlorophyte, Dunaliella tertiolecta Butcher, the spectrally averaged m vivo absorption cross section, normalized to chlorophyll a (so-called a* values), vary two-fold in response to changes in growth irradiance. We used a kinetic approach to examine the specific factors which account for these changes in optical properties as cells photoadapt. Using Triton X-100 to solubilize membranes, we were able to differentiate between “package” effects and pigmentation effects. Our analyses suggest that 43–49% of the variability in a* is due to changes in pigmentation, whereas 51–57% is due to the “package” effect. Further analyses revealed that changes in cell sue did not significantly affect packaging, while thylakoid stacking and the transparency of thylakoid membranes were important factors. Our results suggest that thylakoid membrane protein/lipid ratios change during photoadaptation, and these changes influence the effective rate of light harvesting per unit chlorophyll a. 相似文献
109.
Arland T. Hotchkiss Michael R. Gretz Kevin B. Hicks R. Malcolm Brown 《Journal of phycology》1989,25(4):646-654
The two-layered, fibrillar cell wall of Mougeotia C. Agardh sp. consisted of 63.6% non-cellulosic carbohydrates and 13.4% cellulose. The orientation of cellulose microfibrils in the native cell wall agrees with the multinet growth hypothesis, which has been employed to explain the shift in microfibril orientation from transverse (inner wall) toward axial (outer wall). Monosaccharide analysis of isolated cell walls revealed the presence of ten sugars with glucose, xylose and galactose most abundant. Methylation analysis of the acid-modified, 1 N NaOH insoluble residue fraction showed that it was composed almost exclusively of 4-linked glucose, confirming the presence of cellulose. The major hemicellulosic carbohydrate was semi-purified by DEAE Sephacel (Cl?) anion-exchange chromatography of the hot 1 N NaOH soluble fraction. This hemicellulose was a xylan consisting of a 4-xylosyl backbone and 2,4-xylosyl branch points. The major hot water soluble neutral polysaccharide was identified as a 3-linked galactan. Mougeotia cell wall composition is similar to that of (Charophyceae) and has homologies with vascular plant cell walls. Our observations support transtructural evidence which suggests that members of the Charophyceae represent the phylogenetic line that gave rise to vascular plants. Therefore, the primary cell walls of vascular plants many have evolved directly from structures typical of the filamentous green algal cell walls found in the Charophyceae. 相似文献
110.
Alicia Mathis Kevin L. Murray & Caleb R. Hickman 《Ethology : formerly Zeitschrift fur Tierpsychologie》2003,109(2):159-170
Prey may experience ontogenetic changes in vulnerability to some predators, either because of changes in morphology or experience. If prey match their level of antipredator behavior to the level of predatory threat, prey responses to predators should reflect the appropriate level of threat for their stage of development. For larval salamanders, responses to predators may change with body size because larger larvae are less vulnerable to predation by gape‐limited predators or because fleeing responses by large salamanders may be more effective than for smaller salamanders. In a field experiment, small larval ringed salamanders, Ambystoma annulatum, responded to chemical stimuli (‘kairomones’) from predatory newts, Notophthalmus viridescens, with an antipredator response (decreased activity). Laboratory‐reared larvae decreased their activity following exposure to newt kairomones, indicating that larval ringed salamanders do not require experience with newts to recognize them as predators. In both experiments, larvae distinguished between chemical stimuli from newts and stimuli from tadpoles (non‐predators) and a blank control. In a third experiment, field‐caught (experienced) larvae showed a graded response to newt kairomones based on their body size: small larvae tended to decrease their activity while larger larvae showed no change or an increase in activity. This graded response was not observed for neutral stimuli, indicating that it is predator‐specific. Therefore, ringed salamander larvae exhibit threat‐sensitive ontogenetic changes in their response to chemical stimuli from predatory newts. 相似文献