Sequence-dependent pausing of single lambda exonuclease molecules     

Perkins TT  Dalal RV  Mitsis PG  Block SM 《Science (New York, N.Y.)》2003,301(5641):1914-1918

Lambda exonuclease processively degrades one strand of duplex DNA, moving 5'-to-3' in an ATP-independent fashion. When examined at the single-molecule level, the speeds of digestion were nearly constant at 4 nanometers per second (12 nucleotides per second), interspersed with pauses of variable duration. Long pauses, occurring at stereotypical locations, were strand-specific and sequence-dependent. Pause duration and probability varied widely. The strongest pause, GGCGAT TCT, was identified by gel electrophoresis. Correlating single-molecule dwell positions with sequence independently identified the motif GGCGA. This sequence is found in the left lambda cohesive end, where exonuclease inhibition may contribute to the reduced recombination efficiency at that end.  相似文献   

Bioelectric phenomena related to protein-fixed charge in a crab nerve fiber     

E M Lieberman  M S Perkins  T Tomita  E B Wright 《Science (New York, N.Y.)》1967,156(772):240-242

The bioelectrical characteristics of a crab nerve fiber subjected to solutions containing the impermeant anions ferrocyanide, glutamate, or ethanolsulfate substituted for chloride include prolonged slow depolarizations that elicit prolonged trains of impulses sulperimposed on the plateau portion. Propionate and nitrate depress repetitive firing while thiocyancite has only a slighit effect. Nerves treated with ferrocyanide lose potassium and gain sodium, which fact may account for their depolarization. On the other hand, studies with interference microscopy reveal shift by ferrocyanide in the interference fringe pattern of the sheath material; this suggests a reorientation of fixed charges in the protein layers by direct action of the anion on these layers. This may also account for the electrical manifestations observed.  相似文献   

Chemical detection with a single-walled carbon nanotube capacitor     

Snow ES  Perkins FK  Houser EJ  Badescu SC  Reinecke TL 《Science (New York, N.Y.)》2005,307(5717):1942-1945

We show that the capacitance of single-walled carbon nanotubes (SWNTs) is highly sensitive to a broad class of chemical vapors and that this transduction mechanism can form the basis for a fast, low-power sorption-based chemical sensor. In the presence of a dilute chemical vapor, molecular adsorbates are polarized by the fringing electric fields radiating from the surface of a SWNT electrode, which causes an increase in its capacitance. We use this effect to construct a high-performance chemical sensor by thinly coating the SWNTs with chemoselective materials that provide a large, class-specific gain to the capacitance response. Such SWNT chemicapacitors are fast, highly sensitive, and completely reversible.  相似文献   

Stage of pregancy and foetal loss following exposure of mares to processionary caterpillars     

A.J. Cawdell-Smith  K.H. Todhunter  N.R. Perkins  W.L. Bryden 《Journal of Equine Veterinary Science》2009

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2-Methylisoborneol disposition in three strains of catfish: absence of biotransformation     

Daniel Schlenk  Bryan DeBusk  Everett J. Perkins 《Fish physiology and biochemistry》2000,23(3):225-232

2-Methylisoborneol (MIB) and structurally related terpenoid compounds are responsible for millions of dollars of lost revenue to catfish farmers. In an attempt to determine enzymatic pathways of biotransformation and elimination of MIB, the in vitro metabolism of MIB was examined in the Ulvade strain of channel catfish (Ictalurus punctatus). Although cytochrome P450 (CYP) activities were observed and correlated with expression of specific isoforms (i.e. steroid hydroxylation and CYP3A expression), no metabolites of MIB were observed. To determine whether extrahepatic biotransformation may be occurring the in vivo metabolism and disposition of ¹⁴C-MIB was examined in Uvalde, USDA-103 channel catfish, and a channel catfish X blue catfish (Ictalurus furcatus) hybrid species. Confirming in vitro hepatic studies, no metabolites were observed in plasma from animals treated with an intra-arterial dose of ¹⁴C-MIB. ¹⁴C-MIB elimination was predicted using a two compartment model in each strain of fish. There was no significant difference in terminal half-lives between strains but possible differences in total body clearance and apparent volumes of distribution which may be related to higher lipid content in the hybrids. Results of these studies indicate biotransformation has no involvement in MIB elimination and that other physiological processes may play a more significant role in MIB disposition within Ictalurid fish species.  相似文献   

The early performance of deciduous trees grown in acidic minespoil ameliorated with pulverized fuel ash     

P. V. Perkins  A. R. Vann 《Land Degradation \u0026amp; Development》1995,6(1):57-67

A field trial investigated the early performance of six tree species planted into an acidic minespoil ameliorated by ploughing or incorporation of 10 per cent by weight of pulverized fuel ash (PFA). Common alder, Italian alder and silver birch produced the greatest total shoot extensions for PFA-ameliorated minespoil. Rowan, sycamore and aspen were unsatisfactory for both ameliorative treatments. PFA increased the pH of the minespoil for a period of time potentially beneficial for the establishment of the tree species. For the rate of PFA application studied, general salinity effects and cold-water-soluble boron both presented problems for successful tree growth.  相似文献   

Pulmonary function and metabolic physiology of theropod dinosaurs     

JA Ruben  Dal Sasso C  NR Geist  WJ Hillenius  TD Jones  M Signore 《Science (New York, N.Y.)》1999,283(5401):514-516

Ultraviolet light analysis of a fossil of the theropod dinosaur Scipionyx samniticus revealed that the liver subdivided the visceral cavity into distinct anterior pleuropericardial and posterior abdominal regions. In addition, Scipionyx apparently had diaphragmatic musculature and a dorsally attached posterior colon. These features provide evidence that diaphragm-assisted lung ventilation was present in theropods and that these dinosaurs may have used a pattern of exercise physiology unlike that in any group of living tetrapods.  相似文献   

Effect of ethanol, clofibric acid and temperature on the uptake and elimination of 2-methylisoborneol in channel catfish (Ictalurus punctatus)     

D. Schlenk  E.J. Perkins  W.B. Hawkins 《Fish physiology and biochemistry》1999,21(2):173-178

Off-flavor, due to organoleptic compounds such as 2-methylisoborneol (MIB), is the single largest detriment to the harvest, production and profit from the channel catfish aquaculture industry of the Southeastern United States. Methods to increase the metabolism and/or elimination of compounds like MIB would provide a means towards improving catfish rearing practices. Previous studies indicated one or more forms of cytochrome P450 monooxygenase (CYP) may be involved in the biotransformation and elimination of specific organoleptic compounds, such as 2-methylisoborneol (MIB). In order to determine the role of CYP in the elimination of MIB, various compounds that have been shown to modulate cytochrome P450 expression in catfish were administered before and after exposure to ¹⁴C-MIB. Uptake and elimination was monitored in fish over 24 and 48 h, respectively. Pretreatment with clofibric acid (100 mg kg^–1-gavage) which induces a CYP2K-like isoform, and ethanol (1.0% v/v-aqueous) a CYP2 represser, alone and with enhanced temperature (added 10 °C) failed to affect uptake of MIB. Pretreatment with these compounds and conditions also failed to enhance elimination of MIB from channel catfish. However, when fish were treated with 1.0% ethanol after MIB exposure (i.e., during depuration), beta elimination halflives were changed from 144±35 to 71±13 h. in sexually mature animals but unchanged (191±113) in juveniles. The failure of CYP-modulating agents to alter MIB elimination in catfish suggests MIB may not readily undergo Phase I oxidation via CYP. The enhanced elimination of MIB in adults by ethanol warrants further study as to its potential use in aquaculture in purging MIB and related compounds prior to fish processing.  相似文献   

Spreading deserts or spatially confined environmental impacts? land degradation and cattle ranching in the kalahari desert of botswana     

J. S. Perkins  D. S. G. Thomas 《Land Degradation \u0026amp; Development》1993,4(3):179-194

The Kalahari Desert of Botswana is being increasingly used for livestock production, particularly through the establishment of ranches dependent on borehole water supplies. Reports of the environmental impacts of land use practices in such drylands are conflicting, with livestock systems supplied by boreholes and wells being regarded in some studies as centres for the spread of desertification and in others as having environmental impacts that are overridden by the effects of natural climatic variability. the results of an environmental study focused on a block of borehole ranches established in the eastern Kalahari as part of Botswana's Tribal Grazing Lands Policy dating from the 1970s are reported. the investigations concentrated on ecological, soil chemistry and soil erosion impacts on the study ranches. Although important changes in the ecology and soil nutrient status of the ranches are identified, their contribution to environmental degradation must be viewed in terms of stocking levels, The radial nature of range usage, giving rise to marked spatial variations in impact, and natural climatic variability. Changes are not solely detrimental, but long-term monitoring, especially in relation to climatic fluctuations, needs to be undertaken to fully establish the dynamic interactions of variables.  相似文献