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41.
Summary Irradiation of Escherichia coli with UV light causes a transient inhibition of DNA replication. This effect is generally thought to be accounted for by blockage of the elongation of DNA replication by UV-induced lesions in the DNA (a cis effect). However, by introducing an unirradiated E. coli origin (oriC)-dependent replicon into UV-irradiated cells, we have been able to show that the environment of a UV-irradiated cell inhibits initiation of replication from oriC on a dimer-free replicon. We therefore conclude that UV-irradiation of E. coli leads to a trans-acting inhibition of initiation of replication. The inhibition is transient and does not appear to be an SOS function. 相似文献
42.
43.
Actinomycetes were isolated from the upper 1 - 3 cm of the soil layer in a well-developed forest and in an adjacent clearcut
area where Douglas-fir [Pseudotsuga menziesii (MIRB.) Franco] regeneration had been impaired for two decades. The population density in the clearcut area was two times
as high as that in the forested area. The percentage of actinomycetes that inhibited seed germination of the test plants was
significantly higher in isolates obtained from the clearcut area than in those obtained from the forested area, and isolates
from the clearcut showed five times the phytotoxic effect of those from the forest. Some actinomycete isolates, 4 % from the
clearcut and 2.6 % from the forest, significantly reduced in vitro growth of two common ectomycorrhizal fungi of Douglas-fir,Laccaria laccata andHebeloma ovstuliniforme. Two actinomycete isolates from the clearcut reduced fungal growth by 40 % and 73 %. Reduction of the nutrient in the growth
medium did not affect the antifungal activity of the actinomycetes. The data support the idea that, along with other factors,
phytotoxic and antifungal actinomycetes may suppress natural regeneration or establishment of planted seedlings - either directly
or. indirectly - through inhibition of seed germination or of mycorrhizal fungi. 相似文献
44.
Action potentials initiated by single channels opening in a small neuron (rat olfactory receptor). 总被引:10,自引:0,他引:10
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Rat olfactory receptor neurons were enzymatically dissociated and studied with the cell-attached configuration of the patch-clamp technique. Biphasic current waveforms induced across the membrane patch by intracellular action potentials were observed in approximately 5% of cells studied. In one cell in particular, current injected by the opening of a single channel initiated an action potential in the remainder of the cell each time the channel opened. A conventional type of electrical model of the cell and patch allowed the accurate modeling of cell excitability. The same model was used to explain the shape of the action potential current waveforms induced across the patch. The analysis indicated that the whole cell resistance (Ro) was approximately 40 G omega and the membrane capacitance (Co) was close to the standard value of 1 microF.cm-2. In addition, the threshold potential change necessary to initiate an action potential (Vth) was approximately 13 mV and a minimum current injection of 1 pA was required to depolarize the cell to spike threshold. When the smaller size of mammalian receptors are taken into account, membrane electrical properties were found to be consistent with those of salamander cells investigated by others using whole-cell recording. The analysis also revealed possible errors in the determination of single-channel conductances and reversal potentials by cell-attached recording from small cells. 相似文献
45.
Ronald J. Hauptman Brian A. Perry David G. Capco 《Development, growth & differentiation》1989,31(2):157-164
Proteins of the detergent-resistant cytoskeleton fraction and the detergent-soluble fraction from Xenopus oocytes and embryos are examined using a procedure which allows rapid and uniform extraction of tissues and large, single cells. SDS-polyacrylamide gels reveal only a few prominent cytoskeletal proteins in the early embryo, however qualitatively different proteins begin to appear after gastrulation. Incorporation of [35 S]-methionine into newly synthesized proteins indicates that there is synthesis and assembly of proteins into the cytoskeleton, but the amount remains low until after gastrulation. The use of nucleic acid probes for alpha-tubulin and actin mRNA indicates that about 80% of these mRNAs in the oocyte and meiotically mature egg are bound to the detergent-resistant cytoskeleton. 相似文献
46.
Soluble derivatives of the beta amyloid protein precursor of Alzheimer's disease are labeled by antisera to the beta amyloid protein 总被引:6,自引:0,他引:6
M R Palmert S L Siedlak M B Podlisny B Greenberg E R Shelton H W Chan M Usiak D J Selkoe G Perry S G Younkin 《Biochemical and biophysical research communications》1989,165(1):182-188
The amyloid deposited in Alzheimer's disease (AD) is composed primarily of a 39-42 residue polypeptide (beta AP) that is derived from a larger beta amyloid protein precursor (beta APP). In previous studies, we and others identified full-length, membrane-associated forms of the beta APP and showed that these forms are processed into soluble derivatives that lack the carboxyl-terminus of the full-length forms. In this report, we demonstrate that the soluble approximately 125 and approximately 105 kDa forms of the beta APP found in human cerebrospinal fluid are specifically labeled by several different antisera to the beta AP. This finding indicates that both soluble derivatives contain all or part of the beta AP sequence, and it suggests that one or both of these forms may be the immediate precursor of the amyloid deposited in AD. 相似文献
47.
We investigated the inhibitory effects of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) on ATP-dependent H+ accumulation by membrane vesicles prepared from the turtle urinary bladder epithelium. NBD-Cl at 30 microM was found to completely inhibit the vanadate-insensitive component of H+ transport, with half-maximal inhibition occurring at 4.2 to 5.4 microM. In contrast, the vanadate-inhibitable component was unaffected by 30 microM NBD-Cl. At high concentrations (300 microM), both components were fully inhibited. The results confirm the presence of two distinct H+ transport processes in turtle bladder membranes and identify selective inhibitors, NBD-Cl and vanadate, for each process. 相似文献
48.
Water oxidation in photosystem II: from radical chemistry to multielectron chemistry 总被引:8,自引:0,他引:8
G T Babcock B A Barry R J Debus C W Hoganson M Atamian L McIntosh I Sithole C F Yocum 《Biochemistry》1989,28(25):9557-9565
49.
Characteristics of tripeptide transport in human jejunal brush-border membrane vesicles 总被引:3,自引:0,他引:3
These studies are aimed at characterizing the transport of the tripeptide, glycylglycyl-L-proline (GlyGlyPro) across human jejunal brush-border membrane vesicles. GlyGlyPro (0.65 mM) was hydrolyzed by brush-border membrane vesicles with the extent of hydrolysis per mg protein being 23% at 0.5 min, 57% at 1 min and complete hydrolysis at 60 min. Treatment of the membrane vesicles with gel-complexed papain (to remove membrane peptidases) resulted in minimal hydrolysis of GlyGlyPro up to 10 min of incubation. Measurement of GlyGlyPro influx with papain-treated vesicles in the presence of increasing medium osmolarity showed that uptake occurred into an osmotically reactive intravesicular space. Transport of GlyGlyPro with normal and papain-treated membrane vesicles was similar in the presence of an inward Na+ or K+ gradient. No overshoot phenomenon was observed in the presence of an inward proton gradient (extravesicular pH 5.5; intravesicular pH 7.5). An interior negative membrane potential induced by a K+ diffusion potential in the presence of valinomycin stimulated the uptake of the peptide. The effect of increasing concentrations on initial rates of GlyGlyPro uptake revealed the presence of a saturable component as well as a diffusional component. Preloading the membrane vesicles with 20 mM glycylsarcosylsarcosine stimulated uptake by 4-fold. Uptake of GlyGlyPro was inhibited greater than 50% by dipeptides and tripeptides and less than 15% by free amino acids. These results indicate that GlyGlyPro uptake in jejunal brush-border membrane vesicles is not energized by a Na+ or proton gradient and that transport occurs by carrier-mediated and diffusional processes. 相似文献
50.
The combined effects of chlorine and ammonia on litter breakdown in outdoor experimental streams 总被引:2,自引:0,他引:2
The response of Potamogeton crispus L. breakdown to controlled doses of different levels of chlorine and chlorine + ammonia was investigated over two years in
outdoor experimental streams. In 1985, downstream riffles of 2 streams were dosed (observed in-stream concentrations) at ca.
10 μg/L Total Residual Chlorine (TRC), one stream at 64 μg/L TRC and one stream at 230 μg/L TRC. Two control streams were
not dosed and the upstream riffles of each stream served as within stream controls. In 1986, the downstream riffle of one
stream was dosed at 70 μg/L TRC and a second stream was dosed at 200 μg/L TRC. Four streams were also dosed with 2.5 mg/L
NH3-N: one stream with no chlorine, one stream with ca. 10 μg/L TRC, one with 56 μg/L TRC, and one with 150 μg/L TRC. A seventh
stream was dosed for 2 h at 2000 μg/L TRC and 2.5 mg/L ammonia and then allowed to recover (recovery stream). Each year, litter
decomposition (degree day k values) was measured during two 35 day trials (Jun–Jul and Aug–Sep). In 1985, when streams were dosed with chlorine alone,
decomposition was significantly reduced with the high (230 μg/L TRC) chlorine dose. Downstream decomposition was 27% (Jun–Jul)
and 59% (Aug–Sep) of the upstream (control) rate. No other chlorine effects were found during this period. In Jun–Jul 1986,
there was significantly lower decomposition in the downstream dosed sites of the 200 μg/L TRC alone stream, the 146 μg/L TRC
+ ammonia stream and the recovery stream; downstream decay rates were (respectively) 56%, 42% and 64% of the upstream control
sites. No other up-down pairs were different in July 1986. In Aug–Sep, all three streams with chlorine + ammonia (6, 56 and
146 μg/L TRC + 2,5 mg/L ammonia) and the 70 μg/L TRC alone stream had significantly lower decomposition rates in the downstream
dosed sites. For these streams, downstream decay rates ranged from 46% (high chlorine + ammonia) to 73% (low chlorine + ammonia)
of the upstream control rates. No other up-down pairs were different during this trial. Up and downstream sites of the stream
dosed with 2.5 mg/L ammonia alone were nearly identical for both trials (< 3% difference). These results indicate that TRC
at less than 250 μg/L can significantly reduce litter decomposition and strongly suggest that addition of ammonia to chlorinated
water can increase the toxic effect of chlorine.
currently at the Department of Fisheries and Wildlife
currently at the Department of Fisheries and Wildlife 相似文献