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31.
The cyanophage AN-15 was found to have a requirement for either 1 mM calcium or 1 mM magnesium ions to maintain viral stability,
whereas 1 mM calcium ions alone were essential for the infection process to proceed in Anabaena sp. strain PCC 7120. Following
prolonged incubation, phage-resistant cells were detected at a high frequency (approximately 10-5) in lysates, as either renewed growth in liquid cultures, or as colonies in confluently lysed lawns. Southern hybridisation
failed to detect AN-15 DNA in any of the resistant strains, implying that resistance is unlikely to be due to the presence
of temperate phages. A high rate of spontaneous mutation is therefore likely to be the cause of resistance. Two classes of
resistant cells were identified; those in which AN-15 failed to attach to host cells, and those in which attachment occurred,
but subsequent replication was defective. However, it was possible to overcome phage resistance by the isolation of spontaneous
mutants of AN-15, capable of infecting phage-resistant cells. These observations imply that if cyanophages are to be assessed
as a means of controlling cyanobacterial blooms in freshwater bodies, the ionic (notably calcium) concentration of the water
must be considered, together with the possible need to employ alternative cyanophage strains if resistance to the original
one arises.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
32.
The effects of the calmodulin antagonists W-7 and trifluoperazine have been measured on the Ca2+-activated potassium channel in the membrane surrounding protoplasmic drops expressed from internodal cells of charophyte
plants. The large-conductance (170 pS), voltage- and Ca2+-dependent gating, and prominent conductance substrate of this channel shows a strong kinetic resemblance to those of the
Maxi-K channel from animal cells.
This is the first study of the action of calmodulin antagonists which measures their effects on the most populated substates
as well as the closed and main open states of Maxi-K channels. The substate analysis provides new evidence for different modes
of action of- and different bindings sites for these calmodulin antagonists.
Neither antagonist produces the simple closure of the channel reported previously as its effect on the Maxi-K channel, though
both do induce flicker-block, reducing the mean current to near zero at high concentrations following an inverted Michaelis-Menten
curve.
W-7 reduces residence time in the fully open state, thus raising, in the same proportions, the probabilities of finding the
channel in the closed state or a pre-existing substate. Its binding to the channel is voltage- and calcium-dependent.
In contrast, trifluoperazine reduces residence in the open state and promotes an apparently new substate which overlaps the
closed state at −50 mV but is distinguishable from it at voltages more negative than −100 mV. This substate may represent
times that trifluoperazine is bound to the channel.
Both antagonists have effects clearly distinguishable from that of withdrawing calcium from the channel, which does not affect
open state residence time but increases closed state residence time. Thus neither antagonist reverses the activating effect
of Ca2−. This is good kinetic evidence against the view that the channel is activated by Ca2+-calmodulin and that the effect of a calmodulin antagonist is to reverse this process by making Ca2−-calmodulin less available.
Received: 26 August 1996/Revised: 7 October 1996 相似文献
33.
Analysis of DNA sequences of 132 introns and 140 exons from 42 pairs of orthologous genes of mouse and rat was used to compare
patterns of evolutionary change between introns and exons. The mean of the absolute difference in length (measured in base
pairs) between the two species was nearly five times as high in the case of introns as in the case of exons. The average rate
of nucleotide substitution in introns was very similar to the rate of synonymous substitution in exons, and both were about
three times the rate of substitution at nonsynonymous sites in exons. G+C content of introns and exons of the same gene were
correlated; but mean G+C content at the third positions of exons was significantly higher than that of introns or positions
1–2 of exons from the same gene. G+C content was conserved over evolutionary time, as indicated by strong correlations between
mouse and rat; but the change in G+C content was greatest at position 3 of exons, intermediate in introns, and lowest at positions
1–2 in introns.
Received: 23 December 1996 / Accepted: 1 April 1997 相似文献
34.
Meredith F. Small 《International journal of primatology》1981,2(1):91-95
Using weight and skinfold thickness to calculate relative body fat, the fat content of 21 captive female rhesus macaques was
estimated. Although age and. pregnancy had no effect on fat, rank was significant, especially during winter, with high-rank
females having the highest fat scores. Rank therefore has a significant effect on the health of captive animals and may also
affect the health of individuals in feral groups. 相似文献
35.
K Hoffken I D Meredith R A Robins R W Baldwin C J Davies R W Blamey 《BMJ (Clinical research ed.)》1977,2(6081):218-220
In a retrospective study in women with breast cancer circulating immune complex levels were measured by radioimmunoprecipitation with 125I-Clq. Before operation all the patients showed plasma immune complex levels significantly higher than those in controls. Twelve months after mastectomy patients identified clinicopathologically as having a good prognosis had almost normal levels of immune complexes. By contrast, patients with detectable dissemination on diagnosis or those who died within 22 months after mastectomy had significantly raised plasma levels. The tumour-specific nature of the immune complexes detected remains to be shown and suggestions about the applicability of this test not only for prognosis but also for monitoring the course of malignant diseases need to be confirmed by further investigations. 相似文献
36.
The report of R. J. Gillies, M. P. Rosenberg, and D. W. Deamer (1981, J. Cell. Phys., 108, 115–122) that sea urchin fertilization acid is anaerobically produced CO2, was reinvestigated by inseminating Strongylocentrotus purpuratus eggs in HCO?3-free seawater, then bubbling the seawater with N2 to remove volatile acid. Fertilization acid production occurred in HCO?3-free seawater and with N2-bubbling, the pH rose 0.28 ± 0.08 unit, significantly less than the rise of 0.63 ± 0.14 unit during N2-bubbling of HCO?3-free seawater that had been acidified with CO2 and similar to the rise of 0.18 ± 0.07 unit when acidification was with HCl. We conclude that most, if not all, of the sea urchin fertilization acid is nonvolatile and thus is not CO2; since it is not a weak acid, it must be H+. 相似文献
37.
A review of the salt sensitivity of the Australian freshwater biota 总被引:13,自引:7,他引:6
Barry T. Hart Paul Bailey Rick Edwards Kent Hortle Kim James Andrew McMahon Charles Meredith Kerrie Swadling 《Hydrobiologia》1991,210(1-2):105-144
In Victoria, Australia, both dryland salinity and salinity in irrigation regions are serious agricultural problems. One option
to control the latter is to pump groundwater to maintain it below the surface. However, this leaves a saline wastewater for
disposal, probably into local streams or wetlands. This review of the salt sensitivity of the biota of Australian streams
and wetlands gives information of interest to those responsible for developing controls on these discharges. The review addresses
the lethal and sub-lethal effects of salinity on microbes (mainly bacteria), macrophytes and micro-algae, riparian vegetation,
invertebrates, fish, amphibians, reptiles, mammals, and birds. Data suggest that direct adverse biological effects are likely
to occur in Australian river, stream and wetland ecosystems if salinity is increased to around 1 000 mg L−1. The review highlights a general lack of data on the sensitivity of freshwater plants and animals to salinity increases. 相似文献
38.
Refined atomic structures of N9 subtype influenza virus neuraminidase and escape mutants 总被引:8,自引:0,他引:8
W R Tulip J N Varghese A T Baker A van Donkelaar W G Laver R G Webster P M Colman 《Journal of molecular biology》1991,221(2):487-497
The crystal structure of the N9 subtype neuraminidase of influenza virus was refined by simulated annealing and conventional techniques to an R-factor of 0.172 for data in the resolution range 6.0 to 2.2 A. The r.m.s. deviation from ideal values of bond lengths is 0.014 A. The structure is similar to that of N2 subtype neuraminidase both in secondary structure elements and in their connections. The three-dimensional structures of several escape mutants of neuraminidase, selected with antineuraminidase monoclonal antibodies, are also reported. In every case, structural changes associated with the point mutation are confined to the mutation site or to residues that are spatially immediately adjacent to it. The failure of antisera to cross-react between N2 and N9 subtypes may be correlated with the absence of conserved, contiguous surface structures of area 700 A2 or more. 相似文献
39.
The nanH genes of Vibrio cholerae and Salmonella typhimurium LT2 coding neuraminidase were cloned separately in Escherichia coli, and the expression products purified. Single crystals of the V. cholerae neuraminidase were obtained using the hanging drop vapour diffusion method with polyethylene glycol as precipitant at pH 7.2. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with unit cell dimensions a = 71.9 A, b = 79.0 A, c = 165.7 A, and with one molecule in the asymmetric unit. Diffraction extends to at least 2.5 A. Single crystals of the S. typhimurium neuraminidase were obtained by hanging drop with potassium phosphate as precipitant at pH 7.2. The crystals also belong to the orthorhombic space group P2(1)2(1)2(1), with unit cell dimensions a = 47.4 A, b = 82.8 A, c = 92.4 A, and with one molecule in the asymmetric unit. Diffraction extends to at least 1.8 A. 相似文献
40.
D R Laver 《The Journal of general physiology》1992,100(2):269-300
The patch-clamp technique is used to investigate divalent ion block of the large-conductance K+ channel from Chara australis. Block by Ba2+, Ca2+, Mg2+, and Pt(NH3)4(2+) from the vacuolar and cytoplasmic sides is used to probe the structure of, and ion interactions within, the pore. Five divalent ion binding sites are detected. Vacuolar Ca2+ reduces channel conductance by binding to a site located 7% along the membrane potential difference (site 1, delta = 0.07; from the vacuolar side); it also causes channel closures with mean a duration of approximately 0.1-1 ms by binding at a deeper site (site 2, delta = 0.3). Ca2+ can exit from site 2 into both the vacuolar and cytoplasmic solutions. Cytoplasmic Ca2+ reduces conductance by binding at two sites (site 3, delta = -0.21; site 4, delta = -0.6; from the cytoplasmic side) and causes closures with a mean duration of 10-100 ms by binding to site 5 (delta = -0.7). The deep sites exhibit stronger ion specificity than the superficial sites. Cytoplasmic Ca2+ binds sequentially to sites 3-5 and Ca2+ at site 5 can be locked into the pore by a second Ca2+ at site 3 or 4. Ca2+ block is alleviated by increasing [K+] on the same side of the channel. Further, Ca2+ occupancy of the deep sites (2, 4, and 5) is reduced by K+, Rb+, NH4+, and Na+ on the opposite side of the pore. Their relative efficacy correlates with their relative permeability in the channel. While some Ca2+ and K+ sites compete for ions, Ca2+ and K+ can simultaneously occupy the channel. Ca2+ binding at site 1 only partially blocks channel conduction. The results suggest the presence of four K+ binding sites on the channel protein. One cytoplasmic facing site has an equilibrium affinity of 10 mM (site 6, delta = -0.3) and one vacuolar site (site 7, delta less than 0.2) has low affinity (greater than 500 mM). Divalent ion block of the Chara channel shows many similarities to that of the maxi-K channel from rat skeletal muscle. 相似文献