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1.
2.
The electron transfer resulting from illumination and dark storage of PS II has been studied using EPR signals from several electron carriers. The recombination of D+ (Signal II) and Q−A formed by illumination occurred during dark storage at 77 K and was used to deplete reaction centres of D+. The donor D was then shown to be oxidized in the dark by the S2 state of the oxygen-evolving complex. A slow change which occurred during dark storage of PS II samples was detected using the power saturation characteristics of D. We interpret this effect on D to be an indirect result of a rearrangement of the manganese complex during long-term dark adaptation. A role for D in the stability, protection and perhaps initial manganese binding of the oxygen-evolving complex is suggested. 相似文献
3.
A mutant muskmelon (Cucumis melo L.) with characteristic Fe-deficiency chlorosis symptoms was compared to related cultivars in its ability to obtain Fe via the widely known Fe-stress response mechanisms of dicotyledonous plants. The three cultivars (fefe, the Fe-inefficient mutant; Mainstream and Edisto, both Fe efficient plants) were grown in nutrient solution in either 0 or 3.5 mg L-1 Fe as FeCl3. None of the three cultivars released reductants or phytosiderophores, but both Edisto and Mainstream produced massive amounts of H+ ions to reduce and maintain the pH of nutrient solutions below pH 4.0. The roots of these two Fe-efficient cultivars were also capable of reducing Fe3+ to Fe2+. These responses maintained green plants, resulted in high leaf Fe in both Edisto and Mainstream, and produced Mn toxicity in Mainstream. The lack of Fe-deficiency stress response in fefe not only affected leaf Fe concentration and chlorosis, but also resulted in reduced uptake of Mn. The importance of reduced Fe (Fe2+) to the Fe-efficient cultivars was confirmed by growing the cultivars with BPDS (4, 7-diphenyl-1, 10-phenanthroline disulfonic acid, a ferrous chelator) and EDDHA [ethylene-diamine di (0-hydroxphenylacetic acid)] (a ferric chelator), and observing increased chlorosis and reduced Fe uptake in BPDS grown plants. The Fe-deficiency response observed in these cultivars points out the diversity of responses to Fe deficiency stress in plants. The fefe mutant has a limited ability to absorb Fe and Mn and perhaps could be used to better understand Mn uptake in plants. 相似文献
4.
Increasing pressure is being placed on the scientific community to evaluate research activities. Scientific meetings consume a small but important fraction of the research budget. Audit of a well established series of scientific meetings showed that they met their immediate objectives in that they were international and multidisciplinary and provided a forum in which all participants actively contributed to discussion. The meetings had a positive outcome for the participants, leading in many cases to the subsequent exchange of research material (60% of participants) and to the establishment of collaborative research projects (31%). The impact of the meetings on the scientific community at large was assessed by citation analysis, which showed that the proceedings were cited early, often, and over a substantial period. 相似文献
5.
6.
A radioimmunoassay for progesterone was developed which uses one micro-column chromatography for purification of hexane extracts of plasma. 相似文献
7.
Basic fibroblast growth factor (bFGF) binds to cell surface receptor (CSR) proteins and to heparan sulfate proteoglycans (HSPG). On the basis of equilibrium dissociation constants (Kd), the CSR has been considered a "high-affinity" binding site and HSPG a "low-affinity" site. We measured the apparent individual on and off rate constants (kon and koff) for bFGF binding to these two sites on intact cells and to each class of binding site in the absence of the other. While the kon's for CSR and HSPG on intact cells were not statistically different (konC = 2.27 x 10(8) M-1 min-1; konH = 0.90 x 10(8) M-1 min-1), the koff for the HSPG was 22.7-fold greater than that for the CSR (koffC = 0.003 min-1; koffH = 0.68 min-1). Thus, the difference in Kd's appears to result from the faster rate at which bFGF is released from the HSPG sites compared to the CSR. The kon's for isolated CSR and HSPG, and the koff for isolated HSPG, did not differ significantly from those for intact cells konC = 2.50 x 10(8) M-1 min-1; konH = 0.92 x 10(8) M-1 min-1; koffH = 0.095 min-1). However, the off rate for isolated CSR (koffC = 0.048 min-1) was statistically indistinguishable from the off rate for HSPG and 16-fold greater than the off rate for CSR on intact cells. The "high-affinity" binding of bFGF to intact cells probably refers only to a complex of bFGF with both CSR and HSPG, and not to the CSR alone. 相似文献
8.
Detecting non-neutral heterogeneity across a region of DNA sequence in the ratio of polymorphism to divergence 总被引:11,自引:4,他引:7
Natural selection, in the form of balancing selection or selective sweeps,
can result in a decoupling of the amounts of molecular polymorphism and
divergence. Thus natural selection can cause some areas of DNA sequence to
have greater silent polymorphism, relative to divergence between species,
than other areas. It would be useful to have a statistical test for
heterogeneity in the polymorphism to divergence ratio across a region of
DNA sequence, one that could identify heterogeneity greater than that
expected from the neutral processes of mutation, drift, and recombination.
The only currently available test requires that a region be arbitrarily
divided into sections that are compared with each other, and the
subjectivity of this division could be problematic. Here a test is proposed
in which runs of polymorphic and fixed sites are counted, where a "run" is
a set of one or more sites of one type preceded and followed by the other
type. The number of runs is smaller than otherwise expected if
polymorphisms are clumped together. By simulating neutral evolution and
comparing the observed number of runs to the simulations, a statistical
test is possible which does not require any a priori decisions about
subdivision.
相似文献
9.
Summary The cellular retinoic acid-binding proteins (CRABPs) are thought to modulate the responsiveness of cells to retinoic acid
(RA). We have previously shown that primary cultures of murine embryonic palate mesenchymal (MEPM) cells express both CRABP-I
and CRABP-II genes and that this expression is regulated by RA and transforming growth factor β (TGF-β). These cells also
express high levels of TGF-β3, which is also regulated by RA and TGF-β. We have used an antisense strategy to investigate
the role of the CRABPs in retinoid-induced gene expression. Subconfluent cultures of MEPM cells were treated for several days
with phosphorothioate modified 18-mer oligonucleotides antisense to CRABP-I or CRABP-II and then with all-trans-retinoic acid at a concentration of 3.3 μM or 0.33 μM for 5 or 22 h. Total RNA was then extracted and the expression of TGF-β3, retinoic acid receptor β (RAR-β), and tenascin
was assessed by northern blot analysis. Antisense oligonucleotides to CRABP-I partially inhibited the RA-induced TGF-β3, RAR-β,
and tenascin mRNA expression. The corresponding mis-sense oligonucleotides were without effect. Antisense oligonucleotides
to CRABP-II also partially inhibited RA-induced expression of these genes. As with the CRABP-I antisense, mis-sense oligonucleotides
to CRABP-II had no effect. These data suggest that both CRABPs modulate the responsiveness of MEPM cells to retinoic acid.
Inhibition of endogenous CRABP expression renders MEPM cells less responsive to RA with respect to induction of TGF-β3, RAR-β,
and tenascin gene expression. These results have important implications for our understanding of the role of the CRABPs in
retinoid teratology. 相似文献
10.
Building the State, Making the Nation: The Bases and Limits of State Centralization in "Modern" Peru 总被引:1,自引:0,他引:1
Most contemporary discussions of the nature of the state, state-building, and national consciousness formation are based on an oppositional model of state-society relations. In such models, state-building is depicted as a process in which the state must impose its central institutions and cultural/moral values on the recalcitrant local populations found within its territorial boundaries who, in resisting the state, cling to a myriad of local, oppositional identities. The present article critiques the oppositional model, proposes an alternative conceptualization that encompasses dimensions of cooperation and conflict in state-society relations, and thus points toward the forces that underlie the fragmentation and consolidation of states and national cultures. 相似文献