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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.
We have isolated the delta-globin gene of the New-World spider monkey,
Ateles geoffroyi, and compared its nucleotide sequence with those of other
primate delta- and beta-globin genes. Among primate delta-globin genes, the
rate of nonsynonymous substitutions is much less than the rate of
synonymous substitutions. This suggests that primate delta- globin genes
may remain under evolutionary conservation, perhaps because hemoglobin A2
has an as yet unknown physiological importance.
相似文献
4.
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
5.
Biochemical pathways in prokaryotes can be traced backward through evolutionary time 总被引:10,自引:0,他引:10
For the first time, a credible prokaryotic phylogenetic tree is being
assembled by Woese and others using quantitative sequence analysis of
oligonucleotides in the highly conservative rRNA. This provides an
evolutionary scale against which the evolutionary steps that led to the
arrangement and regulation of contemporary biochemical pathways can be
measured. This paper presents an emerging evolutionary picture of aromatic
amino acid biosynthesis within a large superfamily assemblage of
prokaryotes that is sufficiently developed to illustrate a new perspective
that will be applicable to many other biochemical pathways.
相似文献
6.
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
7.
A radioimmunoassay for progesterone was developed which uses one micro-column chromatography for purification of hexane extracts of plasma. 相似文献
8.
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
9.
In Drosophila pseudoobscura, the amylase (Amy) multigene family is
contained within a series of inversions, or gene arrangements, on the third
chromosome. The Standard (ST), Santa Cruz (SC), and Tree Line (TL)
inversions are central to the phylogeny of arrangements, and have clusters
of other arrangements derived from them. The gene arrangements belonging to
each of these three clusters have a characteristic number of Amy genes,
ranging from three in ST to two in SC to one in TL. This distribution
pattern can reflect a history of either duplications or deletions, although
the data available in the past did not permit a decision between these
alternatives. We provide unambiguous evidence that three Amy genes were
present before the divergence of the ST, SC, and TL arrangements. Thus, the
current status of the Amy multigene family is the result of deletions in
the TL and SC arrangements, which created three new pseudogenes: TL
Amy2-psi, TL Amy3-psi, and SC Amy3- psi. Analysis of pseudogene sequences
revealed that, in the SC and ST arrangements, pseudogene evolution has been
retarded, most likely due to the homogenization effect of gene conversion.
Finally, by determining the original copy number, we have reconstructed the
evolutionary history of the Amy multigene family and linked it with the
evolution of the central gene arrangements.
相似文献
10.
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