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181.
In acute experiments on cats, we studied the impulse activity of 262 neurons of the parietal associative zone (PAZ, field 5). Among them, 129 cells [100 silent units and 29 units generating background activity (BA)] were identified as output neurons, while 133 cells with the BA were interneurons of the intrinsic cortical neuronal circuits. Electrical stimulation of the primary visual, auditory, or somatosensory cortices evoked no impulse responses in silent output PAZ neurons, while output neurons with the BA and interneurons (more than 65 and 80% of the cell units, respectively) generated clear responses (more frequently, phasic). Stimulation of the auditory and visual cortices exerted mostly inhibitory effects, while stimulation of the somatosensory cortex provided mostly excitatory influences. The ratios of neurons generating primary excitatory and inhibitory responses to stimulation of the visual, auditory, and somatic cortices were 0.3:1, 0.6:1, and 3.2:1, respectively. More than 95% of the field-5 neurons were influenced from the primary sensory zones via di- and/or polysynaptic pathways. Monosynaptic excitatory inputs from the visual cortex were identified for 3.8% of interneurons and 6.9% of output PAZ neurons; for the auditory cortical inputs, the respective figures were 1.7 and 3.5%. Monosynaptic connections with the somatic cortex were found only for 4% of the interneurons under study. It has been concluded that interaction of heteromodal signals coming to the PAZ via the corticopetal and associative inputs occurs on neurons of all the cortical layers. 相似文献
182.
Three maize root-specific genes are not correctly expressed in regenerated caps in the absence of the quiescent center 总被引:6,自引:0,他引:6
The quiescent center is viewed as an architectural template in the root apical meristem of all angiosperm and gymnosperm
root tips. In roots of Arabidopsis thaliana (L.) Heynh., the quiescent center inhibits differentiation of contacting initial cells and maintains the surrounding initial
cells as stem cells. Here, the role of the quiescent center in the development of the maize (Zea mays L.) root cap has been further explored. Three maize root-specific genes were identified. Two of these were exclusively expressed
in the root cap and one of them encoded a GDP-mannose-4,6-dehydratase. Most likely these two genes are structural, tissue-specific
markers of the cap. The third gene, a putative glycine-rich cell wall protein, was expressed in the cap and in the root epidermis
and, conceivably is a positional marker of the cap. Microsurgical and molecular data indicate that the quiescent center and
cap initials may regulate the positional and structural expression of these genes in the cap and thereby control root cap
development.
Received: 22 September 1999 / Accepted: 9 November 1999 相似文献
183.
Primary charge separation in Photosystem II 总被引:3,自引:3,他引:0
In this Minireview, we discuss a number of issues on the primary photosynthetic reactions of the green plant Photosystem II.
We discuss the origin of the 683 and 679 nm absorption bands of the PS II RC complex and suggest that these forms may reflect
the single-site spectrum with dominant contributions from the zero-phonon line and a pronounced ∼80 cm−1 phonon side band, respectively. The couplings between the six central RC chlorins are probably very similar and, therefore,
a `multimer' model arises in which there is no `special pair' and in which for each realization of the disorder the excitation
may be dynamically localized on basically any combination of neighbouring chlorins. The key features of our model for the
primary reactions in PS II include ultrafast (<500 fs) energy transfer processes within the multimer, `slow' (∼20 ps) energy
transfer processes from peripheral RC chlorophylls to the RC multimer, ultrafast charge separation (<500 fs) with a low yield
starting from the singlet-excited `accessory' chlorophyll of the active branch, cation transfer from this `accessory' chlorophyll
to a `special pair' chlorophyll and/or charge separation starting from this `special pair' chlorophyll (∼8 ps), and slow relaxation
(∼50 ps) of the radical pair by conformational changes of the protein. The charge separation in the PS II RC can probably
not be described as a simple trap-limited or diffusion-limited process, while for the PS II core and larger complexes the
transfer of the excitation energy to the PS II RC may be rate limiting.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
184.
Neerken S Ma YZ Aschenbrücker J Schmidt KA Nowak FR Permentier HP Aartsma TJ Gillbro T Amesz J 《Photosynthesis research》2000,65(3):261-268
Properties of the excited states in reaction center core (RCC) complexes of the green sulfur bacterium Prosthecochloris aestuarii were studied by means of femtosecond time-resolved isotropic and anisotropic absorption difference spectroscopy at 275 K.
Selective excitation of the different transitions of the complex resulted in the rapid establishment of a thermal equilibrium.
At about 1 ps after excitation, the energy was located at the lowest energy transition, BChl a 835. Time constants varying between 0.26 and 0.46 ps were observed for the energy transfer steps leading to this equilibrium.
These transfer steps were also reflected in changes in polarization. Our measurements indicate that downhill energy transfer
towards excited BChl a 835 occurs via the energetically higher spectral forms BChl a 809 and BChl a 820. Low values of the anisotropy of about 0.07 were found in the ‘two-color’ measurements at 820 and 835 nm upon excitation
at 800 nm, whereas the ‘one-color’ kinetics showed much higher anisotropies. Charge separation occurred with a time constant
varying between 20 and 30 ps.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
185.
Naoto Kobayashi Jochen Reiser Wilhelm Kriz Ryoko Kuriyama Peter Mundel 《The Journal of cell biology》1998,143(7):1961-1970
Podocytes are unique cells that are decisively involved in glomerular filtration. They are equipped with a complex process system consisting of major processes and foot processes whose function is insufficiently understood (Mundel, P., and W. Kriz. 1995. Anat. Embryol. 192:385–397). The major processes of podocytes contain a microtubular cytoskeleton. Taking advantage of a recently established cell culture system for podocytes with preserved ability to form processes (Mundel, P., J. Reiser, A. Zúñiga Mejía Borja, H. Pavenstädt, G.R. Davidson, W. Kriz, and R. Zeller. 1997b. Exp. Cell Res. 36:248–258), we studied the functional significance of the microtubular system in major processes. The following data were obtained: (a) Microtubules (MTs) in podocytes show a nonuniform polarity as revealed by hook-decoration. (b) CHO1/ MKLP1, a kinesin-like motor protein, is associated with MTs in podocytes. (c) Treatment of differentiating podocytes with CHO1/MKLP1 antisense oligonucleotides abolished the formation of processes and the nonuniform polarity of MTs. (d) During the recovery from taxol treatment, taxol-stabilized (nocodazole- resistant) MT fragments were distributed in the cell periphery along newly assembled nocodazole-sensitive MTs. A similar distribution pattern of CHO1/MKLP1 was found under these circumstances, indicating its association with MTs. (e) In the recovery phase after complete depolymerization, MTs reassembled exclusively at centrosomes. Taken together, these findings lead to the conclusion that the nonuniform MT polarity in podocytes established by CHO1/MKLP1 is necessary for process formation. 相似文献
186.
187.
Nabedryk Eliane Breton Jacques Okamura Melvin Y. Paddock Mark L. 《Photosynthesis research》1998,55(2-3):293-299
In bacterial reaction centers (RCs), changes of protonation state of carboxylic groups, of quinone-protein interactions as well as backbone rearrangements occuring upon QB photoreduction can be revealed by FTIR difference spectroscopy. The influence of compensatory mutations to the detrimental Asp L213 Asn replacement on QB
–/QB FTIR spectra of Rb. sphaeroides RCs was studied in three double mutants carrying a Asn M44 Asp, Arg M233 Cys, or Arg H177 His suppressor mutation. The proton uptake by Glu L212 upon QB
– formation, as reflected by the positive band at 1728 cm–1, is increased in the Asn M44 Asp and Arg H177 His suppressor RCs with respect to native RCs, and remains comparable to that observed in Asp L213 Asn mutant RCs. Only the Arg M233 Cys suppressor mutation affected the 1728 cm–1 band, reducing its amplitude to near native level. Thus, there is no clear correlation between the apparent extent of proton uptake by Glu L212 and the recovery of the proton transfer RC function. In all of the mutant spectra, several protein (amide I and amide II) and quinone anion (C...O/C...C) modes are perturbed compared to the spectrum of native RCs. These IR data show that all of the compensatory mutations alter the semiquinone-protein interactions and the backbone providing direct evidence of structural changes accompanying the restoration of efficient proton transfer in RCs containing the Asp L213 Asn lesion. 相似文献
188.
We have analyzed the rate of electron transfer between the tetrahemic cytochrome and the primary electron donor in isolated reaction centers of Roseobacter denitrificans as a function of the ambient redox potential. Three different phases are observed: a slow phase (half-time > ms), and two fast phases with half-times of 5 µs and 380 ns. The slow phase is present at high redox potential, it corresponds to the kinetics of charge recombination between the photo-oxidized primary electron acceptor P+ and the reduced primary acceptor (Q
A
–
). The 5 µs phase titrates with the reduction of the highest potential heme (HP1). This phase corresponds to the electron transfer between heme HP1 and P+. At redox potentials where the second high potential heme HP2 becomes reduced, the 5 µs phase disappears and is replaced by the 380 ns phase, which is therefore related to the electron transfer between the high potential heme HP2 and P+. To explain the large difference in the rate of oxidation of HP1 and HP2 we propose a tentative model where the heme HP2 is closest to P. 相似文献
189.
Song Jai Young Choi Dong-Woog Lee Jong Seob Kwon Young Myung Kim Sang-Gu 《Plant molecular biology》1998,38(5):735-742
The characterization of a cDNA clone encoding non-specific lipid transfer protein (PvLTP, formerly named PVR3) in the roots of bean seedlings has been previously reported. In this study, we examined the temporal and spatial accumulation of PvLTP mRNA and the effect of the auxin naphthaleneacetic acid (NAA) on the accumulation of PvLTP mRNA during root development. In situ hybridization showed that accumulation of PvLTP mRNA is highly tissue-specific. Accumulation was detected in the cortical tissue, but not in other tissues of root, including the quiescent center and root cap. Within the cortical tissue, accumulation of PvLTP mRNA was developmentally regulated; accumulation of PvLTP mRNA was high in the cortical tissue of the proximal and ground meristem and declined as cortical tissue developed further. Since the appropriate distribution of auxin is an important factor responsible for the maintenance of root meristem organization. We examined effect of auxin on the accumulation of PvLTP mRNA in relation to the development of cortical tissue. In bean seedlings grown on medium supplemented with 5 M NAA, morphological alternations, including radial root expansion and abnormal tissue organization in the root apical meristem, were observed. Only faint accumulation signals of PvLTP mRNA were observed in the cortical tissue of proximal meristem region, indicating that cortical tissue development was repressed by exogenous NAA. However, our results suggest that the change in accumulation of PvLTP mRNA is not direct regulatory effect but reflective effect of altered development of cortical tissue that was induced by exogenous NAA. The temporal and spatial accumulation of PvLTP mRNA indicates that PvLTP is a useful marker for the development of cortical tissue in the root tip in bean seedlings. 相似文献
190.