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1.
A malate dehydrogenase was purified from the unicellular green alga Chlorogonium elongatum Dangeard. The enzyme was localized in the mitochondria by immunogold electron microscopy and was found to be present on the cristae. The concentration of the enzyme is regulated by acetate and light. In cells cultured heterotrophically with acetate as carbon source the activity and the concentration of the enzyme is 5- to 6-fold higher than in autotrophic cells. In mixotrophically cultured cells (light and acetate) the enzyme level attains only half of the value of that in heterotrophic cells. Acetate induces an increase of the enzyme concentration while light has an inhibitory effect on this process. 相似文献
2.
The nucleotide sequences of the plastid 16S rDNA of the multicellular red alga Antithamnion sp. and the 16S rDNA/23S rDNA intergenic spacers of the plastid DNAs of the unicellular red alga Cyanidium caldarium and of Antithamnion sp. were determined. Sequence comparisons support the idea of a polyphyletic origin of the red algal and the higher-plant chloroplasts. Both spacer regions include the unsplit tRNAIle (GAU) and tRNAAla (UGC) genes and so the plastids of both algae form a homogeneous group with those of chromophytic algae and Cyanophora paradoxa characterized by small-sized rDNA spacers in contrast to green algae and higher plants. Nevertheless, remarkable sequence differences within the rRNA and the tRNA genes give the plastids of Cyanidium caldarium a rather isolated position. 相似文献
3.
The structural transitions occurring during the assembly and disassembly of pig brain microtubule protein were investigated by time-resolved X-ray scattering using synchrotron radiation. The reactions were introduced by a slow temperature scan (2 deg.C/min) from 0 °C to 37 °C and back. Several structurally distinct states could be resolved during one cycle of assembly/disassembly. During the temperature rise, one observes four main phases: prenucleation events, microtubule nucleation, growth, and postassembly events.Heating from 0 °C to 22 °C results in a biphasic breakdown of rings and other aggregates, while the apparent mean diameter increases from 38 to 41 nm. Parallel time-resolved electron microscopic observations suggest that the initial solution contains several types of aggregates, mostly double concentric and single rings, but also rod-like particles, clusters of rings and other aggregates. All of these tend to break down with increasing temperature. Double concentric rings seem to dissociate into large and small single rings before both types of rings break down into protofilament fragments and tubulin subunits. From the breakdown products, associations of several protofilament fragments are formed, which are important for initiating microtubule nucleation. Assembly of nuclei begins around 22 °C. Microtubule elongation takes place between 25 and 30 °C. They grow mainly by addition of tubulin subunits but not via rings.During the reverse temperature scan, microtubules shorten by the release of subunits and/or small protofilament fragments from their ends. The degree of disassembly is strongly increased below 22 °C. Below about 10 °C rings are reformed, probably from the fragments, but their final number is much less than initially.Conditions that prevent microtubule nucleation such as GDP or Ca2+ also stabilize rings, even at 37 °C. Thus, rings are viewed as storage aggregates of tubulin and microtubule associated proteins, whose breakdown is a prerequisite for microtubule formation, and whose reformation is independent of microtubule breakdown.The midpoints of microtubule growth and breakdown differ by about 12 deg.C so that the system shows hysteresis-like behavior. It is dependent on microtubule formation and is not seen when the temperature is cycled below that required for nucleation. Thus, even during a slow temperature scan, microtubule assembly is kinetically limited by nucleation. By contrast, depolymerization proceeds close to equilibrium.The radius of gyration of the tubulin heterodimers is 3.1 nm. The weight average diameter of rings in cold solutions is 38 nm, that of microtubules is 24.5 nm.At radiation dose rates of about 100 rad/s. radiation damage is of minor importance, as judged by the criterion of polymerizability. Total doses of up to 500,000 rad can be applied.Some concepts of analyzing time-resolved X-ray scattering data are presented. They make use of the fact that the scattering intensities vary continuously both with scattering angle and time. Cross-correlation of different regions of the pattern, and comparison of their temperature derivatives, reveals structural transitions not seen by other techniques. 相似文献
4.
the entire mitochondrial genome (mt genome) of the unicellular green alga Platymonas subcordiformis (synonym Tetraselmis subcordiformis; Prasinophyceae) was cloned and a physical map for the four restriction enzymes Hind III, Eco RI, Bgl II and Xba I was constructed. The mt genome of P. subcordiformis is a 42.8 kb circular molecule, coding for at least 23 genes. Hybridization and sequence analysis revealed the presence of a ca. 1.5 kb inverted repeat on the mt genome of P. subcordiformis. Phylogenetic analyses based on sequences of several coxI genes were carried out. Our data indicate that mitochondria from P. subcordiformis and from land plants form a natural, monophyletic group. 相似文献
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Phyto- and protozooplankton biomass during austral summer in surface waters of the Weddell Sea and vicinity 总被引:3,自引:3,他引:0
Summary Phyto- and protozooplankton were sampled in the upper 10 m of the water column in austral summer during a cruise of RV Polarstern from January 6 to February 20 1985 in the eastern Bransfield Strait vicinity and in the northern, southeastern (off Vestkapp, twice: I and II) and southern Weddell Sea (Vahsel Bay across the Filchner Depression to Gould Bay). The plankton assemblages are discussed in relation to physical, chemical and biological factors in the different geographical areas in summer. Phytoplankton biomass (Phytoplankton carbon, PPC) ranged from 4–194 g carbon/l and consisted on average of 65% diatoms and 35% autotrophic flagellates. Whereas in the northwest phytoplankton assemblages were dominated by small nanoflagellates (78% of PPC), higher biomass of diatoms (54–94% of PPC) occurred at the other sampling sites. In general autotrophic flagellates and small pennate diatoms dominated at oceanic stations; in neritic areas large centric diatoms prevailed. Chlorophyll a concentrations ranged from 0.25–3.14/g chl a/l with a mean of 1.13/gmg chlorophyll a/l and an average phytoplankton carbon/chlorophyll a ratio of 39. Protozooplankton biomass (Protozooplankton carbon, PZC) ranged from 0–67 g carbon/l and consisted of 49% ciliates, 49% heterotrophic dinoflagellates and 2% tintinnids. Heterotrophic dinoflagellates were more important in the northern investigation areas (58%–84% of PZC). Ciliates dominated the protozooplankton in the southeast and south (56%–65% of PZC); higher abundances of tintinnids were observed only in the south (11% of PZC). The most remarkable feature of the surface waters was the high protozooplankton biomass: protozooplankton amounted to 25% on an average of the combined biomass of PPC plus PZC for the entire investigation period. Protozoan biomass in the southeastern and southern Weddell Sea occasionally exceeded phytoplankton biomass. Temperature, salinity, and inorganic nutrients were generally lower in the southern regions; at most of these stations a meltwater layer occurred in the upper meters of the water column. We suggest that this physical regime allows a well developed summer system with a high proportion of heterotrophic microplankton. In the eastern Bransfield Strait, in the northern Weddell Sea and close to the coast off Vestkapp (I), however, early summer conditions occurred with less protozooplankton contribution.Contribution no. 427 from the Alfred-Wegener-Institute for Polar and Marine Research 相似文献
8.
In the green flagellate Chlorogonium elongatum the cAMP concentration of the cells depends on the culture conditions of the alga. Autotrophically cultured cells possess three times more cAMP than cells cultured heterotrophically (in the dark with acetate). The activity of adenylate cyclase in a membrane fraction of Chlorogonium is distinctly stimulated by GTPγS. Such a stimulation is generally regarded as an indication of the participation of heterotrimeric G-proteins in this regulatory process. The activity of phosphodiesterase is stimulated in vitro by calmodulin and inhibited by trifluoperazin and EGTA. 相似文献
9.
Nitrogen turnover in soil and global change 总被引:3,自引:0,他引:3
Ollivier J Töwe S Bannert A Hai B Kastl EM Meyer A Su MX Kleineidam K Schloter M 《FEMS microbiology ecology》2011,78(1):3-16
Nitrogen management in soils has been considered as key to the sustainable use of terrestrial ecosystems and a protection of major ecosystem services. However, the microorganisms driving processes like nitrification, denitrification, N-fixation and mineralization are highly influenced by changing climatic conditions, intensification of agriculture and the application of new chemicals to a so far unknown extent. In this review, the current knowledge concerning the influence of selected scenarios of global change on the abundance, diversity and activity of microorganisms involved in nitrogen turnover, notably in agricultural and grassland soils, is summarized and linked to the corresponding processes. In this context, data are presented on nitrogen-cycling processes and the corresponding microbial key players during ecosystem development and changes in functional diversity patterns during shifts in land use. Furthermore, the impact of increased temperature, carbon dioxide and changes in precipitation regimes on microbial nitrogen turnover is discussed. Finally, some examples of the effects of pesticides and antibiotics after application to soil for selected processes of nitrogen transformation are also shown. 相似文献
10.
Carsten Frühbeis Dominik Fr?hlich Wen Ping Kuo Jesa Amphornrat Sebastian Thilemann Aiman S. Saab Frank Kirchhoff Wiebke M?bius Sandra Goebbels Klaus-Armin Nave Anja Schneider Mikael Simons Matthias Klugmann Jacqueline Trotter Eva-Maria Kr?mer-Albers 《PLoS biology》2013,11(7)
Reciprocal interactions between neurons and oligodendrocytes are not only crucial for myelination, but also for long-term survival of axons. Degeneration of axons occurs in several human myelin diseases, however the molecular mechanisms of axon-glia communication maintaining axon integrity are poorly understood. Here, we describe the signal-mediated transfer of exosomes from oligodendrocytes to neurons. These endosome-derived vesicles are secreted by oligodendrocytes and carry specific protein and RNA cargo. We show that activity-dependent release of the neurotransmitter glutamate triggers oligodendroglial exosome secretion mediated by Ca2+ entry through oligodendroglial NMDA and AMPA receptors. In turn, neurons internalize the released exosomes by endocytosis. Injection of oligodendroglia-derived exosomes into the mouse brain results in functional retrieval of exosome cargo in neurons. Supply of cultured neurons with oligodendroglial exosomes improves neuronal viability under conditions of cell stress. These findings indicate that oligodendroglial exosomes participate in a novel mode of bidirectional neuron-glia communication contributing to neuronal integrity. 相似文献