Structural characterization of myosin from bovine brain

Myosins isolated from bovine brain, rabbit skeletal muscle, and chicken gizzard smooth muscle and their heavy meromyosin and light meromyosin fractions were studied in the electron microscope by negative staining with uranyl acetate. Under similar conditions of preparation and polymerization, the three myosins formed paracrystals of different structures. The light meromyosin portion of the skeletal muscle myosin also assembled in a different fashion than the brain or smooth muscle light meromyosins; the latter two assembled similarly. The heavy meromyosin portion from each of the three myosins was shown to interact with the actins isolated from each of the three tissue sources by the formation of the characteristic arrowhead patterns with similar periodicities. The brain heavy meromyosin attachment to both skeletal and brain actins was dissociated by ATP. It is suggested that differences in the light meromyosin portions of the three myosins may account in part for their differences in assembly in vivo.     

应用电子探针对植物根际和根内营养元素微区分布的探讨

用电子探针可检测出玉米、大豆根际和根内含有Na,Mg,Al,Si,P,S,Cl,K,Ca,Ti,Fe,Cu和Zn 13种元素。这些元素在根际土壤、粘液层和根组织内的含量分布有一定的规律性。除Si,Al,Ca,Fe在根际土壤中峰值较高外,Ti仅在土壤中达到可检测量;S,Fe和Zn富集在粘液层,Mg,P,Cl只在根组织内才有较明显的峰。这些规律可作为区分根—土界面的参考指标。K含量在根内明显高于根际土壤,并由表皮层到中柱径向增加;Ca则与K不同,且受植物种类的影响。     

Compartmentation of glutamic acid metabolism in brain slices

— (1) Compartmentation of glutamate metabolism in brain cortex previously observed only in vivo, has now been demonstrated in vitro.This was shown by using [U-¹⁴C]aspartate and [U-¹⁴C]glutamate as tracer substrates. (2) Preparation and maintenance of the slices at 0° resulted in reversible inhibition of glutamine synthesis. Preincubation at 37° for 10 min or preparation of the slices at room temperature partially overcame this inhibition. (3) Transfer to fresh medium after preincubation had an added stimulatory effect on glutamine synthesis. (4) Incubation in high K⁺ medium (27 mm ) altered the relative specific activity of glutamine. (5) The data are in keeping with the postulate of the existence of at least two different pools of citric acid cycle intermediates in the cerebral cortex.     

Evidence for a new type of endosymbiotic organization in a population of the ciliate Mesodinium rubrum from British Columbia

The ciliate Mesodinium rubrum has excited considerable interest because it has been reported to contain cryptophycean endoxymbionts which lack nuclei (Taylor et al., 1969, 1971). Hibberd (1977), however, has recently shown that in Mesodinium from the United Kingdom the cryptophycean chloroplasts and mitochondria share a common cytoplasm with an apparent cryptophycean nucleus. This cytoplasm is separated from the ciliate cytoplasm by a single membrane. Here we confirm, through extensive observation including serial sectioning, the presence of multiple, discrete, anucleate chloroplast-mitochondrial complexes in a population of Mesodinium from British Columbia. In addition to the normal ciliate macro- and micronuclei, however, these cells contain a third type of nucleus, possibly of cryptophycean origin, which is apparently in its own membrane-bound cytoplasm. This suggests the possibility of a new type of endosymbiotic organization in which endosymbiont organelles have become separated into discrete cytoplasmic compartments.     

Alteration in Neuronal-Glial Metabolism of Glutamate by the Neurotoxin Kainic Acid

The effect of the excitotoxin kainic acid on glutamate and glutamine metabolism was studied in cerebellar slices incubated with D-[2-14C]glucose, [U-14C]gamma-aminobutyric acid, [3H]acetate, [U-14C]glutamate, and [U-14C]glutamine as precursors. Kainic acid (1 mM) strongly inhibited the labeling of glutamine relative to that of glutamate from all precursors except [2-14C]glucose and [U-14C]glutamine. Kainic acid did not inhibit glutamine synthetase directly. The data indicate that in the cerebellum kainic acid inhibits the synthesis of glutamine from the small pool of glutamate that is thought to be associated with glial cells. Kainic acid also markedly stimulated the efflux of glutamate from cerebellar slices and this release was not sensitive to tetrodotoxin. Kainic acid stimulated efflux of both glucose- and acetate-labeled glutamate. In contrast, veratridine released glucose-labeled glutamate preferentially via a tetrodotoxin-sensitive mechanism. Kainic acid did not release [U-14C]glutamate from synaptosomal fractions. These results suggest that the bulk of the glutamate released from cerebellar slices by kainic acid comes from nonsynaptic pools.     

Evidence for a double-helically coiled toroidal chromonema in the dinoflagellate chromosome

The structure of the dinoflagellate chromosome is examined using spread whole mounts and serial sectioning. Evidence is presented that the chromosome is composed of a toroidal bundle of DNA strands which is coiled plectonemically into a tight double helix. This structure is apparent in interphase whole-mount chromosomes and by careful analysis of serial sectioning can be shown in all mitotic stages. It is particularly apparent in the arms of replicating Y shaped chromosomes which often uncoil slightly (either naturally or as a result of the fixation used). A model is presented which is based on these results and recent genetic and biochemical data.     

The tetrameric pheromone module SteC‐MkkB‐MpkB‐SteD regulates asexual sporulation,sclerotia formation and aflatoxin production in Aspergillus flavus

For eukaryotes like fungi to regulate biological responses to environmental stimuli, various signalling cascades are utilized, like the highly conserved mitogen‐activated protein kinase (MAPK) pathways. In the model fungus Aspergillus nidulans, a MAPK pathway known as the pheromone module regulates development and the production of secondary metabolites (SMs). This pathway consists five proteins, the three kinases SteC, MkkB and MpkB, the adaptor SteD and the scaffold HamE. In this study, homologs of these five pheromone module proteins have been identified in the plant and human pathogenic fungus Aspergillus flavus. We have shown that a tetrameric complex consisting of the three kinases and the SteD adaptor is assembled in this species. It was observed that this complex assembles in the cytoplasm and that MpkB translocates into the nucleus. Deletion of steC, mkkB, mpkB or steD results in abolishment of both asexual sporulation and sclerotia production. This complex is required for the positive regulation of aflatoxin production and negative regulation of various SMs, including leporin B and cyclopiazonic acid (CPA), likely via MpkB interactions in the nucleus. These data highlight the conservation of the pheromone module in Aspergillus species, signifying the importance of this pathway in regulating fungal development and secondary metabolism.     

Cytoplasmic dynein's mitotic spindle pole localization requires a functional anaphase-promoting complex, gamma-tubulin, and NUDF/LIS1 in Aspergillus nidulans

In Aspergillus nidulans, cytoplasmic dynein and NUDF/LIS1 are found at the spindle poles during mitosis, but they seem to be targeted to this location via different mechanisms. The spindle pole localization of cytoplasmic dynein requires the function of the anaphase-promoting complex (APC), whereas that of NUDF does not. Moreover, although NUDF's localization to the spindle poles does not require a fully functional dynein motor, the function of NUDF is important for cytoplasmic dynein's targeting to the spindle poles. Interestingly, a gamma-tubulin mutation, mipAR63, nearly eliminates the localization of cytoplasmic dynein to the spindle poles, but it has no apparent effect on NUDF's spindle pole localization. Live cell analysis of the mipAR63 mutant revealed a defect in chromosome separation accompanied by unscheduled spindle elongation before the completion of anaphase A, suggesting that gamma-tubulin may recruit regulatory proteins to the spindle poles for mitotic progression. In A. nidulans, dynein is not apparently required for mitotic progression. In the presence of a low amount of benomyl, a microtubule-depolymerizing agent, however, a dynein mutant diploid strain exhibits a more pronounced chromosome loss phenotype than the control, indicating that cytoplasmic dynein plays a role in chromosome segregation.