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41.
To initiate DNA replication, DnaA recognizes and binds to specific sequences within the Escherichia coli chromosomal origin (oriC), and then unwinds a region within oriC. Next, DnaA interacts with DnaB helicase in loading the DnaB-DnaC complex on each separated strand. Primer formation by primase (DnaG) induces the dissociation of DnaC from DnaB, which involves the hydrolysis of ATP bound to DnaC. Recent evidence indicates that DnaC acts as a checkpoint in the transition from initiation to the elongation stage of DNA replication. Freed from DnaC, DnaB helicase unwinds the parental duplex DNA while interacting the cellular replicase, DNA polymerase III holoenzyme, and primase as it intermittently forms primers that are extended by the replicase in duplicating the chromosome.  相似文献   
42.
43.
We investigated the effects of Fe and Cu status of pea (Pisum sativum L.) seedlings on the regulation of the putative root plasma-membrane Fe(III)-chelate reductase that is involved in Fe(III)-chelate reduction and Fe2+ absorption in dicotyledons and nongraminaceous monocotyledons. Additionally, we investigated the ability of this reductase system to reduce Cu(II)-chelates as well as Fe(III)-chelates. Pea seedlings were grown in full nutrient solutions under control, -Fe, and -Cu conditions for up to 18 d. Iron(III) and Cu(II) reductase activity was visualized by placing roots in an agarose gel containing either Fe(III)-EDTA and the Fe(II) chelate, Na2bathophenanthrolinedisulfonic acid (BPDS), for Fe(III) reduction, or CuSO4, Na3citrate, and Na2-2,9-dimethyl-4,7-diphenyl-1, 10-phenanthrolinedisulfonic acid (BCDS) for Cu(II) reduction. Rates of root Fe(III) and Cu(II) reduction were determined via spectrophotometric assay of the Fe(II)-BPDS or the Cu(I)-BCDS chromophore. Reductase activity was induced or stimulated by either Fe deficiency or Cu depletion of the seedlings. Roots from both Fe-deficient and Cu-depleted plants were able to reduce exogenous Cu(II)-chelate as well as Fe(III)-chelate. When this reductase was induced by Fe deficiency, the accumulation of a number of mineral cations (i.e., Cu, Mn, Fe, Mg, and K) in leaves of pea seedlings was significantly increased. We suggest that, in addition to playing a critical role in Fe absorption, this plasma-membrane reductase system also plays a more general role in the regulation of cation absorption by root cells, possibly via the reduction of critical sulfhydryl groups in transport proteins involved in divalent-cation transport (divalent-cation channels?) across the root-cell plasmalemma.  相似文献   
44.
Intracellular phospholipase A(2) (PLA(2)) is responsible for releasing arachidonic acid from cellular phospholipids, and is thought to be the first step in eicosanoid biosynthesis. Intracellular PLA(2)s have been characterized in fat body and hemocytes from tobacco hornworms, Manduca sexta. Here we show that bacterial challenge stimulated increased PLA(2) activity in isolated hemocyte preparations, relative to control hemocyte preparations that were challenged with water. The increased activity was detected as early as 15 s post-challenge and lasted for at least 1 h. The increased activity depended on a minimum bacterial challenge dose, and was inhibited in reactions conducted in the presence of oleyoxyethylphosphorylcholine, a site-specific PLA(2) inhibitor. In independent experiments with serum prepared from whole hemolymph, we found no PLA(2) activity was secreted into serum during the first 24 h following bacterial infection. We infer that a hemocytic intracellular PLA(2) activity is increased immediately an infection is detected. The significance of this enzyme lies in its role in launching the biosynthesis of eicosanoids, which mediate cellular immune reactions to bacterial infection.  相似文献   
45.
We describe a new class of reagents for identifying substrates, adaptors, and regulators of HECT and RING E3s. UBAITs (Ub iquitin‐A ctivated I nteraction T raps) are E3‐ubiquitin fusion proteins and, in an E1‐ and E2‐dependent manner, the C‐terminal ubiquitin moiety forms an amide linkage to proteins that interact with the E3, enabling covalent co‐purification of the E3 with partner proteins. We designed UBAITs for both HECT (Rsp5, Itch) and RING (Psh1, RNF126, RNF168) E3s. For HECT E3s, trapping of interacting proteins occurred in vitro either through an E3 thioester‐linked lariat intermediate or through an E2 thioester intermediate, and both WT and active‐site mutant UBAITs trapped known interacting proteins in yeast and human cells. Yeast Psh1 and human RNF126 and RNF168 UBAITs also trapped known interacting proteins when expressed in cells. Human RNF168 is a key mediator of ubiquitin signaling that promotes DNA double‐strand break repair. Using the RNF168 UBAIT, we identify H2AZ—a histone protein involved in DNA repair—as a new target of this E3 ligase. These results demonstrate that UBAITs represent powerful tools for profiling a wide range of ubiquitin ligases.  相似文献   
46.
Stress granules are large messenger ribonucleoprotein (mRNP) aggregates composed of translation initiation factors and mRNAs that appear when the cell encounters various stressors. Current dogma indicates that stress granules function as inert storage depots for translationally silenced mRNPs until the cell signals for renewed translation and stress granule disassembly. We used RasGAP SH3-binding protein (G3BP) overexpression to induce stress granules and study their assembly process and signaling to the translation apparatus. We found that assembly of large G3BP-induced stress granules, but not small granules, precedes phosphorylation of eIF2α. Using mouse embryonic fibroblasts depleted for individual eukaryotic initiation factor 2α (eIF2α) kinases, we identified protein kinase R as the principal kinase that mediates eIF2α phosphorylation by large G3BP-induced granules. These data indicate that increasing stress granule size is associated with a threshold or switch that must be triggered in order for eIF2α phosphorylation and subsequent translational repression to occur. Furthermore, these data suggest that stress granules are active in signaling to the translational machinery and may be important regulators of the innate immune response.  相似文献   
47.
It has been suggested that the absence of floral rewards in many orchid species causes pollinators to probe fewer flowers on a plant, and thus reduces geitonogamy, i.e. self-pollination between flowers, which may result in inbreeding depression and reduced pollen export. We examined the effects of nectar addition on pollinator visitation and pollen transfer by tracking the fate of colour-labelled pollen in Anacamptis morio, a non-rewarding orchid species pollinated primarily by queen bumble-bees. Addition of nectar to spurs of A. morio significantly increased the number of flowers probed by bumble-bees, the time spent on an inflorescence, pollinarium removal and the proportion of removed pollen involved in self-pollination through geitonogamy, but did not affect pollen carryover (the fraction of a pollinarium carried over from one flower to the next). Only visits that exceeded 18 s resulted in geitonogamy, as this is the time taken for removed pollinaria to bend into a position to strike the stigma. A mutation for nectar production in A. morio would result in an initial 3.8-fold increase in pollinarium removal per visit, but also increase geitonogamous self-pollination from less than 10% of pollen depositions to ca. 40%. Greater efficiency of pollen export will favour deceptive plants when pollinators are relatively common and most pollinaria are removed from flowers or when inbreeding depression is severe. These findings provide empirical support both for Darwin's contention that pollinarium bending is an anti-selfing mechanism in orchids and for the idea that floral deception serves to maximize the efficiency of pollen export.  相似文献   
48.
The effects of the abundance of acorns of the oak, Quercus crispula, on the population dynamics of three rodent species (Apodemus speciosus, A. argenteus, and Clethrionomys rufocanus) were analyzed using time series data (1992–2006). The data were obtained in a forest in northern Hokkaido, Japan, by live trapping rodents and directly counting acorns on the ground. Apodemus speciosus generally increased in abundance following acorn masting. However, the clear effect of acorn abundance was not detected for the other two rodent species. Acorns of Q. crispula contain tannins, which potentially have detrimental effects on herbivores. Apodemus speciosus may reduce the damage caused by acorn tannins with tannin-binding salivary proteins and tannase-producing bacteria, whereas such physiological tolerance to tannins is not known in the other two rodent species. The differences in the effects of acorns between the three species may be due to differences in their physiological tolerance to tannins.  相似文献   
49.
Bioethanol is mainly produced from food crops such as sugar cane and maize, and this has been held partly responsible for the rise of food commodity prices. Tobacco, integrated in biorefinery facilities for the extraction of different compounds, could become an alternative feedstock for biofuel production. When grown for energy production, using high plant densities and several mowings during the growing season, tobacco can produce large amounts of inexpensive green biomass. We have bred two commercial tobacco cultivars (Virginia Gold and Havana 503B) to increase the carbohydrate content by the overexpression of thioredoxin f in the chloroplast. Marker-free transplastomic plants were recovered and their agronomic performance under field conditions was evaluated. These plants were phenotypically equivalent to their wild types yet showed increased starch (up to 280 %) and soluble sugar (up to 74 %) contents in leaves relative to their control plants. Fermentable sugars released from the stalk were also higher (up to 24 %) for transplastomic plants. After heat pretreatment, enzymatic hydrolysis and yeast fermentation of leaf and stalk hydrolysates, an average of 20–40 % more ethanol was obtained from transplastomic plants than their wild-type controls. We propose an integral exploitation of the entire tobacco plant managed as a forage crop (harvesting sugar and starch-rich leaves and lignocellulosic stalks) that could considerably cheapen the entire production process.  相似文献   
50.
A bioluminescence chemical oceanography research cruise (Varifront III) through the Sea of Cortez from November through December 1981 provided an opportunity to investigate plankton associated with a brilliant and extensive display of surface water bioluminescence at the north end of Balleñas Channel. New observations of bioluminescence were made on larval stages of the euphausiid Nyctiphanes simplex Hansen (Calyptopis II, Furcilia I, II, and III, and juveniles) and Euphausia eximia Hansen (Calyptopis I), the Calanoida copepods Centropages furcatus Dana, Paracalanus indiens Wolfenden, Acrocalanus longicornis Giesbrecht, the Cyclopoida copepods Corycaeus (Corycaeus) speciousus Dana, Corycaeus (Onychocorycaeus) latus Dana, and several dinoflagellates Ceratium breve Ostenfeld and Schmidt, Ceratium horridum Gran, and Ceratium gibberum Gourret. These observations indicate the increasing importance of some of the smaller copepods and larval euphausiids contributing to surface bioluminescence.  相似文献   
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