We have previously reported the partial purification of a Ca2+- independent phosphoenolpyruvate carboxylase (PEPC) protein-serine/threonine kinase (PEPC-PK) from illuminated leaves of N-sufficient tobacco (Nicotiana tabacum L.) plants (Y.-H. Wang, R. Chollet [1993] FEBS Lett 328: 215–218). We now report that this C3 PEPC-kinase is reversibly light activated in vivo in a time-dependent manner. As the kinase becomes light activated, the activity and L-malate sensitivity of its target protein increases and decreases, respectively. The light activation of tobacco PEPC-PK is prevented by pretreatment of detached leaves with various photosynthesis and cytosolic protein-synthesis inhibitors. Similarly, specific inhibitors of glutamine synthetase block the light activation of tobacco leaf PEPC-kinase under both photorespiratory and nonphotorespiratory conditions. This striking effect is partially and specifically reversed by exogenous glutamine, whereas it has no apparent effect on the light activation of the maize (Zea mays L.) leaf kinase. Using an in situ “activity-gel” phosphorylation assay, we have identified two major Ca2+-independent PEPC-kinase catalytic polypeptides in illuminated tobacco leaves that have the same molecular masses (approximately 30 and 37 kD) as found in illuminated maize leaves. Collectively, these results indicate that the phosphorylation of PEPC in N-sufficient leaves of tobacco (C3) and maize (C4) is regulated through similar but not identical light-signal transduction pathways.
We have used a reductive technique known to produce highly reactive metals to fabricate nickel and nickel-based nanostructured materials. The strong dependence of the magnetic, chemical, electrical, and optical properties of nanostructured materials are intimately correlated with material structure; thus, thorough knowledge of the effect of synthesis parameters on the structure is critical for the refinement of fabrication techniques. X-ray diffraction and electron microscopy are used to determine the effect of the synthetic conditions and subsequent processing on the material structure. Characteristic lengths of these materials range from 3 to 50 nm, depending on synthesis and annealing conditions. Annealing produces a metastable Ni 3 C phase that forms only in the presence of active carbon, suggesting that not only active nickel but also active carbon results from this process. The addition of P(Ph) 3 affects the time and temperature dependence of the nickel crystallite growth, the temperature at which Ni 3 C crystallites are first observed and the maximum temperature to which Ni 3 C can be retained.
Chemical synthesis at elevated temperature (200 °C) produces a highly disordered form of cobalt similar to that produced by mechanical milling. Annealing of the disordered phase produces material with different face-centered cubic (fcc) to hexagonal close-packed (hcp) ratios, depending on the particular thermal treatment and the amount of disorder prior to annealing. The fcc-to-hcp ratio changes with the molecular weight (or boiling point) of the solvent, but this correlation does not appear to be linear. The coercivity and remanence ratio of chemically synthesized cobalt depend on the crystallite size, phases present, and amount of disorder.
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